Biogas Upgrading via Hydrogenotrophic Methanogenesis in Two

Sep 21, 2015 - This study proposes an interesting solution to store the electricity as CH4 and, together, upgrade biogas to higher CH4 content. Anaero...
1 downloads 15 Views 1MB Size
Subscriber access provided by University of Otago Library

Article

Biogas upgrading via hydrogenotrophic methanogenesis in two-stage Continuous Stirred Tank Reactors at mesophilic and thermophilic conditions Ilaria Bassani, Panagiotis G Kougias, Laura Treu, and Irini Angelidaki Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.5b03451 • Publication Date (Web): 21 Sep 2015 Downloaded from http://pubs.acs.org on September 24, 2015

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 30

Environmental Science & Technology

1

Biogas upgrading via hydrogenotrophic

2

methanogenesis in two-stage Continuous Stirred

3

Tank Reactors at mesophilic and thermophilic

4

conditions

5

Ilaria Bassani, Panagiotis G. Kougias, Laura Treu, Irini Angelidaki*

6

Department of Environmental Engineering, Technical University of Denmark, Kgs. Lyngby,

7

Denmark

8

KEYWORDS biogas upgrading, methane, anaerobic digestion, methanogens, metagenomics,

9

random DNA sequencing, full-length 16S rRNA, hydrogenotrophic archaea

10

ACS Paragon Plus Environment

1

Environmental Science & Technology

Page 2 of 30

11

ABSTRACT This study proposes an innovative setup composed by two stage reactors to

12

achieve biogas upgrading coupling the CO2 in the biogas with external H2 and subsequent

13

conversion into CH4 by hydrogenotrophic methanogenesis. In this configuration, the biogas

14

produced in the first reactor was transferred to the second one, where H2 was injected. This

15

configuration was tested at both mesophilic and thermophilic conditions. After H2 addition, the

16

produced biogas was upgraded to average CH4 content of 89% in the mesophilic reactor and

17

85% in the thermophilic. At thermophilic conditions, a higher efficiency of CH4 production and

18

CO2 conversion was recorded. The consequent increase of pH did not inhibit the process

19

indicating adaptation of microorganisms to higher pH levels. The effects of H2 on the microbial

20

community were studied using high-throughput Illumina random sequences and full-length 16S

21

rRNA genes extracted from the total sequences. The relative abundance of archaeal community

22

markedly increased upon H2 addition with Methanoculleus as dominant genus. The increase of

23

hydrogenotrophic methanogens and syntrophic Desulfovibrio and the decrease of aceticlastic

24

methanogens indicate a H2-mediated shift towards the hydrogenotrophic pathway enhancing

25

biogas upgrading. Moreover, Thermoanaerobacteraceae were likely involved in syntrophic

26

acetate oxidation with hydrogenotrophic methanogen in absence of aceticlastic methanogenesis.

27

INTRODUCTION

28

Wind and biomass are promoted worldwide as sustainable forms of energy. The Danish energy

29

policy has set a goal to cover 50% of electricity demand in 2020 by exploitation of wind energy

30

(1). Moreover, it is a goal to use up to 50% of the manure for bioenergy production. With the

31

expansion of the wind mill sector, the necessity for electricity storage has arisen. This study

32

proposes an interesting solution to store the electricity as CH4 and, together, upgrade biogas to

33

higher CH4 content. Anaerobic digestion (AD) of biomass produces biogas with ~50-70% CH4

ACS Paragon Plus Environment

2

Page 3 of 30

Environmental Science & Technology

34

and 30-50% CO2. Biogas containing higher concentrations of CH4 (>90%) has higher heating

35

value and can be used as fuel for cars or transported through the national gas grid (2).

36

Nevertheless, although several biogas upgrading methods exist, their cost is commonly high (3).

37

Renewable electricity utilization is expanding worldwide and uneven production of wind and

38

solar energy can result in excess of resources. This surplus can be used to electrolyze water to

39

produce H2. Nevertheless, H2 as fuel presents some drawbacks related to its low volumetric

40

energy content and difficulty in storage and transport (4).

41

Biological biogas upgrading, coupling the H2, produced by water electrolysis, with the CO2 in

42

biogas and converting it to CH4, has been recently reported (5). Further studies showed that the

43

H2 injection into the reactor can convert >40% of the CO2 present in biogas (6). Although

44

biological biogas upgrading has several advantages, the direct H2 injection in the reactor (in-situ

45

upgrading) can cause technical challenges. CO2 removal could lead to a substantial increase of

46

pH negatively affecting the process. In addition, H2 low gas-liquid mass transfer rate is a

47

process’ limiting factor (7), because it must be dissolved in reactor’s liquid phase to be utilized

48

by microorganisms.

49

AD process occurs by a combination of pathways assigned to an extremely complex and

50

specialized microbial consortium determined by bacteria-archaea interactions, resilient to process

51

variations (8). Specifically, bacteria hydrolyze polymers into monomers, and then ferment them

52

to lactate, volatile fatty acids (VFA) and alcohols. These products are further fermented by

53

syntrophic bacteria to acetate, formate, H2, and CO2 that are used as substrates by methanogens.

54

Syntrophic interactions are necessary because these reactions are thermodynamically

55

unfavorable unless their products are kept at low concentrations by a second microorganism,

56

such as methanogens, that utilizes them (9). For example, dissolved H2 concentration has a role

ACS Paragon Plus Environment

3

Environmental Science & Technology

Page 4 of 30

57

in products’ levels regulation. High H2 partial pressure leads to propionate and butyrate

58

accumulation, while low H2 partial pressure enhances CO2 and CH4 production (9). Normally,

59

most of the CH4 is formed from acetate (70%), mainly by aceticlastic methanogenesis, e.g.

60

Methanosarcinales, and bacterial syntrophic acetate oxidation (SAO, i.e. oxidation of acetate to

61

CO2 and H2) and only the remaining 30% is produced directly from H2/CO2 (8, 10).

62

Methanogenesis from H2/CO2 is mainly carried out by hydrogenotrophic methanogens that

63

reduce CO2 with H2 to produce CH4. Therefore, we hypothesized that the H2 addition changes the

64

microbial community composition promoting the hydrogenotrophic methanogenic pathway and

65

the CO2 consumption. Previous studies reported that, in H2-mediated upgraded reactors,

66

dominant archaeal species belonged to Methanobacteriales, i.e. Methanothermobacter

67

thermautotrophicus (11). However, because of the complexity of the microbial community, most

68

of the biogas reactor population is still unidentified and poorly characterized.

69

In this experiment a novel reactor configuration was designed to investigate the effect of the

70

H2 addition on AD process performance. Moreover, different bioinformatics approaches were

71

combined to achieve a more complete insight into the microbial community composition. The

72

setup was composed by two serial-connected continuous stirred-tank reactors (CSTRs), treating

73

cattle manure. The first reactor was the main biogas producer, while the second one, where H2

74

was injected, was treating the effluent from the first reactor and serving as upgrading chamber,

75

responsible for the conversion of CO2 to CH4. Mesophilic and thermophilic conditions were

76

applied to investigate the effect of the temperature on the process. The microbial community was

77

analyzed with three innovative metagenomic approaches: (I) taxonomy assignment performed

78

directly on Total Random Sequences (TRS), (II) 16S rRNA Shotgun Reads (16S SR) extracted

79

from TRS, (III) Assembled Full-Length 16S rRNA gene sequences (16S AFL) from 16S SR.

ACS Paragon Plus Environment

4

Page 5 of 30

Environmental Science & Technology

80

TRS were mapped against unique clade-specific marker genes to achieve an accurate

81

classification level and relative abundance estimation. Moreover, the recently developed method

82

(12) that aligns short-read DNA sequences to a large 16S rRNA database, to reconstruct the

83

complete 16S rRNA gene, was, for the first time, applied to the study of the biogas reactor

84

community.

85 86

MATERIALS AND METHODS

87

Substrate characteristics and feedstock preparation

88

The cattle manure substrate was obtained from the Hashøj biogas plant, Denmark, sieved

89

through a 2 mm net to remove large particles, such as barley residues, and stored at -20°C, in 5 L

90

bottles. Before usage, the substrate was thawed at 4°C for 1-2 days. The manure had a pH of

91

7.44, total solids (TS) and volatile solids (VS) content of 47.40±1.86 and 34.56±1.40 g/L,

92

respectively. The total Kjeldahl Nitrogen (TKN) and ammonium nitrogen NH4+ (NH4–N) were

93

3.03 ± 0.10 and 2.07 ± 0.01 g-N/L, respectively. The concentration of total VFA was 6.83 ± 0.48

94

g/L.

95 96

Reactor’s setup and operation

97

The setup consisted of two analogous two-stage CSTR, each with a total working volume of

98

3.5 L. The working volume of primary (R1 and R2) and secondary (SR1 and SR2) reactors were

99

1.5 L and 2 L, respectively. Both primary and secondary reactors were filled with inoculum,

100

obtained from Hashøj (mesophilic) and Snertinge (thermophilic) biogas plants, Denmark. Main

101

substrates treated by these biogas plants are animal slurry (pig and cattle) and wastes from food

102

industries. Each reactor was mixed by magnetic stirrers. The temperature of R1 and SR1 was

ACS Paragon Plus Environment

5

Environmental Science & Technology

Page 6 of 30

103

maintained at mesophilic conditions (35 ± 1 °C), while R2 and SR2 were kept at thermophilic

104

conditions (55 ± 1 °C) using thermal jackets. Primary reactors were fed twice per day with cattle

105

manure, while the secondary were fed with the effluent from the primary. Moreover, the biogas

106

produced in the primary reactors was transferred to the secondary ones. The hydraulic retention

107

times (HRTs) were selected to simulate full-scale applications conditions and set to 25 days for

108

R1, 33 days for SR1, 15 days for R2 and 20 days for SR2. The total organic loading rate (OLR)

109

was 0.6 and 1 gVS/Lday for mesophilic and the thermophilic reactor system, respectively.

110

Although the inocula were originating from reactors operated at same temperature, the initial

111

HRT was dedicated to allow adaptation of the inocula to the operational conditions. The

112

reactors’ working conditions were ensured during the whole experiment, avoiding leakages and

113

filtering substrate and inoculum, to minimize fibers accumulation of consequent working volume

114

reduction. Once steady state conditions were achieved, H2 was continuously injected to the

115

secondary reactors through a diffuser placed at the bottom of the reactor. The H2 flow rate was

116

established according to the stoichiometry of hydrogenotrophic methanogenesis reaction: per

117

each mole of CO2 contained in the biogas before the H2 addition, 4 moles of H2 were added, i.e.

118

~192 and 510 mL/Lday for mesophilic and thermophilic reactor, respectively.

119 120

Analytical methods

121

The biogas production of both reactors was measured daily by an automated gas meter with a

122

100 mL reversible cycle and registration (13). TS, VS, NH4–N and TKN were measured

123

according to the Standard Methods for Examination of Water and Wastewater (14). Samples

124

from primary and secondary reactors were collected for pH and VFA analysis twice per week.

125

The pH was measured immediately after the collection to avoid the CO2 removal from the liquid

ACS Paragon Plus Environment

6

Page 7 of 30

Environmental Science & Technology

126

phase, by a digital PHM210 pH meter connected to the Gel pH electrode (pHC3105–8;

127

Radiometer analytical). VFA samples were prepared according to Kougias and coworkers (15).

128

VFA concentration was determined using a gas chromatograph (GC-2010; Shimadzu) with a

129

flame ionization detector and FFAP column as described previously (15) The biogas composition

130

was measured twice per week by a Gas Chromatograph equipped with a Thermal Conductivity

131

Detector (GC-TCD) as described previously (16). For batches assays, the CH4 production was

132

measured regularly by a gas Chromatograph with a Flame Ionization Detector (FID), as

133

described previously (17). Detention limits for the measurement of CH4, CO2 and H2 by GC

134

were defined by the calibration curve (5-100%), while the detection limits for VFA were 5-1500

135

mg/L.

136 137

Effect of pH on specific hydrogenotrophic methanogenic activity

138

A batch assay was performed to determine the hydrogenotrophic methanogenic activity of the

139

microbial community of SR1 and SR2 at different pH values, at steady state after the H2

140

addition. Inoculum from the secondary reactors was obtained and immediately transferred to 118

141

mL serum bottles (20 mL of inoculum each batch), flushing with N2. The pH was adjusted to 6.0,

142

7.0, 8.0, 8.5 and 10.0 using N2:CO2 (80:20) or HCl or NaOH 2 M. All the batches were prepared

143

in triplicates, sealed with rubber stoppers (Rubber B.V., Hilversum, NL) and aluminum caps

144

(Wheaton, Millville, NJ). 1 atm of H2 and CO2 (80:20) were injected in the batches and they

145

were incubated at 35°C and 55°C and 200 rpm. The results were expressed as CH4 yield

146

(mL/gVS).

147 148

Microbial Community Composition

ACS Paragon Plus Environment

7

Environmental Science & Technology

Page 8 of 30

149

Samples were obtained during reactors’ steady state operation to ensure representative process

150

conditions and microbial community stability. Genomic DNA was extracted from the secondary

151

reactors with RNA PowerSoil® DNA Elution Accessory Kit (MO BIO Laboratories, Carlsbad,

152

CA). An initial filtration step, through a 100 µm nylon cell strainer filter, was introduced to

153

remove the plant residues originating from animal feed (i.e. barley plant). The sample was

154

centrifuged at 2500 g for 10 min and the supernatant was discarded recovering ~2 g of pellet.

155

Quality and quantity of the DNA extracted were determined using NanoDrop (ThermoFisher

156

Scientific, Waltham, MA) and Qbit fluorimeter (Life Technologies, Carlsbad, CA). Samples

157

were sequenced by the Ramaciotti Centre for Gene Function Analysis (UNSW, Sydney), using

158

NextSeq 500 sequencing technology and Nextera XT kit (Illumina, San Diego, CA) for library

159

preparation (150+150 bp).

160

Rarefaction curves and alpha diversity indexes were calculated with MG-RAST toolkit (18) on

161

16S SR as described by Kougias and co-workers (19), except for the minimum alignment length

162

that was 150 bp.

163

Total raw reads were filtered to remove the low quality sequences using Trimmomatic (20).

164

The rRNA-like sequences were extracted using riboPicker (21), by aligning the TRS with

165

SILVA and Ribosomal Database Project (RDP) 16S rRNA gene sequences databases. Extracted

166

sequences were assembled into the complete 16S rRNA using EMIRGE (12). 16S AFL were

167

taxonomically assigned by RDP classifier (cutoff 0.8; 22). A further alignment was performed

168

using BLASTN against NCBI 16S rRNA database (23). The taxonomic level was assigned

169

according to the identity thresholds reported by Yarza and co-workers (24). Phylogenetic trees

170

representing complete BLAST results were drawn using MEGAN software (25).

ACS Paragon Plus Environment

8

Page 9 of 30

Environmental Science & Technology

171

The composition of the microbial community was further determined from TRS using

172

Metagenomic Phylogenetic Analysis (MetaPhlAn) tool (distance function "braycurtis",

173

"correlation" method; 26). The minimum value of relative abundance considered was 0.01%. The

174

microbial relative abundance is indicated in the results as percentage of the total community. The

175

phylogenetic tree representing the microbial community was drawn using GraPhlAn

176

(https://bitbucket.org/nsegata/graphlan). Finally, heat maps representing the relative abundance

177

and the folds change of microorganisms were drawn using Multiexperiment viewer (MeV; 27).

178

Raw Illumina sequences were submitted to the NCBI sequence read archive database (SRA)

179

with accession number SRP058235. 16S AFL data were submitted to MG-RAST with accession

180

numbers

181

(thermophilic preH2) and 4623917.3 (thermophilic postH2).

4623916.3

(mesophilic

preH2),

4623915.3

(mesophilic

postH2),

4624043.3

182 183

RESULTS AND DISCUSSION

184

Process monitoring and biogas upgrade

185

Operational data from the reactors under steady state conditions before and after the H2

186

addition are summarized in Table 1. The biogas produced by the primary reactor alone

187

constituted 97% and 74% of the total biogas, in mesophilic and thermophilic reactor,

188

respectively. Before the H2 addition, the CH4 content of the mesophilic reactor was ~70%. After

189

the H2 addition, the CH4 production rate increased by 53% resulting in an average CH4 content of

190

~89% (with a maximum of 92%). Correspondingly, the CO2 production rate decreased by 65%,

191

due to the conversion of CO2 to CH4 resulting in an average CO2 content of 9% (with a minimum

192

of 5%) (Figure S1a). Similarly, in the thermophilic reactor before the H2 addition, the CH4

193

content was ~67%. After the H2 addition, the CH4 production rate increased by 45% leading to an

ACS Paragon Plus Environment

9

Environmental Science & Technology

Page 10 of 30

194

average CH4 content of ~85% (with a maximum of 91%). Correspondingly, the CO2 production

195

rate decreased by 77% leading to an average CO2 content of 7% (with a minimum of 6%) (Figure

196

S1b). In both reactors, the decrease of CO2 production rate in the biogas was higher than the

197

increase of CH4 production rate. This was likely due to the concomitant pH increase, resulting in

198

a larger portion of CO2 dissolved in the reactor liquid phase, as bicarbonate.

199

Notably, after the H2 addition, the CH4 yield derived from the H2-mediated CO2 conversion to

200

CH4 represented ~25% of the total CH4 yield, in both reactors. Moreover, the CH4 yield resulting

201

from the degradation of manure increased by 14% and by 7% in mesophilic and thermophilic

202

reactor, respectively (Figure S2a and b).

203

The biogas quality achieved fulfills the objective of >90% CH4 content, at both temperature

204

conditions, increasing biogas heating value and extending its possible usage as energy carrier (2).

205

In the mesophilic reactor, 99% of the injected H2 (with a maximum of 100%) was utilized and

206

54% of the CO2 (with a maximum of 77%) was converted to CH4 (Figure S3a). Similarly, in the

207

thermophilic, 92% of the H2 injected (with a maximum of 100%) was utilized and the 62% of the

208

CO2 (with a maximum of 73%) was converted to CH4 (Figure S3b). The incomplete conversion

209

of CO2 could be due to inadequate amount of injected H2. As previously described the H2 flow

210

rate was set according to the stoichiometry of hydrogenotrophic methanogenesis reaction.

211

However, an excess of biogas, and thus CO2, was produced after the H2 addition, compared to

212

the pre H2 phase, resulting in a surplus of unconverted CO2 (Table 1).

213

Besides the incomplete removal of CO2, the unutilized H2 found in the gas phase was likely

214

due to the low hydrogen gas-liquid mass transfer rate (7). As previously observed the gas transfer

215

to the liquid phase plays an important role for H2 microbial uptake and is also influenced by the

ACS Paragon Plus Environment

10

Page 11 of 30

Environmental Science & Technology

216

stirring speed (28). In this experiment, the H2 gas transfer was not fast enough leading to the

217

accumulation of not dissolved H2 to the reactor’s head space.

218

In both primary reactors pH and VFA levels remained stable for the whole experiment (Figure

219

S4a and c). In the secondary, VFA levels remained stable, while the pH, upon the H2 addition,

220

increased to approximately 8.2 in SR1 and more markedly to 8.5 in SR2 (Figure S4b and d). The

221

lower increase found in SR1 is likely due to the lower conversion efficiency observed in the

222

mesophilic reactor. Although methanogenesis normally occurs in a pH range between 6.5 and 8.5

223

and the process can be severely affected if the pH is below 6 or above 8.5 (29), in this

224

experiment, no process inhibition or reduction in the conversion of CO2 to CH4 were observed.

225

To verify this finding a batch assay was performed, in which the hydrogenotrophic

226

methanogenic activity of inocula from SR1 and SR2 at steady state after the H2 addition was

227

tested at different pH values. The results from this test confirmed the feasibility of

228

biomethanation process at a maximum pH of 8.5 for both temperature conditions (although with

229

significantly decreased CH4 yields) stating the adaptation of microorganisms to the increased pH

230

values. Conversely, pH levels above 8.5 resulted in fatal deterioration of the process (Figure 1).

231 232

Comparison of mesophilic versus thermophilic reactor performances

233

Upon the H2 addition, a significant increase in CH4 content was achieved at both temperature

234

conditions. Nevertheless, although in the mesophilic reactor a higher percentage of H2 was

235

utilized, the CO2 converted was lower (Table 1 and Figure S3a and b).

236

Thermophilic conditions are commonly associated with higher CH4 yields and production rates

237

(30). By comparing VS of the primary reactors, 20% less organic material was detected at

238

thermophilic conditions, meaning that at higher temperature 20% more biomass was degraded

ACS Paragon Plus Environment

11

Environmental Science & Technology

Page 12 of 30

239

(data not shown). This is in accordance with previous report where degradation of organic matter

240

at thermophilic conditions was more efficient than at mesophilic (30). This result partially

241

validates the different performances of continuous reactors. However, because the OLR was

242

different, a direct comparison of the performances was not feasible. The methanogenic activity

243

of mesophilic and thermophilic inocula used in the continuous reactors was tested in a batch

244

assay with glucose as substrate (Supporting Information). By comparing the resulting CH4

245

production rates, the CH4 potential of the glucose, at standard temperature and pressure

246

conditions, resulted in 54% higher CH4 productivity at 55°C than 37°C (data not shown).

247

Therefore, these results attribute to the inoculum an important role in the discrepancies observed

248

in reactors’ process stating its importance in process performances.

249 250

Microbial Community Composition

251

Rarefaction curves and alpha diversity indexes showed high community dynamicity, with

252

higher diversity in mesophilic reactor (Table 2 and Figure S5). After the H2 addition, the

253

diversity decreased at both temperature conditions resulting into a more specialized community.

254

Although the taxonomic assignment based on a single gene has various disadvantages (PCR

255

biases, ambiguous assignments), the 16S rRNA gene is currently the only extensively used and

256

sufficiently informative marker available in high-quality databases (24). In this study, the full-

257

length 16S rRNA gene was reconstructed providing a perspective of the taxonomic diversity,

258

including undiscovered taxa, together with microorganisms’ richness and relative abundances

259

(12).

260

Interestingly, from the present application of the 16S rRNA gene assembly on average 70

261

sequences per sample were obtained (Table 2). Because only the 16S rRNA sequences of the

ACS Paragon Plus Environment

12

Page 13 of 30

Environmental Science & Technology

262

most abundant microorganisms are expected to be assembled, they can be considered as the most

263

relevant community members. Complete results of microbial community taxonomy according to

264

16S AFL are reported in Figure S6 and Datasets S1-S6. Because most of biogas reactor’s

265

community is still uncharacterized, the taxonomic assignment provided by the used algorithms

266

was, for some microorganisms involved in biogas upgrading, uncertain, asserting the necessity

267

for further investigation. In particular, in the thermophilic reactor, the classification of the most

268

abundant microorganism, accounting for the half of the community, was unclear. While RDP

269

classifier pointed out a similarity to Acetomicrobium (phylum Bacteroidetes) with a low

270

threshold (0.37), BLAST assigned it to order Clostridiales (86% similarity).

271

The low phylogenetic resolution observed underlines the limits of single gene sequencing

272

analysis and the necessity of TRS information. From the application of the TRS strategy, higher

273

microbial community richness was detected at every classification level, displaying, for instance,

274

numerous phyla not detected by 16S AFL analysis (Figure S7). In accordance with previous

275

studies, Bacteroidetes, Proteobacteria, Firmicutes and Actinobacteria were dominant in the AD

276

process, likely involved in polysaccharides and proteins hydrolysis (31-34). Additionally,

277

Firmicutes and Proteobacteria include acetogenic and syntrophic bacteria that can degrade VFA

278

(32). 16S AFL analysis indicated only Firmicutes and Bacteroidetes as dominant phyla

279

presenting more than 40% relative abundance (Figure S8). According to TRS analysis,

280

Proteobacteria represented the most abundant phylum in both reactors (~21 and 36%,

281

respectively), while Firmicutes did not account for more than 11% of the total community

282

(Figure S7).

283

In particular, an unclassified genus of Desulfobulbaceae was found to be very abundant and

284

further increasing after the H2 addition to 28% (2-folds) in the thermophilic reactor (Figures 2c

ACS Paragon Plus Environment

13

Environmental Science & Technology

Page 14 of 30

285

and S9). Relevant species such as Desulfobulbus propionicus and Desulfurivibrio alkaliphilus

286

were also detected, though in low abundance. The former decreased ~3-folds at both temperature

287

conditions, the latter, coherently with the higher pH observed, increased at thermophilic

288

conditions. The decrease of D. propionicus can be explained with its ability to use H2 in absence

289

of sulfate to convert acetate and CO2 to propionate (35). In fact, the selective stimulation of

290

hydrogenotrophic methanogens could have caused a competition for the H2.

291

Several species of Desulfovibrio were identified, with D. desulfuricans as the most abundant

292

(1.5-2%). Under sulfate limited conditions, these species can produce acetate, H2 and CO2 in co-

293

occurrence with a hydrogenotrophic methanogens (36, 37). The concomitant increase in

294

abundance of Desulfovibrio spp. and of methanogens (>1-fold) observed in the thermophilic

295

reactor (Figures 2c and S9) highlights a possible syntrophic association of great importance for

296

biogas production and upgrading.

297

Moreover, microorganisms similar to Pseudomonas and Acinetobacter were identified by both

298

16S AFL and TRS analysis (0.5-3%; Figures 2c, 3, S6 and S9). These genera, together with

299

phylum Actinobacteria and genus Advenella, were previously detected in anaerobic reactors (19,

300

38, 39) and are involved in recalcitrant compounds decomposition producing enzymes for

301

lignocellulose degradation (40).

302

Among Bacteroidetes, the most representative genus was an unclassified member of

303

Sphingobacteriaceae accounting for 15% and 5% of the total community, in mesophilic and

304

thermophilic reactor respectively, prior the H2 addition. This genus was found to decrease after

305

the addition of H2. Unclassified species of Bacteroides, Cellulophaga and Flavobacterium were

306

found with >3% relative abundance in the mesophilic reactor and 18% relative abundance, is also involved in synergistic

311

cellulose degradation (43).

312

Firmicutes were mainly represented by Clostridiales belonging to genus Clostridium and to an

313

unclassified species of Alkaliphilus. While in the thermophilic reactor their relative abundance

314

remained quite stable (~7%) they halved in the mesophilic (from 8 to 4%). Because of the

315

difficulty in taxonomic classification of Clostridia (21), TRS results are in disagreement with

316

16S AFL analysis, where Clostridiales were found to double in the mesophilic reactor and

317

halved in the thermophilic (Figure 2). These microorganisms are known to play a fundamental

318

role in cellulosome-mediated cellulose hydrolysis, but they do not present the β-sugar

319

consumption pathway.

320

Sphingobacteriales acting synergistically with Clostridiales and whose function is crucial for

321

cellulose hydrolysis (44, 45). This hypothesis can reasonably explain their concomitant change in

322

abundance observed in this experiment.

323 324

This reaction is, thus, carried out by Thermotogales and

Further interesting microorganisms, found by TRS, were Aminobacterium colombiense and Mycoplasma, which role still remains unclear.

325

Families Porphyromonadaceae, Rikenellaceae and order Cytophagales, abundant in 16S AFL

326

results, were underrepresented in TRS, maybe due to their low occurrence within the database

327

used or to misassignments. These microorganisms, together with Erysipelotrichacales, are

328

involved in carbohydrates and proteins degradation and VFA production (41, 46, 47). Their

329

relative abundance changed maintening levels of VFA degraders and producers constant.

ACS Paragon Plus Environment

15

Environmental Science & Technology

Page 16 of 30

330

Moreover, 16S AFL analysis revealed in the mesophilic reactor, after the H2 addition, a species

331

similar to Candidatus Cloacamonas acidaminovorans (92% identity) accounting for 1.7% of the

332

community. This recently characterized microorganism, might oxidize propionate and ferment

333

sugars and aminoacids to produce H2 and CO2 indicating this species as a possible syntrophic

334

bacterium (46, 48).

335

Other abundant microorganisms found were identified by 16S AFL analysis as similar to

336

genera Halocella and Sedimentibacter and species Advenella faeciporci (99% identity) and

337

Tissierella creatinini (95% identity).

338

Generally, bacteria involved in the first steps of the AD decreased, while hydrogenotrophic

339

methanogens and syntrophic bacteria increased. These findings, together with the decreased

340

microbial diversity, state the role of the H2 driving the AD towards the final steps enhancing the

341

methanogenic process.

342

Concerning the archaeal community, 16S AFL analysis attributed to Euryarchaeota a relative

343

abundance of 1-2%. Moreover, TRS clearly ascertained their predominant position as their

344

relative abundance increased from 17 to 45% (~3-folds) and from 27 to 36%, in mesophilic and

345

thermophilic reactor, respectively (Figure S7). In accordance with a previous study,

346

Methanoculleus was found as predominant genus (46). In particular, according to TRS it

347

increased from 8 to 36% (4.5-folds) in the mesophilic reactor and from 17 to 24% (1.5-folds) in

348

the thermophilic (Figures 2, 3 and S9). Interestingly, in both TRS and 16S AFL analysis it was

349

identified at species level as M. marisnigri (>97% similarity).

350

The known hydrogenotrophic methanogens (49, 50), Methanocorpusculum labreanum,

351

Methanogenium sp., (1-3%) and an unknown genus of Methanoregulaceae (6-7%), were also

352

found (Figures 2, 3 and S9).

ACS Paragon Plus Environment

16

Page 17 of 30

Environmental Science & Technology

353

Genus Methanosarcina was detected only by TRS (1% of the total community). M. barkeri and M. acetivorans were also found, but less

355

represented (Figures 2c and 3). The low abundance of genus Methanosarcina can be explained

356

by the low VFA concentration and particularly acetate in the reactors. High acetate levels are, in

357

fact, known to favor the selective proliferation of aceticlastic methanogens (34). Moreover,

358

aceticlastic methanogens are more sensitive to pH and ammonia levels than hydrogenotrophic

359

(50). In their absence, acetate consumption and CH4 formation are mainly carried out through

360

SAO and reduction of CO2/H2 to CH4 by a hydrogenotrophic methanogen (10). A possible

361

candidate for SAO could belong to Thermoanaerobacteraceae (51), which, according to 16S

362

AFL, was very abundant in both reactors (>20% and >7%, respectively) after the H2 addition

363

(Figure 2a and b). At both temperature conditions aceticlastic methanogens decreased, although

364

more markedly at mesophilic conditions.

365

The increase of Methanoculleus and Mehanoregulaceae confirms our hypothesis delineating a

366

selective action of the H2 on hydrogenotrophic methanogens stimulating CO2 consumption and

367

biogas upgrading.

368

In conclusion, the results clearly state the feasibility of H2-mediated biogas upgrading, at both

369

mesophilic and thermophilic conditions with higher biomethanation and CO2 conversion

370

efficiency at thermophilic. Moreover, H2 transfer to the liquid phase was an important factor

371

limiting the H2 availability for microorganisms. Concerning the effect of H2 on microbiome, the

372

innovative metagenomic methods complemented each other providing a better characterization

373

of the microbial community and an understanding of its complexity. Specifically, the decrease of

374

hydrolytic and fermentative bacteria and aceticlastic methanogens and the increase of

375

hydrogenotrophic methanogens and syntrophic bacteria assert the selective pressure of the H2

ACS Paragon Plus Environment

17

Environmental Science & Technology

Page 18 of 30

376

towards the hydrogenotrophic pathway enhancing the CO2 consumption and consequently the

377

biogas upgrading.

378

ACS Paragon Plus Environment

18

Page 19 of 30

Environmental Science & Technology

379

Table 1: Mesophilic and thermophilic reactor performances at steady state conditions before and

380

after the H2 addition Mesophilic

Thermophilic

Pre H2

Post H2

Pre H2

Post H2

94±20

110±13

368±42

385±24

n.a.

91

n.a.

274

CH4

69.7±0.3

88.9±2.4

67.1±0.8

85.1±3.7

CO2

30.3±0.3

8.8±3.2

32.9±0.9

6.6±0.9

0

2.3±1.8

0

8.3±3.6

CH4 yield manure (mL/gVS)

111±24

130±23

249±27

267±24

Total CH4 yield (mL/gVS)

111±24

168±21

249±27

359±22

CH4 rate (mL/Lday)

66±14

100±12

247±27

359±20

CO2 rate (mL/Lday)

29±6

10±3

121±15

28±5

CO2 conversion (mL/Lday)

0

23±3

0

93±5

H2 flow rate (mL/Lday)

0

192±28

0

510±32

H2 consumption (mL/Lday)

0

178±26

0

470±35

Biogas rate (mL/Lday) Biogas rate R1 (mL/Lday) Biogas composition (%)

H2

pH of the R1 and R2

7.74±0.16

7.78±0.04 7.82±0.16 7.95±0.03

pH of the SR1 and SR2

7.73±0.15

8.17±0.13 7.89±0.17 8.49±0.04

total VFA R1 and R2 (g/L)

0.16± 0.04

0.17±0.04 1.18±0.84 0.19±0.07

total VFA SR1 and SR2 (g/L)

0.09±0.05

0.16±0.03 0.28±0.17 0.38±0.07

ACS Paragon Plus Environment

19

Environmental Science & Technology

Page 20 of 30

381 382

Table 2: Summary of sequencing results and alpha diversity indexes. Mesophilic

Thermophilic

Pre H2

Post H2

Pre H2

Post H2

87,819,274

65,063,014

69,103,700

73,977,902

High quality sequences

80%

80%

81%

81%

Extracted rRNA-like sequences

171,275

143,980

178,823

207,687

Full-length 16S rRNA

79

70

70

62

70.45

86.09

58.95

49.60

Illumina total random sequences

assembled sequences Alpha diversity 383 384

ASSOCIATED CONTENT

385

Supporting Information. Reactor’s performances (Figures S1-S4), microbial community

386

composition (Figures S5-S9 and Datasets S1-S6), and additional material and methods are

387

available free of charge via the Internet at http://pubs.acs.org.

388

AUTHOR INFORMATION

389

Corresponding Author

390

*e-mail: [email protected] ; phone: +45 45 25 14 29; fax: +45 45 93 28 50

391

Author Contributions

ACS Paragon Plus Environment

20

Page 21 of 30

Environmental Science & Technology

392

The manuscript was written through contributions of all authors. All authors have given approval

393

to the final version of the manuscript.

394

ACKNOWLEDGMENT

395

We thank Hector Garcia and Hector Diaz for technical assistance and Hugo Maxwell Connery

396

for the IT support. This work was supported by the Danish Council for Strategic Research under

397

the project “SYMBIO – Integration of biomass and wind power for biogas enhancement and

398

upgrading via hydrogen assisted anaerobic digestion”, contract 12-132654.

399 400

ABBREVIATIONS

401

AD, anaerobic digestion; VFA, volatile fatty acids; CSTR, continuous stirred-tank reactors; TRS,

402

Random Sequences; 16S SR, 16S rRNA Shotgun Reads; 16S AFL, Assembled Full-Length 16S

403

rRNA gene; R1, primary mesophilic reactor; R2, primary thermophilic reactor; SR1, secondary

404

mesophilic reactor; SR2, secondary thermophilic reactor.

405

REFERENCES

406

1. Hamelin, L.; Wesnæs, M.; Wenzel, H.; Petersen, B. M. Environmental consequences of

407

future biogas technologies based on separated slurry. Environ. Sci. Technol. 2011, 45 (13),

408

5869-5877.

409 410

2. Deng, L.; Hägg, M. B. Techno-economic evaluation of biogas upgrading process using CO2 facilitated transport membrane. Int. J. Greenh. Gas Control 2010, 4 (4), 638-646.

411

3. Nordberg, Å.; Edström, M.; Uusi-Penttilä, M.; Rasmuson, Å. C. Selective desorption of

412

carbon dioxide from sewage sludge for in-situ methane enrichment: Enrichment experiments

413

in pilot scale. Biomass Bioenergy 2012, 37, 196–204.

ACS Paragon Plus Environment

21

Environmental Science & Technology

Page 22 of 30

414

4. Granovskii, M.; Dincer, I.; Rosen, M. A. Economic and environmental comparison of

415

conventional hybrid electric and hydrogen fuel cell vehicles. J Power Sources 2006, 159 (2),

416

1186-1193.

417

5. Luo, G.; Johansson, S.; Boe, K.; Xie, L.; Zhou, Q.; Angelidaki, I. Simultaneous hydrogen

418

utilization and in situ biogas upgrading in an anaerobic reactor. Biotechnol. Bioeng. 2012,

419

109 (4), 1088-1094.

420

6. Luo, G.; Angelidaki, I. Co-digestion of manure and whey for in situ biogas upgrading by the

421

addition of H2: Process performance and microbial insights. Appl. Microbiol. Biotechnol.

422

2013, 97 (3), 1373–1381.

423 424 425 426 427 428 429 430

7. Guiot, S. R.; Cimpoia, R.; Carayon, G. Potential of wastewater-treating anaerobic granules for biomethanation of synthesis gas. Environ. Sci. Technol. 2011, 45 (5), 2006–2012. 8. Angelidaki, I.; Karakashev, D.; Batstone, D. J.; Plugge, C. M.; Stams, A. J. Biomethanation and its potential. Meth. Enzymol. 2011, 494, 327-351. 9. Liu, Y.; Whitman, W. B. Metabolic; phylogenetic; and ecological diversity of the methanogenic archaea. Ann. N. Y. Acad. Sci. 2008, 1125 (1), 171–189. 10. Schnürer, A.; Zellner, G.; Svensson, B. H. Mesophilic syntrophic acetate oxidation during methane formation in biogas reactors. FEMS Microbiol. Ecol. 1999, 29 (3), 249–261.

431

11. Luo, G.; Angelidaki, I. Integrated biogas upgrading and hydrogen utilization in an anaerobic

432

reactor containing enriched hydrogenotrophic methanogenic culture. Biotechnol. Bioeng.

433

2012, 109 (11), 2729–2736.

ACS Paragon Plus Environment

22

Page 23 of 30

Environmental Science & Technology

434

12. Miller, C. S.; Baker, B. J.; Thomas, B. C.; Singer, S. W.; Banfield, J. F. EMIRGE:

435

reconstruction of full-length ribosomal genes from microbial community short read

436

sequencing data. Genome Biol. 2011, 12 (5), 44.

437

13. Angelidaki, I.; Ellegaard, L.; Ahring, B. K. Compact automated displacement gas metering

438

system for measurement of low gas rates from laboratory fermentors. Biotechnol. Bioeng.

439

1992, 39 (3), 351–353.

440 441

14. APHA (American Public Health Association). Standard Methods for the Examination of Water and Wastewater. APHA: Washington, DC, 2005.

442

15. Kougias, P. G.; Boe, K.; Angelidaki, I. Effect of organic loading rate and feedstock

443

composition on foaming in manure-based biogas reactors. Bioresour. Technol. 2013, 144, 1–

444

7.

445

16. Kougias, P. G.; Boe, K.; Tsapekos, P.; Angelidaki, I. Foam suppression in overloaded

446

manure-based biogas reactors using antifoaming agents. Bioresour. Technol. 2014, 153, 198-

447

205.

448

17. Kougias, P. G.; Boe, K.; Einarsdottir, E. S.; Angelidaki, I. Counteracting foaming caused by

449

lipids or proteins in biogas reactors using rapeseed oil or oleic acid as antifoaming agents.

450

Water Res. 2015, 79, 119-127.

451

18. Meyer, F.; Paarmann, D.; D'Souza, M.; Olson, R.; Glass, E. M.; Kubal, M.; Paczian, T.;

452

Rodriguez, A.; Stevens, R.; Wilke, A.; Wilkening, J.; Edwards, R. A. The metagenomics

453

RAST server–a public resource for the automatic phylogenetic and functional analysis of

454

metagenomes. BMC Bioinf. 2008, 9 (1), 386.

ACS Paragon Plus Environment

23

Environmental Science & Technology

455

Page 24 of 30

19. Kougias, P. G.; De Francisci, D.; Treu, L.; Campanaro, S.; Angelidaki, I. Microbial analysis

456

in biogas reactors suffering by foaming incidents. Bioresour. Technol. 2014, 167, 24-32.

457

20. Bolger, A. M.; Lohse, M.; Usadel, B. Trimmomatic: a flexible trimmer for Illumina sequence

458 459 460

data. Bioinformatics 2014, 30 (15), 2114-2120. 21. Schmieder, R.; Lim, Y. W.; Edwards, R. Identification and removal of ribosomal RNA sequences from metatranscriptomes. Bioinformatics 2012, 28 (3), 433-435.

461

22. Wang, Q.; Garrity, G. M.; Tiedje, J. M.; Cole, J. R. Naive Bayesian classifier for rapid

462

assignment of rRNA sequences into the new bacterial taxonomy. Appl. Environ. Microbiol.

463

2007, 73 (16), 5261-5267.

464 465

23. Zhang, Z.; Schwartz, S.; Wagner, L.; Miller, W. A greedy algorithm for aligning DNA sequences. J. Comput. Biol. 2000, 7 (1-2), 203-214.

466

24. Yarza, P.; Yilmaz, P.; Pruesse, E.; Glöckner, F. O.; Ludwig, W.; Schleifer, K-H.; Whitman,

467

W. B.; Euzéby, J.; Amann, R.; Rosselló-Móra, R. Uniting the classification of cultured and

468

uncultured bacteria and archaea using 16S rRNA gene sequences. Nat. Rev. Microbiol. 2014,

469

12 (9), 635–645.

470 471

25. Huson, D. H.; Auch, A. F.; Qi, J.; Schuster, S. C. MEGAN analysis of metagenomic data. Genome Res. 2007, 17 (3), 377-386.

472

26. Segata, N.; Waldron, L.; Ballarini, A.; Narasimhan, V.; Jousson, O.; Huttenhower, C.

473

Metagenomic microbial community profiling using unique clade-specific marker genes. Nat.

474

Methods 2012, 9 (8), 811-814.

ACS Paragon Plus Environment

24

Page 25 of 30

Environmental Science & Technology

475

27. Howe, E.; et al. Mev: multiexperiment viewer. In Biomedical Informatics for Cancer

476

Research; Ochs, M. F., Casagrande, J. T., Davuluri R. V., Eds.; Springer US: New York

477

Dordrecht Heidelberg London 2010; pp. 267-277.

478 479 480 481

28. Luo, G.; Angelidaki, I. Hollow fiber membrane based H₂ diffusion for efficient in situ biogas upgrading in an anaerobic reactor. Appl. Microbiol. Biotechnol. 2013, 97 (8), 3739–44. 29. Weiland, P. Biogas production: current state and perspectives. Appl. Microbiol. Biotechnol. 2010, 85 (4), 849-860.

482

30. Levén, L.; Eriksson, A. R. B.; Schnürer, A. Effect of process temperature on bacterial and

483

archaeal communities in two methanogenic bioreactors treating organic household waste.

484

FEMS Microbiol. Ecol. 2007, 59 (3), 683–693.

485

31. De Francisci, D.; Kougias, P. G.; Treu, L.; Campanaro, S.; Angelidaki, I. Microbial diversity

486

and dynamicity of biogas reactors due to radical changes of feedstock composition.

487

Bioresour. Technol. 2015, 176, 56–64.

488

32. Krakat, N.; Schmidt, S.; Scherer, P. Potential impact of process parameters upon the bacterial

489

diversity in the mesophilic anaerobic digestion of beet silage. Bioresour. Technol. 2011, 102

490

(10), 5692–5701.

491

33. Yi, J.; Dong, B.; Jin, J.; Dai, X. Effect of increasing total solids contents on anaerobic

492

digestion of food waste under mesophilic conditions: Performance and microbial

493

characteristics

494

10.1371/journal.pone.0102548.

analysis.

PLoS

One

2014,

ACS Paragon Plus Environment

9

(7),

e102548;

DOI:

25

Environmental Science & Technology

Page 26 of 30

495

34. Wirth, R,; Kovács, E.; Maróti, G.; Bagi, Z.; Rákhely, G.; Kovács, K. L. Characterization of a

496

biogas-producing microbial community by short-read next generation DNA sequencing.

497

Biotechnol. Biofuels 2012, 5 (1), 41.

498

35. Laanbroek, H. J.; Abee, T.; Voogd, I. L. Alcohol conversion by Desulfobulbus propionicus

499

Lindhorst in the presence and absence of sulfate and hydrogen. Arch. Microbiol. 1982, 133

500

(3), 178-184.

501

36. Bryant, M. P.; Campbell, L. L.; Reddy, C. A.; Crabill, M. R. Growth of Desulfovibrio in

502

lactate or ethanol media low in sulfate in association with H2-utilizing methanogenic

503

bacteria. Appl. Environ. Microbiol. 1977, 33 (5), 1162-1169.

504 505

37. Muyzer, G.; Stams, A. J. The ecology and biotechnology of sulphate-reducing bacteria. Nature Reviews Microbiology 2008, 6 (6), 441-454.

506

38. Hanreich, A.; Schimpf, U.; Zakrzewski, M.; Schlüter, A.; Benndorf, D.; Heyer, R.; Rapp, E.;

507

Pühler, A.; Reichl, U.; Klocke, M. Metagenome and metaproteome analyses of microbial

508

communities in mesophilic biogas-producing anaerobic batch fermentations indicate

509

concerted plant carbohydrate degradation. Syst. Appl. Microbiol. 2013, 36 (5), 330–338.

510

39. Xu, S.; Fu, B.; Zhang, L.; Liu, H. Bioconversion of H2/CO2 by acetogen enriched cultures for

511

acetate and ethanol production: the impact of pH. World J. Microbiol. Biotechnol. 2015, 31

512

(6), 941-950.

513 514

40. Ntougias, S.; Bourtzis, K.; Tsiamis, G. The microbiology of olive mill wastes. BioMed Res. Int. 2013, 784591; DOI:10.1155/2013/784591.

ACS Paragon Plus Environment

26

Page 27 of 30

Environmental Science & Technology

515

41. Wang, C.; Zuo, J.; Chen, X.; Xing, W.; Xing, L.; Li, P.; Lu, X.; Li, C. Microbial community

516

structures in an integrated two-phase anaerobic bioreactor fed by fruit vegetable wastes and

517

wheat straw. J. Environ. Sci. 2014, 26 (12), 2484-2492.

518

42. Traversi, D.; Villa, S.; Lorenzi, E.; Degan, R.; Gilli, G. Application of a real-time qPCR

519

method to measure the methanogen concentration during anaerobic digestion as an indicator

520

of biogas production capacity. J. Environ. Manage. 2012, 111, 173-177.

521

43. Hung, C. H.; Chang, Y. T.; Chang, Y. J. Roles of microorganisms other than Clostridium and

522

Enterobacter in anaerobic fermentative biohydrogen production systems–a review.

523

Bioresour. Technol. 2011, 102 (18), 8437-8444.

524

44. Xia, Y.; Wang, Y.; Fang, H. H.; Jin, T.; Zhong, H.; Zhang, T. Thermophilic microbial

525

cellulose

decomposition

526

metatranscriptomic

527

DOI:10.1038/srep06708.

and

and

methanogenesis

metagenomic

analysis.

pathways Sci.

Rep.

recharacterized 2014,

4,

by 6708;

528

45. Jiménez, D. J.; Dini-Andreote, F.; Van Elsas, J. D. Metataxonomic profiling and prediction

529

of functional behaviour of wheat straw degrading microbial consortia. Biotechnol. Biofuels

530

2014, 7 (1), 1-18.

531

46. Stolze, Y.; Zakrzewski, M.; Maus, I.; Eikmeyer, F.; Jaenicke, S.; Rottmann, N.; Siebner, C.;

532

Pühler, A.; Schlüter, A. Comparative metagenomics of biogas-producing microbial

533

communities from production-scale biogas plants operating under wet or dry fermentation

534

conditions. Biotechnol. Biofuels 2015, 8 (1), 14.

ACS Paragon Plus Environment

27

Environmental Science & Technology

Page 28 of 30

535

47. Hahnke, S.; Maus, I.; Wibberg, D.; Tomazetto, G.; Pühler, A.; Klocke, M.; Schlüter, A.

536

Complete genome sequence of the novel Porphyromonadaceae bacterium strain ING2-E5B

537

isolated from a mesophilic lab-scale biogas reactor. J. Biotechnol. 2015, 193, 34-36.

538

48. Pelletier, E.; Kreimeyer, A.; Bocs, S.; Rouy, Z.; Gyapay, G.; Chouari, R.; Rivière, D.;

539

Ganesan, A.; Daegelen, P.; Sghir, A.; Cohen, G. N.; Médigue, C.; Weissenbach, J.; Le

540

Paslier, D. Candidatus Cloacamonas acidaminovorans: genome sequence reconstruction

541

provides a first glimpse of a new bacterial division. J. Bacteriol. 2008, 190 (7), 2572-2579.

542

49. Garcia, J. L.; Bharat, K. C. P.; Bernard, O. Taxonomic, phylogenetic, and ecological

543

diversity of methanogenic Archaea. Anaerobe 2000, 64 (4), 205-226.

544

50. Oren, A. The Family Methanoregulaceae. In The Prokaryotes: Other Major Lineages of

545

Bacteria and The Archaea; Rosenberg, E., DeLong, E. F., Lory, S., Stackebrandt, E.,

546

Thompson, F., Eds.; Springer: Berlin Heidelberg 2014; pp. 253-258.

547

51. Lü, F.; Bize, A.; Guillot, A.; Monnet, V.; Madigou, C.; Chapleur, O.; Mazéas, L.; He, P.;

548

Bouchez, T. Metaproteomics of cellulose methanisation under thermophilic conditions

549

reveals a surprisingly high proteolytic activity. ISME J. 2014, 8 (1), 88–102

550 551

For table of content only

ACS Paragon Plus Environment

28

Page 29 of 30

Environmental Science & Technology

Figure 1: CH4 yield of the secondary mesophilic (grey column) and thermophilic reactor (black column) at steady state after the H2 addition, measured in batch experiment at different initial pH values.

Figure 2: Heat maps of relative abundance (>1%; left part of each panel) and folds change (log2>1; right part of each panel) of the most interesting microorganisms populating mesophilic and

ACS Paragon Plus Environment

Environmental Science & Technology

Page 30 of 30

thermophilic reactors at steady state before and after the H2 addition. Correspondence between colors and relative abundance or fold change is reported in the scale at the top of each panel. Folds change is represented in red and green for increased and decreased microorganisms, respectively. The results are obtained according to 16S AFL (a and b) and TRS analysis (c).

Figure 3: Phylogenetic tree of the whole AD microbial community both of mesophilic and thermophilic reactors at steady states, according to TRS analysis. In color are reported microorganism names identified at family and genus level, while, in black, those identified at inferior classification levels. The size of the bullets indicates the abundance of each taxon considering together all samples. Black branches represent microorganisms with an occurrence lower than the threshold imposed (max_annot_clades 50).

ACS Paragon Plus Environment