Correspondence/Rebuttal pubs.acs.org/est
Comment on Microbial Community Composition Is Unaffected by Anode Potential
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it should be possible to use anode potential as a mechanism to select various ARB communities with different electron transfer properties. ARB communities appear to adjust to anode potential to some extent, but it is still unknown whether this adjustment is primarily due to selection of particular strains and species within the communities that have different optimal electron transfer potentials; or whether it is through physiological adaptation of
hile bioelectrochemical systems (BESs) such as microbial fuel cells (MFCs) or microbial electrolysis cells (MECs) remain promising technologies, their widespread use for the sustainable production of energy from wastewaters is yet to be realized. Establishment of anode-respiring bacteria (ARB) at the anode surface is considered an important determinant of BES performance. Because ARB use the anode for respiration,
Figure 1. Phylogenetic tree of 16S rRNA gene sequences from different strains of Geobacter psychrophilus selected with acetate on electrodes poised at −0.36 V and −0.25 V vs Ag/AgCl. * indicates the replicate samples. Multiple alignments performed with MUSCLE algorithm and the tree built using Geneious 7.2.1 software with Jukes Cantor genetic distance model and UPGMA method. The letter A to E on the right side indicate sequences from the NCBI database producing the best alignment with the 16S rRNA gene sequences of various strains of G. psychrophilus (accession numbers of A: [EF527233.1]; B: [GQ339169.1]; C: [FR667837.1], D: [JX225411.1], E: [HE974841.1]). The scale bar indicates the distances with 0.003 equiv to 0.3% difference. Published: December 5, 2014 © 2014 American Chemical Society
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Environmental Science & Technology
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species that have the capacity to adapt to different potentials. For this reason, we welcome the attempt of Zhu et al.1 to address the controversy regarding the impact of anode potential on microbial community composition and performance. Zhu and colleagues1 claim that set anode potential does not affect the composition of anode respiring bacterial (ARB) communities selected in single-chambered microbial electrolysis cells (MECs). For anode potentials of −0.25 V, −0.09 V, 0.21 V, 0.51 V or 0.81 V vs a standard hydrogen electrode (SHE), Zhu et al. found that the ARB community was dominated at a genus level by Geobacter species. This is of relevance since electrical current production in BES is generally associated with communities dominated by Geobacter sp.2,3 Although their communities at different potentials were all Geobacter sp. dominated, they exhibited disparities in maximum current, biomass and exocellular electron transfer components (EETCs), as shown by nonturnover cyclic voltammograms. Zhu et al. use this finding to support their hypothesis of a physiological adaptation of the dominant Geobacter species to anode potential. However, they do not discuss the possibility of selection below the genus level and present no evidence that could discriminate between strain selection and physiological adaptation of the resident taxa. Zhu et al. cite the work of Torres et al.2 who did find that anode potential influenced the composition of the anode respiring communities. Zhu et al. explain the different results by suggesting a possible “crosstalk” between Torres’ electrodes that were incubated in the same reactor chamber. Unfortunately, Zhu et al. overlook our recent publication3 in which we use a similar experimental set up to them and report that anode potential does affect community composition at the strain level. Indeed, our study suggests genetic selection rather than physiological adaptation to anode potential. In reply to Zhu et al.’s work, we have further examined the data from our paper3 which records 16S rRNA gene sequences from four independent ARB communities, selected at either −0.25 V or −0.36 V vs Ag/AgCl (−0.16 V and −0.05 V vs SHE, respectively). Multiple alignment of this sequence data provides evidence that community composition varies below species level among MECs fixed at different anode potential, including among the members of the Geobacter sp. (Figure 1). In conclusion, we do not think that there is yet definite proof that “Microbial community composition is unaffected by anode potential”. A comparison between our data and that of Zhu et al. analyzed and presented in a similar manner, could shed much needed light on whether or not adaptation to anode potential is due to a physiological response, strain-level population selection, or a more complex combination of interacting factors.
Correspondence/Rebuttal
REFERENCES
(1) Zhu, X.; Yates, M. D.; Hatzell, M. C.; Ananda Rao, H.; Saikaly, P. E.; Logan, B. E. Microbial community composition is unaffected by anode potential. Environ. Sci. Technol. 2014, 48 (2), 1352−1358. (2) Torres, C. I.; Krajmalnik-Brown, R.; Parameswaran, P.; Marcus, A. K.; Wanger, G.; Gorby, Y. A.; Rittmann, B. E. Selecting anoderespiring bacteria based on anode potential: Phylogenetic, electrochemical, and microscopic characterization. Environ. Sci. Technol. 2009, 43 (24), 9519−9524. (3) Commault, A. S.; Lear, G.; Packer, M. A.; Weld, R. j. Influence of anode potentials on selection of Geobacter strains in microbial electrolysis cells. Bioresour. Technol. 2013, 139, 226−234.
Audrey S. Commault*,† Gavin Lear‡ Richard J. Weld†
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† Lincoln Agritech Ltd., Lincoln University, Christchurch 7640, New Zealand ‡ School of Biological Sciences, The University of Auckland, Auckland 1010, New Zealand
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest. 14852
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