Subscriber access provided by UNIV OF DURHAM
Article
Relationships Between Dissolved Organic Matter Composition and Photochemistry in Lakes of Diverse Trophic Status Andrew Chapin Maizel, Jing Li, and Christina K. Remucal Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b01270 • Publication Date (Web): 18 Jul 2017 Downloaded from http://pubs.acs.org on July 19, 2017
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 32
Environmental Science & Technology
1
Relationships Between Dissolved Organic
2
Matter Composition and Photochemistry in
3
Lakes of Diverse Trophic Status
4
Andrew C. Maizel,1 Jing Li,1 and Christina K. Remucal1, 2*
5
6 1
7
Department of Civil and Environmental Engineering
8
University of Wisconsin - Madison
9
Madison, Wisconsin 2
10
Environmental Chemistry and Technology Program
11
University of Wisconsin - Madison
12
Madison, Wisconsin
13 14 15
* Corresponding author address: 660 N. Park St., Madison, WI 53706; e-mail:
16
[email protected]; telephone: (608) 262-1820; fax: (608) 262-0454; Twitter: @remucal.
ACS Paragon Plus Environment
1
Environmental Science & Technology
17
Page 2 of 32
Abstract
18
The North Temperate Lakes-Long Term Ecological Research site includes seven
19
lakes in northern Wisconsin that vary in hydrology, trophic status, and landscape
20
position. We examine the molecular composition of dissolved organic matter (DOM)
21
within these lakes using Fourier transform-ion cyclotron resonance mass spectrometry
22
(FT-ICR MS) and quantify DOM photochemical activity using probe compounds.
23
Correlations between the relative intensity of individual molecular formulas and reactive
24
species production demonstrate the influence of DOM composition on photochemistry.
25
For example, highly aromatic, tannin-like formulas correlate positively with triplet
26
formation rates, but negatively with triplet quantum yields, as waters enriched in highly
27
aromatic formulas exhibit much higher rates of light absorption, but only slightly higher
28
rates of triplet production. While commonly utilized optical properties also correlate with
29
DOM composition, the ability of FT-ICR MS to characterize DOM subpopulations
30
provides unique insight into the mechanisms through which DOM source and
31
environmental processing determine composition and photochemical activity.
ACS Paragon Plus Environment
2
Page 3 of 32
32
Environmental Science & Technology
Introduction
33
Dissolved organic matter (DOM) is a compositionally diverse assembly of
34
molecules that is ubiquitous in natural waters. DOM contributes to numerous
35
environmental processes including carbon transport,1 redox cycling,2 and reactions with
36
environmental contaminants.3 Of special interest to lacustrine systems is the ability of
37
DOM to degrade xenobiotic compounds through the photochemical production of
38
reactive triplet states (3DOM)4 and reactive intermediates such as singlet oxygen (1O2)5
39
and hydroxyl radicals.6,7
40
DOM derives from dissimilar sources and, accordingly, is diverse in composition
41
and photochemical behavior.8-11 Broadly, DOM is considered allochthonous when derived
42
from degraded terrestrial plant material or autochthonous when derived from aquatic
43
microorganisms. Allochthonous DOM is typically higher in molecular weight,12 lower in
44
heteroatom content (e.g., N and S),13 and more aromatic than autochthonous DOM.14
45
DOM is further differentiated in natural systems by physical,15 chemical,16 and
46
biological17-19 processing. While it is challenging to apportion individual mechanisms to
47
DOM modification in environmental systems, DOM tends to become more aliphatic, less
48
oxidized, more diverse in elemental composition, and less chromophoric as it moves from
49
uplands to oceans.20-22 Similarly, formulas that are more aliphatic, less oxidized, and N-
50
rich are more persistent in lakes.23 DOM photochemistry also varies with source and environmental processing.
51 52
3
53
allochthonous DOM.24,25 Similarly, 1O2 quantum yields increase while 3DOM and 1O2
54
steady-state concentrations decrease as DOM moves from headwaters to the ocean.26,27
DOM and 1O2 quantum yields are generally higher in autochthonous DOM than
ACS Paragon Plus Environment
3
Environmental Science & Technology
Page 4 of 32
55
DOM photochemistry is commonly evaluated with the 3DOM probes sorbic acid (HDA)
56
and 2,4,6-trimethylphenol (TMP), and the 1O2 probe furfuryl alcohol (FFA).28 While
57
HDA and TMP directly measure 3DOM and 1O2 is formed from the reaction of 3DOM
58
and O2, there is evidence that these probes measure distinct 3DOM subpopulations.15,29
59
Recent reviews have proposed using a combination of multiple probes to better describe
60
3
DOM photoreactivity.30,31
61
The molecular composition of DOM is increasingly assessed with Fourier
62
transform-ion cyclotron resonance mass spectrometry (FT-ICR MS).32,33 The ability of
63
FT-ICR MS to identify individual molecular formulas in DOM has increased our
64
understanding of how physical,34 chemical,35 and biological processes36 modify DOM and
65
how DOM changes across environmental transects.20,21,37 For example, FT-ICR MS can
66
determine the composition of heteroatom-containing formulas and identify the source of
67
specific DOM subpopulations.16 Additionally, FT-ICR MS can detect bimodal
68
distributions in aromaticity and heteroatom composition that are not apparent using bulk
69
measurements.20 However, as with other methods for determining DOM composition,
70
FT-ICR MS is subject to analytical biases, such as the preferential detection of readily
71
ionized formulas.38
72
There is growing interest in relating DOM photochemistry and composition, as
73
xenobiotic compounds are increasingly identified in remote waters.39,40 However, 3DOM
74
production has not been previously related to molecular composition of DOM. Therefore,
75
we combine photochemical analyses with FT-ICR MS to investigate how DOM
76
composition controls photochemical activity in seven diverse lakes. Samples were
77
collected from the North Temperate Lakes-Long Term Environmental Research (NTL-
ACS Paragon Plus Environment
4
Page 5 of 32
Environmental Science & Technology
78
LTER) site in northern Wisconsin. Little is known about the DOM composition and
79
photoreactivity of NTL-LTER lakes, and the diversity of DOM sources and distinct UV-
80
vis absorbance spectra make them an ideal site to relate these attributes.
81 82
Methods
83
Sample Location. The NTL-LTER site includes two dystrophic (Crystal and
84
Trout Bogs), one mesotrophic (Allequash Lake), and four oligotrophic lakes (Big
85
Muskellunge, Crystal, Sparkling, and Trout Lakes). The NTL-LTER lakes vary in surface
86
area and water source, as well as in DOM source and concentration (Table S1).41-44 The
87
site is in a forested region of northern Wisconsin that is dominated by lakes.45
88
Samples were collected from the center of the NTL-LTER lakes between 02/1990
89
and 11/2014 and analyzed within 2-3 weeks for the concentration of dissolved organic
90
carbon ([DOC]) and by UV-visible spectroscopy (UV-vis). Additional samples (1–4 L)
91
were collected from the center of each lake in June 2015 and the edge of each lake in
92
August 2016 for photochemical analysis. Detailed information about sample collection,
93
analysis, and materials is available in Sections S1-S2.
94
UV-Vis Spectroscopy. Absorbance of samples collected in 2015 and 2016 was
95
determined using a Shimadzu UV-2401 PC, relative to a Milli-Q reference. SUVA254 is
96
the ratio of absorbance at 254 nm (A254) to [DOC].14 E2:E3 is the ratio of absorbance at
97
250 nm to 365 nm.22
98
Mass Spectrometry. DOM was extracted from the NTL-LTER samples collected
99
in August 2016 by solid phase extraction (SPE) and analyzed by FT-ICR MS. Details
100
about the SPE protocol are included in Section S3.46 Sample extracts were diluted 1:10 in
ACS Paragon Plus Environment
5
Environmental Science & Technology
Page 6 of 32
101
1:1 acetonitrile:Milli-Q and aspirated with 0.3 psi pressure into an electrospray source
102
with an applied voltage of -1.4 V. Analysis was performed with a SolariX XR 12T FT-
103
ICR MS (Bruker), coupled to a Triversa NanoMate sample delivery system (Advion), as
104
described previously.47 Details about instrument settings and formula identification are
105
available in Section S3. Formula relative intensities were determined by dividing the
106
intensity of each formula by the sum of intensities of all identified formulas in a sample.
107
Weighted average compositional values (e.g., H:Cw.avg) are the average of that
108
compositional value in all formulas, weighted by the relative intensity of each formula. It
109
should be noted that these average compositional values are determined only from
110
formulas identified by FT-ICR MS, rather than bulk elemental analysis. Average
111
compositional values exhibit similar qualitative trends to bulk analysis (e.g., elevated H:C
112
and N:C in autochthonous fulvic acid isolates)13 and provide insight into compositional
113
differences between natural DOM samples.21 Double bond equivalents per carbon
114
(DBE/C) was calculated as: {C – 0.5(H+Cl) + 0.5(N+S) +1}/C.48 Optical properties and
115
photochemical measurements are correlated by Pearson’s coefficients with the relative
116
intensities of individual formulas.
117
Photochemistry. Samples collected in 2015 and 2016 were irradiated in a
118
Rayonet photoreactor with UV-A bulbs (365 ± 9 nm).25,49 The initial photon flux of the
119
experimental apparatus was determined to be 7.35 x 10-8 E cm-2 s-1 with p-nitroanisole
120
(PNA)-pyridine actinomtery.25 Irradiation durations varied according to probe and sample
121
(10 – 360 minutes), and all irradiations were conducted in triplicate. HDA was added at
122
initial concentrations of 10, 100, 250, 500, and 1000 µM. 3DOM quantum yields (Φ3DOM),
123
formation rates (F3DOM), steady-state concentrations ([3DOM]ss) and first-order loss rate
ACS Paragon Plus Environment
6
Page 7 of 32
Environmental Science & Technology
124
constants (kd) were calculated from the isomerization rate of t,t-HDA and previously
125
estimated rate constants.25,50 Additionally, the observed loss rates of the 3DOM probe
126
TMP (initial concentration = 10 µM) were used to calculate 3DOM quantum yield
127
coefficients (fTMP) and steady-state concentrations of
128
estimated rate constants as described previously.51,52 The quantum yields (Φ1O2) and
129
steady-state concentrations of 1O2 ([1O2]ss) were calculated from the observed loss rates of
130
FFA (initial concentration = 10 µM) and previously measured rate constants.53,54
131
Quantum yields and fTMP were calculated with PNA-pyridine actinometry. All
132
calculations are described in detail in Section S4.55 FFA, PNA, TMP, and HDA isomer
133
concentrations were quantified by high-performance liquid chromatography as described
134
previously.25
3
DOM ([3DOM]ss,TMP) using
135 136
Results and Discussion
137
[DOC] and Optical Properties. [DOC] and UV-vis measurements taken from
138
1990 through 2014 distinguish the seven NTL-LTER lakes according to trophic status
139
and reflect the source and transformation of DOM within each lake. [DOC] represents the
140
concentration of bulk DOM, while A254 quantifies light absorption by chromophoric
141
DOM. DOM composition is described by SUVA254, which correlates with aromaticity,14
142
and E2:E3, which is inversely related to molecular weight.25,56 The bogs have the highest
143
mean [DOC], A254, and SUVA254, and lowest mean E2:E3 (Figure 1; Table S2). The high
144
SUVA254 and low E2:E3 values are typical of terrestrially-derived DOM,57-59 and are
145
reflective of the high fraction of DOM from adjacent wetlands (i.e., ~65%), high DOC
146
loading rates, shallow photic zones, and short hydraulic residence times (HRTs).44
ACS Paragon Plus Environment
7
Environmental Science & Technology
Page 8 of 32
147
In contrast, optical properties cannot be used to distinguish the source of DOM in
148
the oligotrophic lakes. These lakes have the lowest [DOC] and A254 due to longer HRTs
149
and lower carbon loading rates (Table S1),44 but are highly variable in SUVA254 and
150
E2:E3. The major sources of DOM to the oligotrophic lakes are precipitation, aerial
151
deposition, surface waters, and groundwater, rather than terrestrially-derived DOM.44
152
Additionally, more DOM is lost to mineralization compared with export and
153
sedimentation, indicating higher rates of DOM processing.44 The optical properties of the
154
oligotrophic lakes could indicate the presence of autochthonous DOM, which typically
155
has higher E2:E3 and lower SUVA254 than allochthonous DOM.14,22,59-61 However,
156
photobleaching of terrestrially-derived DOM decreases SUVA254 and increases E2:E3,15
157
and the presence of measureable autochthonous DOC in these lakes is debated.44,62
158
Collectively, the lower SUVA254 and higher E2:E3 values in oligotrophic lakes could
159
reflect DOM from autochthonous sources or allochthonous DOM that has undergone
160
extensive processing.
161
Mesotrophic Allequash Lake is unique among the seven NTL-LTER lakes.
162
Although the DOC loading rate is similar to the bogs, Allequash Lake has the shortest
163
HRT and, uniquely, exports more DOC than is lost to mineralization and sedimentation.44
164
Additionally, it receives more DOM from adjacent wetlands than the oligotrophic lakes,
165
but less than the bogs (i.e., ~10%).44 These factors produce DOM that shares some
166
properties with the bogs (high SUVA254, low E2:E3) and some properties with the
167
oligotrophic lakes (low [DOC] and A254).
168
Samples were collected in 2016 for photochemistry and FT-ICR MS
169
measurements, and have [DOC], A254, and E2:E3 slightly above historical means (Figure
ACS Paragon Plus Environment
8
Page 9 of 32
Environmental Science & Technology
170
1; Table S2). The elevated [DOC] and A254 of samples collected in 2016 may reflect their
171
collection from the lake edges, rather than centers. However, UV-vis compositional
172
measurements are generally within a standard deviation of historical means, indicating
173
that the photochemistry and FT-ICR MS results are reflective of representative DOM for
174
NTL-LTER lakes. Long-term trends in the optical properties of the NTL-LTER lakes
175
have been discussed previously.63
176
Molecular Composition of DOM. [DOC] and UV-vis measurements differ
177
between lakes according to trophic status, but the cause of these variations is unclear.
178
Analysis of DOM by FT-ICR MS provides more detailed compositional information.
179
3037 unique CHON0-1P0-1S0-1 formulas are identified in the NTL-LTER lakes (Table 1),
180
with 975 - 1791 unique formulas in individual lakes. In each lake, 60 – 81% of identified
181
formulas contain only CHO. The predominance of CHO formulas is typical of
182
allochthonous DOM.13,17 Similarly, the bulk compositional measurements of CHO
183
formulas are characteristic of lignin-like, terrestrially-derived molecules (H:Cw.avg = 1.05
184
– 1.36; O:Cw.avg = 0.46 – 0.56; Figures 2 and S2).21,64-66 While the terms “lignin-like” and
185
“tannin-like” are not definitive (i.e., a lignin-like formula may not necessarily be derived
186
from lignin), these elemental ratio cutoffs are useful in visualizing the FT-ICR MS data.
187
H:Cw.avg increases and DBE/Cw.avg decreases from the bogs to Allequash Lake to the
188
oligotrophic lakes, confirming that the DOM in the bogs is more aromatic (Table 1).
189
O:Cw.avg is highest in the bogs and lower in Allequash Lake and the oligotrophic lakes.
190
CHON formulas comprise 12 – 31% of all identified formulas (Table 1) and
191
occupy similar van Krevelen space as CHO formulas from the same water body (Figures
192
2 and S3; Table S5). Higher fractions of formulas containing N are observed in the
ACS Paragon Plus Environment
9
Environmental Science & Technology
Page 10 of 32
193
oligotrophic lakes (21 – 31%) than in the bogs (12 – 15%) or Allequash Lake (17%). The
194
enrichment of CHON formulas in oligotrophic lakes could derive from the extensive
195
irradiation of allochthonous DOM or the presence of autochthonous DOM.17 However,
196
the compositional similarity of CHON and CHO formulas suggests a terrestrial source.
197
Few CHOP and CHOS formulas are detected (Figures S4 and S5; Tables 1 and
198
S6). Crystal Bog and Allequash Lake have the highest abundance of P-containing
199
formulas (9.4 and 6.1%, respectively), which could indicate preferential uptake of P-
200
containing formulas in the nutrient-poor lakes. Phospholipid formulas have been
201
identified in offshore coastal waters, but the CHOP formulas observed here have higher
202
O:C than expected for phospholipids.21 While few CHOS formulas are identified in NTL-
203
LTER lakes, some are observed at very high intensity, likely reflecting their ease of
204
ionization.38
205
Molecular Composition Differences Among Lakes. Bray-Curtis dissimilarity
206
analysis according to the log-weighted intensity of all identified formulas separates the
207
lakes by trophic status (Figure S6). Crystal and Trout Bogs are highly similar, with
208
Allequash Lake being slightly more dissimilar, while the oligotrophic lakes are highly
209
dissimilar to the bogs. This trend reflects the differences in the 25-year record of optical
210
properties. For example, SUVA254 measurements were highest in bogs and lowest in
211
Crystal Lake (Figure 1).
212
DOM compositional variation with trophic status is apparent in comparisons of
213
CHON0-1 formulas that are commonly identified in 5 or more lakes (n = 844; Figure S7).
214
The 481 CHO and 14 CHON formulas with higher relative intensity in the bogs and
215
Allequash Lake have lower average H:C (1.01 ± 0.24) and higher average O:C (0.54 ±
ACS Paragon Plus Environment
10
Page 11 of 32
Environmental Science & Technology
216
0.15) than 250 CHO and 99 CHON formulas enhanced in the oligotrophic lakes (H:C =
217
1.37 ± 0.02; O:C = 0.47 ± 0.14). 25% of formulas enhanced in oligotrophic lakes have
218
H:C values ≥1.5, which are typical of microbially-derived compounds,64,67 compared with
219
