Mercury Methylation and Demethylation in Anoxic Lake Sediments and by Strictly Anaerobic Bacteria

After spiking anoxic sediment slurries of three acidic oligotrophic lakes with either HgCl₂ at 1.0 μg/ml or CH₃HgI at 0.1 μg/ml, both mercury methylation and demethylation rates were measured. High mercury methylation potentials were accompanied by high demethylation potentials in the same sediment. These high potentials correlated positively with the concentrations of organic matter and dissolved sulfate in the sediment and with mercury levels in fish. Adjustment of the acidic sediment pH to neutrality failed to influence either the methylation or the demethylation rate of mercury. The opposing methylation and demethylation processes converged to establish similar Hg²⁺-CH₃Hg⁺ equilibria in all three sediments. Because of their metabolic dominance in anoxic sediments, mercury methylation and demethylation in pure cultures of sulfidogenic, methanogenic, and acetogenic bacteria were also measured. Sulfidogens both methylated and demethylated mercury, but the methanogen tested only catalyzed demethylation and the acetogen neither methylated nor demethylated mercury.     

Biodegradation of 1-nitropyrene

The metabolism of ¹⁴C-labeled 1-nitropyrene in microcosms containing nonsterile estuarine sediments, and in cultures of a Mycobacterium sp. previously isolated from oil-contaminated sediments was investigated. Although mineralization of 1-nitropyrene by pure cultures of the Mycobacterium sp. totaled only 12.3% after 10 days of incubation, over 80% of the ethyl acetate extractable ¹⁴C-labeled compounds consisted of 1-nitropyrene metabolites. High pressure liquid chromatographic analysis of 1-nitropyrene degradation products indicated that two major metabolites were formed. They were identified as 1-nitropyrene cis-9,10-and 4,5-dihydrodiols, based on their UV-visible, mass and NMR spectra. Time course studies in microcosms showed that 1-nitropyrene was degraded slowly under aerobic and anaerobic conditions in estuarine sediments. Less than 1% had been converted to ¹⁴CO₂ after 8 weeks of aerobic incubation. The addition of 1-nitropyrene to anaerobic sediments resulted in no ¹⁴CO₂ evolution; however, the nitro group of 1-nitropyrene was reduced to form 1-aminopyrene. Although the mineralization of 1-nitropyrene in sediments was slow, the Mycobacterium sp. metabolized 1-nitropyrene in pure culture. This bacterium appears promising for the bioremediation of this ubiquitous pollutant in contaminated waste.Abbreviations DEP Direct exposure probe - HPLC high pressure liquid chromatography - GC/MS gas chromatography/mass spectrometry - Nitro-PAHS nitropolycyclic aromatic hydrocarbons - TLC thin-layer chromatography - UV ultraviolet     

Microbial Decomposition of Synthetic 14C-Labeled Lignins in Nature: Lignin Biodegradation in a Variety of Natural Materials

Lignin biodegradation in a variety of natural materials was examined using specifically labeled synthetic ¹⁴C-lignins. Natural materials included soils, sediments, silage, steer bedding, and rumen contents. Both aerobic and anaerobic incubations were used. No ¹⁴C-labeled lignin biodegradation to labeled gaseous products under anaerobic conditions was observed. Aerobic ¹⁴C-labeled lignin mineralization varied with respect to type of natural material used, site, soil type and horizon, and temperature. The greatest observed degradation occurred in a soil from Yellowstone National Park and amounted to over 42% conversion of total radioactivity to ¹⁴CO₂ during 78 days of incubation. Amounts of ¹⁴C-labeled lignin mineralization in Wisconsin soils and sediments were significantly correlated with organic carbon, organic nitrogen, nitrate nitrogen, exchangeable calcium, and exchangeable potassium.     

Aerobic and anaerobic microbial activity in deep subsurface sediments from the savannah river plant

Methanogenesis, sulfate reduction, and rates of carbon mineralization were determined for samples derived at different depths from four cores drilled at the Savannah River Plant, Aiken South Carolina. Three‐gram subsamples of the sediments were dispensed to 10‐mL serum bottles under 5% H₂/95% N₂ and amended with 0.5 mL degassed distilled water with or without the following solutes: formate plus acetate, bicarbonate, lactate, and radiolabeled sulfate, glucose, and Índole. After incubating 1 to 5 days, the sediments were assayed for methane, H₂, ³⁵S, and ^I4CO₂. No methanogenesis was detected at any depth in any core and sulfate was rarely reduced. Evolution of ¹⁴CO₂ from glucose and indole was detected in sediments as deep as 262 and 259 m, respectively. At some depths the ¹⁴CO₂ evolution rate was comparable to that of surface soils; however, at other depths no ¹⁴CO₂ evolution could be detected. Injection of sterile air into anaerobic incubations increased rates of carbon mineralization at all depths that had demonstrated anaerobic activity and stimulated mineralization activity in sediments that were inactive anaerobically, suggesting a predominance of aerobic metabolism. Increasing the concentration of added glucose and indole often increased the resulting rates of ¹⁴CO₂ evolved from these substrates. Our data indicate that both aerobic and anaerobic microorganisms are present and metabolically active in samples from deep subsurface environments.     

Spatial and seasonal variation in greenhouse gas and nutrient dynamics and their interactions in the sediments of a boreal eutrophic lake

Dynamics of greenhouse gases, CH₄, CO₂ and N₂O, and nutrients, NO ₂ ^– + NO ₃ ^– , NH ₄ ⁺ and P, were studied in the sediments of the eutrophic, boreal Lake Kevätön in Finland. Undisturbed sediment cores taken in the summer, autumn and winter from the deep and shallow profundal and from the littoral were incubated in laboratory microcosms under aerobic and anaerobic water flow conditions. An increase in the availability of oxygen in water overlying the sediments reduced the release of CH₄, NH ₄ ⁺ and P, increased the flux of N₂O and NO ₂ ^– + NO ₃ ^– , but did not affect CO₂ production. The littoral sediments produced CO₂ and CH₄ at high rates, but released only negligible amounts of nutrients. The deep profundal sediments, with highest carbon content, possessed the greatest release rates of CO₂, CH₄, NH ₄ ⁺ and P. The higher fluxes of these gases in summer and autumn than in winter were probably due to the supply of fresh organic matter from primary production. From the shallow profundal sediments fluxes of CH₄, NH₄ ⁺ and P were low, but, in contrast, production of N₂O was the highest among the different sampling sites. Due to the large areal extension, the littoral and shallow profundal zones had the greatest importance in the overall gas and nutrient budgets in the lake. Methane emissions, especially the ebullition of CH₄ (up to 84% of the total flux), were closely related to the sediment P and NH ₄ ⁺ release. The high production and ebullition of CH₄, enhances the internal loading of nutrients, lake eutrophication status and the impact of boreal lakes to trophospheric gas budgets.     

Influence of pH on Terminal Carbon Metabolism in Anoxic Sediments from a Mildly Acidic Lake

The carbon and electron flow pathways and the bacterial populations responsible for transformation of H₂-CO₂, formate, methanol, methylamine, acetate, glycine, ethanol, and lactate were examined in sediments collected from Knaack Lake, Wis. The sediments were 60% organic matter (pH 6.2) and did not display detectable sulfate-reducing activity, but they contained the following average concentration (in micromoles per liter of sediment) of metabolites and end products: sulfide, 10; methane, 1,540; CO₂, 3,950; formate, 25; acetate, 157; ethanol, 174; and lactate, 138. Methane was produced predominately from acetate, and only 4% of the total CH₄ was derived from CO₂. Methanogenesis was limited by low environmental temperature and sulfide levels and more importantly by low pH. Increasing in vitro pH to neutral values enhanced total methane production rates and the percentage of CO₂ transformed to methane but did not alter the amount of ¹⁴CO₂ produced from [2-¹⁴C]acetate (~24%). Analysis of both carbon transformation parameters with ¹⁴C-labeled tracers and bacterial trophic group enumerations indicated that methanogenesis from acetate and both heterolactic- and acetic acid-producing fermentations were important to the anaerobic digestion process.     

Formation of Methane and Carbon Dioxide from Dimethylselenide in Anoxic Sediments and by a Methanogenic Bacterium

Anaerobic San Francisco Bay salt marsh sediments rapidly metabolized [¹⁴C]dimethylselenide (DMSe) to ¹⁴CH₄ and ¹⁴CO₂. Addition of selective inhibitors (2-bromoethanesulfonic acid or molybdate) to these sediments indicated that both methanogenic and sulfate-respiring bacteria could degrade DMSe to gaseous products. However, sediments taken from the selenium-contaminated Kesterson Wildlife Refuge produced only ¹⁴CO₂ from [¹⁴C]DMSe, implying that methanogens were not important in the Kesterson samples. A pure culture of a dimethylsulfide (DMS)-grown methylotrophic methanogen converted [¹⁴C]DMSe to ¹⁴CH₄ and ¹⁴CO₂. However, the organism could not grow on DMSe. Addition of DMS to either sediments or the pure culture retarded the metabolism of DMSe. This effect appeared to be caused by competitive inhibition, thereby indicating a common enzyme system for DMS and DMSe metabolism. DMSe appears to be degraded as part of the DMS pool present in anoxic environments. These results suggest that methylotrophic methanogens may demethylate methylated forms of other metals and metalloids found in nature.     

Temperature sensitivity of greenhouse gas production in wetland soils of different vegetation

Organic matter decomposition regulates rates of carbon loss (CO₂ and CH₄) in wetlands and has implications for carbon sequestration in the context of changing global temperature. Here we determined the influence of temperature and vegetation type on both aerobic and anaerobic decomposition of organic matter in subtropical wetland soils. As in many other studies, increased temperature resulted in higher rates of respiration and methanogenesis under both aerobic and anaerobic conditions, and the positive effect of temperature depended on vegetation (source of carbon substrate to soil). Under anaerobic incubations, the proportion of gaseous C (CO₂ and CH₄) lost as CH₄ increased with temperature indicating a greater sensitivity of methanogenesis to temperature. This was further supported by a wider range of Q₁₀ values (1.4–3.6) for methane production as compared with anaerobic CO₂ (1.3–2.5) or aerobic CO₂ (1.4–2.1) production. The increasing strength of positive linear correlation between CO₂:CH₄ ratio and the soil organic matter ligno-cellulose index at higher temperature indicated that the temperature sensitivity of methanogenesis was likely the result of increased C availability at higher temperature. This information adds to our basic understanding of decomposition in warmer subtropical and tropical wetland systems and has implications for C models in wetlands with different vegetation types.     

Methods for Measuring Specific Rates of Mercury Methylation and Degradation and Their Use in Determining Factors Controlling Net Rates of Mercury Methylation

A method was developed to estimate specific rates of demethylation of methyl mercury in aquatic samples by measuring the volatile ¹⁴C end products of ¹⁴CH₃HgI demethylation. This method was used in conjunction with a ²⁰³Hg²⁺ radiochemical method which determines specific rates of mercury methylation. Together, these methods enabled us to examine some factors controlling the net rate of mercury methylation. The methodologies were field tested, using lake sediment samples from a recently flooded reservoir in the Southern Indian Lake system which had developed a mercury contamination problem in fish. Ratios of the specific rates of methylation/demethylation were calculated. The highest ratios of methylation/demethylation were calculated. The highest ratios of methylation/demethylation occurred in the flooded shorelines of Southern Indian Lake. These results provide an explanation for the observed increases in the methyl mercury concentrations in fish after flooding.     

Radiative forcing of methane fluxes offsets net carbon dioxide uptake for a tropical flooded forest

Wetlands are important sources of methane (CH₄) and sinks of carbon dioxide (CO₂). However, little is known about CH₄ and CO₂ fluxes and dynamics of seasonally flooded tropical forests of South America in relation to local carbon (C) balances and atmospheric exchange. We measured net ecosystem fluxes of CH₄ and CO₂ in the Pantanal over 2014–2017 using tower‐based eddy covariance along with C measurements in soil, biomass and water. Our data indicate that seasonally flooded tropical forests are potentially large sinks for CO₂ but strong sources of CH₄, particularly during inundation when reducing conditions in soils increase CH₄ production and limit CO₂ release. During inundation when soils were anaerobic, the flooded forest emitted 0.11 ± 0.002 g CH₄‐C m^?2 d^?1 and absorbed 1.6 ± 0.2 g CO₂‐C m^?2 d^?1 (mean ± 95% confidence interval for the entire study period). Following the recession of floodwaters, soils rapidly became aerobic and CH₄ emissions decreased significantly (0.002 ± 0.001 g CH₄‐C m^?2 d^?1) but remained a net source, while the net CO₂ flux flipped from being a net sink during anaerobic periods to acting as a source during aerobic periods. CH₄ fluxes were 50 times higher in the wet season; DOC was a minor component in the net ecosystem carbon balance. Daily fluxes of CO₂ and CH₄ were similar in all years for each season, but annual net fluxes varied primarily in relation to flood duration. While the ecosystem was a net C sink on an annual basis (absorbing 218 g C m^?2 (as CH₄‐C + CO₂‐C) in anaerobic phases and emitting 76 g C m^?2in aerobic phases), high CH₄ effluxes during the anaerobic flooded phase and modest CH₄ effluxes during the aerobic phase indicate that seasonally flooded tropical forests can be a net source of radiative forcings on an annual basis, thus acting as an amplifying feedback on global warming.     

Microbial Methanogenesis and Acetate Metabolism in a Meromictic Lake

Methanogenesis and the anaerobic metabolism of acetate were examined in the sediment and water column of Knaack Lake, a small biogenic meromictic lake located in central Wisconsin. The lake was sharply stratified during the summer and was anaerobic below a depth of 3 m. Large concentrations (4,000 μmol/liter) of dissolved methane were detected in the bottom waters. A methane concentration maximum occurred at 4 m above the sediment. The production of ¹⁴CH₄ from ¹⁴C-labeled HCOOH, HCO₃⁻, and CH₃OH and [2-¹⁴C]acetate demonstrated microbial methanogenesis in the water column of the lake. The maximum rate of methanogenesis calculated from reduction of H¹⁴CO₃⁻ by endogenous electron donors in the surface sediment (depth, 22 m) was 7.6 nmol/h per 10 ml and in the water column (depth, 21 m) was 0.6 nmol/h per 10 ml. The methyl group of acetate was simultaneously metabolized to CH₄ and CO₂ in the anaerobic portions of the lake. Acetate oxidation was greatest in surface waters and decreased with water depth. Acetate was metabolized primarily to methane in the sediments and water immediately above the sediment. Sulfide inhibition studies and temperature activity profiles demonstrated that acetate metabolism was performed by several microbial populations. Sulfide additions (less than 5 μg/ml) to water from 21.5 m stimulated methanogenesis from acetate, but inhibited CO₂ production. Sulfate addition (1 mM) had no significant effect on acetate metabolism in water from 21.5 m, whereas nitrate additions (10 to 14,000 μg/liter) completely inhibited methanogenesis and stimulated CO₂ formation.     

Bacterioplankton and Bacteriobenthos of Three Floodplain Lakes in the Lower Course of the Amur River

The main structural and functional characteristics of bacterioplankton and bacteriobenthos of three lakes in the lower course of the Amur River are presented: the total number of bacteria (TNB), biomass, the numbers of bacteria of certain aerobic and anaerobic groups; the intensities of methanogenesis (MG), methane oxidation (MO), assimilation of ¹⁴C-compounds, sulfate reduction (SR); and gross estimate of organic matter decomposition (D). Depending on the reservoir type and the anthropogenic load, TNB constituted (2.27 to 16.1) × 10⁶ cells/ml in water and (1.06 to 10.35) × 10⁹cells/cm³ in sediments; MO was 0 to 0.28 ml CH₄/(l day) in water and 0 to 8.4 ml CH₄/(dm³ day) in sediments; MG in sediments was 0.001 to 40 ml CH₄/(dm³ day); SR varied from 0.001 to 24.8 mg S/(dm³ day); D was 0.3 to 25 g C/(m² day) in water and 0.2 to 4.9 g C/(m² day) in sediments. The role of anaerobic microbial processes of organic matter decomposition was shown to increase with an increase in the anthropogenic load, attaining 95% of the total D in the ecosystem of an accumulating pond.     

Anaerobic dechlorination and mineralization of pentachlorophenol and 2,4,6-trichlorophenol by methanogenic pentachlorophenol-degrading granules

Anaerobic granules developed for the treatment of pentachlorophenol (PCP) completely minearilized¹⁴C-labeled PCP to¹⁴CH₄ and¹⁴CO₂. Release of chloride ions from PCP was performed by live cells in the granules under anaerobic conditions. No chloride ions were released under aerobic conditions or by autoclaved cells. Addition of sulfate enhanced the initial chloride release rate and accelerated the process of mineralization of¹⁴C-labeled PCP. Addition of molybdate (10 mM) inhibited the chloride release rate and severely inhibited PCP mineralization. This suggests involvement of sulfate-reducing bacteria in PCP dechlorination and mineralization. Addition of 2-bromoethane sulfonate slightly decreased the chloride release rate and completely stopped production of¹⁴CH₄ and¹⁴CO₂ from [¹⁴C]PCP. 2,4,6-trichlorophenol was observed as an intermediate during PCP dechlorination. On the basis of experimental results, dechlorination of 2,4,6-trichlorophanol by the granules was conducted through 2,4-dichlorophenol, 4-chlorophenol or 2-chlorophenol to phenol at pH 7.0–7.2.     

Methane production in Santa Barbara basin sediments

In vitro incubation of Santa Barbara Basin sediments indicated that methane production occurs at all depths sampled, including those in which sulfate reduction occurs. Methane production in the sulfate zone decreases with depth. U‐¹⁴C‐lactate is readily metabolized in the sulfate‐reducing zone, with ¹⁴CO₂ production being greater than ¹⁴CH₄ production. However, if sulfate is added to incubated sediments that have become depleted in sulfate, the ¹⁴CH₄ production increases dramatically at the expense of ¹⁴CO₂ production. Contrary to what has been observed in other ecosystems, sulfate stimulated methane production, especially from lactate. Experiments using 2‐¹⁴C‐acetate or H¹⁴CO₃ ^‐ have indicated that bicarbonate is the principal source of methane and acetate is oxidized to CO₂ in sediments from the methane‐producing zone.     

Substrates for Sulfate Reduction and Methane Production in Intertidal Sediments

The activity of and potential substrates for methane-producing bacteria and sulfate-reducing bacteria were examined in marsh, estuary, and beach intertidal sediments. Slow rates of methane production were detected in all sediments, although rates of sulfate reduction were 100- to 1,000-fold higher. After sulfate was depleted in sediments, the rates of methane production sharply increased. The addition of methylamine stimulated methanogenesis in the presence of sulfate, and [¹⁴C]methylamine was rapidly converted to ¹⁴CH₄ and ¹⁴CO₂ in freshly collected marsh sediment. Acetate, hydrogen, or methionine additions did not stimulate methanogenesis. [methyl-¹⁴C]methionine and [2-¹⁴C]acetate were converted to ¹⁴CO₂ and not to ¹⁴CH₄ in fresh sediment. No reduction of ¹⁴CO₂ to ¹⁴CH₄ occurred in fresh sediment. Molybdate, an inhibitor of sulfate reduction, inhibited [2-¹⁴C]acetate metabolism by 98.5%. Fluoracetate, an inhibitor of acetate metabolism, inhibited sulfate reduction by 61%. These results suggest that acetate is a major electron donor for sulfate reduction in marine sediments. In the presence of high concentrations of sulfate, methane may be derived from novel substrates such as methylamine.     

Terminal Processes in the Anaerobic Degradation of an Algal-Bacterial Mat in a High-Sulfate Hot Spring

The algal-bacterial mat of a high-sulfate hot spring (Bath Lake) provided an environment in which to compare terminal processes involved in anaerobic decomposition. Sulfate reduction was found to dominate methane production, as indicated by comparison of initial electron flow through the two processes, rapid conversion of [2-¹⁴C]acetate to ¹⁴CO₂ and not to ¹⁴CH₄, and the lack of rapid reduction of NaH¹⁴CO₃ to ¹⁴CH₄. Sulfate reduction was the dominant process at all depth intervals, but a marked decrease of sulfate reduction and sulfate-reducing bacteria was observed with depth. Concurrent methanogenesis was indicated by the presence of viable methanogenic bacteria and very low but detectable rates of methane production. A marked increased in methane production was observed after sulfate depletion despite high concentrations of sulfide (>1.25 mM), indicating that methanogenesis was not inhibited by sulfide in the natural environment. Although a sulfate minimum and sulfide maximum occurred in the region of maximal sulfate reduction, the absence of sulfate depletion in interstitial water suggests that methanogenesis is always severely limited in Bath Lake sediments. Low initial methanogenesis was not due to anaerobic methane oxidation.     

Metabolism of Acetate, Methanol, and Methylated Amines in Intertidal Sediments of Lowes Cove, Maine

The fates and the rates of metabolism of acetate, trimethylamine, methylamine, and methanol were examined to determine the significance of these compounds as in situ methane precursors in surface sediments of an intertidal zone in Maine. Concentrations of these potential methane precursors were generally <3 μM, with the exception of sediments containing fragments of the seaweed Ascophyllum nodosum, in which acetate was 96 μM. [2-¹⁴C]acetate turnover in all samples was rapid (turnover time <2 h), with ¹⁴CO₂ as the primary product. [¹⁴C]trimethylamine and methylamine turnover times were slower (>8 h) and were characterized by formation of both ¹⁴CH₄ and ¹⁴CO₂. Ratios of ¹⁴CH₄/¹⁴CO₂ from [¹⁴C]trimethylamine and methylamine in uninhibited sediments indicated that a significant fraction of these substrates were catabolized via a non-methanogenic process. Data from inhibition experiments involving sodium molybdate and 2-bromoethanesulfonic acid supported this interpretation. [¹⁴C]methanol was oxidized relatively slowly compared with the other substrates and was catabolized mainly to ¹⁴CO₂. Results from experiments with molybdate and 2-bromoethanesulfonic acid suggested that methanol was oxidized primarily through sulfate reduction. In Lowes Cove sediments, trimethylamine accounted for 35.1 to 61.1% of total methane production.     

Evidence for Aceticlastic Methanogenesis in the Presence of Sulfate in a Gas Condensate-Contaminated Aquifer

The anaerobic metabolism of acetate was studied in sediments and groundwater from a gas condensate-contaminated aquifer in an aquifer where geochemical evidence implicated sulfate reduction and methanogenesis as the predominant terminal electron-accepting processes. Most-probable-number tubes containing acetate and microcosms containing either [2-¹⁴C]acetate or [U-¹⁴C]acetate produced higher quantities of CH₄ compared to CO₂ in the presence or absence of sulfate.¹⁴CH₄ accounted for 70 to 100% of the total labeled gas in the [¹⁴C]acetate microcosms regardless of whether sulfate was present or not. Denaturing gradient gel electrophoresis of the acetate enrichments both with and without sulfate using Archaea-specific primers showed identical predominant bands that had 99% sequence similarity to members of Methanosaetaceae. Clone libraries containing archaeal 16S rRNA gene sequences amplified from sediment from the contaminated portion of the aquifer showed that 180 of the 190 clones sequenced belonged to the Methanosaetaceae. The production of methane and the high frequency of sequences from the Methanosaetaceae in acetate enrichments with and without sulfate indicate that aceticlastic methanogenesis was the predominant fate of acetate at this site even though sulfate-reducing bacteria would be expected to consume acetate in the presence of sulfate.     

Regulation of Decomposition and Methane Dynamics across Natural,Commercially Mined,and Restored Northern Peatlands

Abstract We examined aerobic and anaerobic microbial carbon dioxide (CO₂) and methane (CH₄) exchange in peat samples representing different profiles at natural, mined, mined-abandoned, and restored northern peatlands and characterized the nutrient and substrate chemistry and microbial biomass of these soils. Mining and abandonment led to reduced nutrient and substrate availability and occasionally drier conditions in surface peat resulting in a drastic reduction in CO₂ and CH₄ production, in agreement with previous studies. Owing mainly to wetter conditions, CH₄ production and oxidation were faster in restored block-cut than natural sites, whereas in one restored site, increased substrate and nutrient availability led to much more rapid rates of CO₂ production. Our work in restored block-cut sites compliments that in vacuum-harvested peatlands undergoing more recent active restoration attempts. The sites we examined covered a large range of soil C substrate quality, nutrient availability, microbial biomass, and microbial activities, allowing us to draw general conclusions about controls on microbial CO₂ and CH₄ dynamics using stepwise regression analysis among all sites and soil depths. Aerobic and anaerobic decomposition of peat was constrained by organic matter quality, particularly phosphorus (P) and carbon (C) chemistry, and closely linked to the size of the microbial biomass supported by these limiting resources. Methane production was more dominantly controlled by field moisture content (a proxy for anaerobism), even after 20 days of anaerobic laboratory incubation, and to a lesser extent by C substrate availability. As methanogens likely represented only a small proportion of the total microbial biomass, there were no links between total microbial biomass and CH₄ production. Methane oxidation was controlled by the same factors influencing CH₄ production, leading to the conclusion that CH₄ oxidation is primarily controlled by substrate (that is, CH₄) availability. Although restoring hydrology similar to natural sites may re-establish CH₄ dynamics, there is geographic or site-specific variability in the ability to restore peat decomposition dynamics.     

Anaerobic Biodegradation of the Lignin and Polysaccharide Components of Lignocellulose and Synthetic Lignin by Sediment Microflora

Specifically radiolabeled [¹⁴C-lignin]lignocelluloses and [¹⁴C-polysaccharide]lignocelluloses were prepared from a variety of marine and freshwater wetland plants including a grass, a sedge, a rush, and a hardwood. These [¹⁴C]lignocellulose preparations and synthetic [¹⁴C]lignin were incubated anaerobically with anoxic sediments collected from a salt marsh, a freshwater marsh, and a mangrove swamp. During long-term incubations lasting up to 300 days, the lignin and polysaccharide components of the lignocelluloses were slowly degraded anaerobically to ¹⁴CO₂ and ¹⁴CH₄. Lignocelluloses derived from herbaceous plants were degraded more rapidly than lignocellulose derived from the hardwood. After 294 days, 16.9% of the lignin component and 30.0% of the polysaccharide component of lignocellulose derived from the grass used (Spartina alterniflora) were degraded to gaseous end products. In contrast, after 246 days, only 1.5% of the lignin component and 4.1% of the polysaccharide component of lignocellulose derived from the hardwood used (Rhizophora mangle) were degraded to gaseous end products. Synthetic [¹⁴C]lignin was degraded anaerobically faster than the lignin component of the hardwood lignocellulose; after 276 days, 3.7% of the synthetic lignin was degraded to gaseous end products. Contrary to previous reports, these results demonstrate that lignin and lignified plant tissues are biodegradable in the absence of oxygen. Although lignocelluloses are recalcitrant to anaerobic biodegradation, rates of degradation measured in aquatic sediments are significant and have important implications for the biospheric cycling of carbon from these abundant biopolymers.