This review of anaerobic metabolism emphasizes aerobic oxidation, because the two processes cannot be separated in a complete treatment of the topic. It is process oriented and highlights the fascinating microorganisms that mediate anaerobic biogeochemistry. We begin this review with a brief discussion of CO 2 assimilation by autotrophs, the source of most of the reducing power on Earth, and then consider the biological processes that harness this potential energy. Energy liberation begins with the decomposition of organic macromolecules to relatively simple compounds, which are simplified further by fermentation. Methanogenesis is considered next because CH 4 is a product of acetate fermentation, and thus completes the catabolism of organic matter, particularly in the absence of inorganic electron acceptors. Finally, the organisms that use nitrogen, manganese, iron, and sulfur for terminal electron acceptors are considered in order of decreasing free-energy yield of the reactions.

Anaerobic Metabolism: Linkages to Trace Gases and Aerobic Processes

. Global wetlands are believed to be climate sensitive, and are the largest natural emitters of methane (CH4). Increased wetland CH4 emissions could act as a positive feedback to future warming. The Wetland and Wetland CH4 Inter-comparison of Models Project (WETCHIMP) investigated our present ability to simulate large-scale wetland characteristics and corresponding CH4 emissions. To ensure inter-comparability, we used a common experimental protocol driving all models with the same climate and carbon dioxide (CO2) forcing datasets. The WETCHIMP experiments were conducted for model equilibrium states as well as transient simulations covering the last century. Sensitivity experiments investigated model response to changes in selected forcing inputs (precipitation, temperature, and atmospheric CO2 concentration). Ten models participated, covering the spectrum from simple to relatively complex, including models tailored either for regional or global simulations. The models also varied in methods to calculate wetland size and location, with some models simulating wetland area prognostically, while other models relied on remotely sensed inundation datasets, or an approach intermediate between the two. Four major conclusions emerged from the project. First, the suite of models demonstrate extensive disagreement in their simulations of wetland areal extent and CH4 emissions, in both space and time. Simple metrics of wetland area, such as the latitudinal gradient, show large variability, principally between models that use inundation dataset information and those that independently determine wetland area. Agreement between the models improves for zonally summed CH4 emissions, but large variation between the models remains. For annual global CH4 emissions, the models vary by ±40% of the all-model mean (190 Tg CH4 yr−1). Second, all models show a strong positive response to increased atmospheric CO2 concentrations (857 ppm) in both CH4 emissions and wetland area. In response to increasing global temperatures (+3.4 °C globally spatially uniform), on average, the models decreased wetland area and CH4 fluxes, primarily in the tropics, but the magnitude and sign of the response varied greatly. Models were least sensitive to increased global precipitation (+3.9 % globally spatially uniform) with a consistent small positive response in CH4 fluxes and wetland area. Results from the 20th century transient simulation show that interactions between climate forcings could have strong non-linear effects. Third, we presently do not have sufficient wetland methane observation datasets adequate to evaluate model fluxes at a spatial scale comparable to model grid cells (commonly 0.5°). This limitation severely restricts our ability to model global wetland CH4 emissions with confidence. Our simulated wetland extents are also difficult to evaluate due to extensive disagreements between wetland mapping and remotely sensed inundation datasets. Fourth, the large range in predicted CH4 emission rates leads to the conclusion that there is both substantial parameter and structural uncertainty in large-scale CH4 emission models, even after uncertainties in wetland areas are accounted for.

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Present state of global wetland extent and wetland methane modelling: conclusions from a model inter-comparison project (WETCHIMP)

management. How does our behavior (including urban development and land-use, water consumption, pollution) influence the movement of energy, water, and elements at local, regional, national or global scales? Will perturbations to chemical and energy cycles alter existing controls on ecosystem processes, and can we learn enough about them for effective regulation? Plants are critical in regulating biogeochemical cycles. Their growth controls the exchange of gases that support life in our current biosphere, and affects soil development. As primary producers, they influence the distribution of energy for higher trophic levels. Understanding how plants influence ecosystem processes requires a multidisciplinary approach drawing on plant physiology and biochemistry, community ecology, and biogeochemistry. Due to their unique physiology and ecology, bryophytes differ from vascular plants in influencing cycles of elements, energy, and water. For example, bryophytes have evolved an effective water relation system. Poikilohydry and desiccation tolerance allow bryophytes to tolerate longer periods of water stress than vascular plants, and to recover quickly with rehydration. With poorly developed conduction systems, water and solutes are taken up over the entire plant surface. Lack of both gametophyte stomata and effective cuticles in many species allows free exchange of solutions and gases across cell surfaces. Thus bryophytes often serve as effective traps for water and nutrients. This also makes them more sensitive to atmospheric chemical deposition than vascular plants. Bryophytes also can tolerate a wide range of temperatures and are found in almost all terrestrial and aquatic environments, including harsh Antarctic environments where vascular plant cover is low (cf. Fogg 1998; Seppelt 1995). Without roots, bryophytes can colonize hard substrates like rock and wood that are poor habitat for vascular species. Bryophytes stabilize soils and prevent the loss of soil and nutrients via erosion, particularly on sand dunes (Martinez & Maun 1999) and in cryptogamic soil crusts (Eldridge 1999; Evans & Johansen 1999). Cation exchange on Sphagnum cell walls releases protons, generating acidity that may inhibit plant and microbial growth (Clymo 1963; Craigie & Maass 1966; Spearing 1972). Finally, bryophytes influence ecosystem succession (Brock & Bregman 1989) through terrestrialization of water bodies, deposition of benthic organic matter or paludification of upland systems. Bryophyte colonization often precedes the establishment of tree surfaces by other canopy-dwelling plants (Nadkarni et al. 2000). Due to their physiology and life history traits, bryophytes influence ecosystem functions by producing organic matter, stabilizing soils or debris, trapping sediments and water, and providing food and habitat for algae, fungi, invertebrates, and amphibians. In this review, my objectives are to highlight several mechanisms by which bryophytes influence carbon (C) and nitrogen (N) cycles within and fluxes from ecosystems. As such, I will focus on how bryophytes fix, intercept, transform, and/or release C and N. My goals are to 1) introduce important processes controlling inputs and outputs of C and N in both terrestrial and aquatic ecosystems, 2) review work on the growth, decomposition, and leaching of bryophyte material, as well as biotic and abiotic controls on these mechanisms, and 3) suggest areas for future research that would advance our understanding of bryophytes in biogeochemical cycling. Current address: U.S. Geological Survey, 345 Middlefield Rd. MS 962, Menlo Park, CA 94025 U.S.A. e-mail: mturetsky@usgs.gov

The Role of Bryophytes in Carbon and Nitrogen Cycling

The lowland peatlands of south-east Asia represent an immense reservoir of fossil carbon and are reportedly responsible for 30% of the global carbon dioxide (CO2) emissions from Land Use, Land Use Change and Forestry. This paper provides a review and meta-analysis of available literature on greenhouse gas fluxes from tropical peat soils in south-east Asia. As in other parts of the world, water level is the main control on greenhouse gas fluxes from south-east Asian peat soils. Based on subsidence data we calculate emissions of at least 900 g CO2 m−2 a−1 (∼250 g C m−2 a−1) for each 10 cm of additional drainage depth. This is a conservative estimate as the role of oxidation in subsidence and the increased bulk density of the uppermost drained peat layers are yet insufficiently quantified. The majority of published CO2 flux measurements from south-east Asian peat soils concerns undifferentiated respiration at floor level, providing inadequate insight on the peat carbon balance. In contrast to previous assumptions, regular peat oxidation after drainage might contribute more to the regional long-term annual CO2 emissions than peat fires. Methane fluxes are negligible at low water levels and amount to up to 3 mg CH4 m−2 h−1 at high water levels, which is low compared with emissions from boreal and temperate peatlands. The latter emissions may be exceeded by fluxes from rice paddies on tropical peat soil, however. N2O fluxes are erratic with extremely high values upon application of fertilizer to wet peat soils. Current data on CO2 and CH4 fluxes indicate that peatland rewetting in south-east Asia will lead to substantial reductions of net greenhouse gas emissions. There is, however, an urgent need for further quantitative research on carbon exchange to support the development of consistent policies for climate change mitigation.

Greenhouse gas fluxes from tropical peatlands in south‐east Asia

Microbial methane accumulations have been discovered in multiple coal-bearing basins over the past two decades. Such discoveries were originally based on unique biogenic signatures in the stable isotopic composition of methane and carbon dioxide. Basins with microbial methane contain either low-maturity coals with predominantly microbial methane gas or uplifted coals containing older, thermogenic gas mixed with more recently produced microbial methane. Recent advances in genomics have allowed further evaluation of the source of microbial methane, through the use of high-throughput phylogenetic sequencing and fluorescent in situ hybridization, to describe the diversity and abundance of bacteria and methanogenic archaea in these subsurface formations. However, the anaerobic metabolism of the bacteria breaking coal down to methanogenic substrates, the likely rate-limiting step in biogenic gas production, is not fully understood. Coal molecules are more recalcitrant to biodegradation with increasing thermal m...

Biogeochemistry of Microbial Coal-Bed Methane

To elucidate the roles of hydrology and vegetation in below ground carbon cycling within peatlands, radiocarbon values were obtained for pore water dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), CH4, and peat from the Glacial Lake Agassiz peatland. The major implication of this work is that the rate of microbial respiration within a peat column is greater than the peat decomposition rate. The radiocarbon content of DOC at both bog and fen was enriched relative to solid-phase peat by ∼150-300‰ consistent with the advection of recently photosynthesized DOC downward into the peat column. Fen Δ14C values for DIC and CH4 closely track the Δ14C of pore water DOC at depth, indicating that this recent plant production was the predominant substrate for microbial respiration. Aceticlastic methanogenesis apparently dominated the upper third of the peat column (α = 1.05), shifting toward CO2 reduction with depth (1.05 < α < 1.08). Upwelling groundwater contributed as much as 15% of the DIC to the bulk DIC pool at depth in the fen. The similarity of Δ14C values for DIC and CH4 suggests that methanogens utilized DIC from this source as well as DIC produced in situ. Bog Δ14C values for pore water DIC and CH4 differ by ≤ 15‰ at all depths and are depleted in 14C relative to DOC by ∼100‰, suggesting microbial utilization of a mixture of older and modern substrates. CO2 reduction was the primary pathway for methanogenesis at all depths in the bog (α = 1.08), and groundwater influence on bulk DIC was negligible. For both sites, Δ14C−DIC and Δ14C−CH4 are approximately equal at depths where stable isotope data indicate a predominance of CO2 reduction and dissimilar when acetate fermentation is indicated.

https://agupubs.onlinelibrary.wiley.com/doi/epdf/10.1029/1999GB001221

Radiocarbon and stable carbon isotopic evidence for transport and transformation of dissolved organic carbon, dissolved inorganic carbon, and CH4 in a northern Minnesota peatland

Methane Concentration and Stable Isotope Distribution as Evidence of Rhizospheric Processes: Comparison of a Fen and Bog in the Glacial Lake Agassiz Peatland Complex

Raised bogs and municipal waste landfills harbor large populations of methanogens within their domed deposits of anoxic organic matter Although the methane emissions from these sites have been estimated by various methods, limited data exist on the activity of the methanogens at depth We therefore analyzed the stable isotopic signature of the pore waters in two raised bogs from northern Minnesota to identify depth intervals in the peat profile where methanogenic metabolism occurs Methanogenesis enriched the deuterium ( 2 H) content of the deep peat pore waters by as much as +11‰ (Vienna Standard Mean Sea Water), which compares to a much greater enrichment factor of +70‰ in leachate from New York City's Fresh Kills landfill The bog pore waters were isotopically dated by tritium ( 3 H) to be about 35 years old at 15 m depth, whereas the landfill leachate was estimated as ∼17 years old from Darcy flow calculations According to an isotopic mass balance the observed deuterium enrichment indicates that about 12 g of CH 4 m -3 d -1 were produced within the deeper peat, compared to about 28 g CH 4 m -3 d -1 in the landfill The values for methane production in the bog peat are substantially higher than the flux rates measured at the surface of the bogs or at the landfill, indicating that deeper methane production may be much higher than was previously assumed

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/2000GB001329

Estimating methane production rates in bogs and landfills by deuterium enrichment of pore water

The probable limits of the carbon budget of the Rapid River Watershed, within the greater Glacial Lake Agassiz Peatland in northern Minnesota, were evaluated using a Monte Carlo simulation approach. Carbon enters the peatlands in groundwater, precipitation, and primary productivity. Carbon leaves the peatlands by groundwater and surface water outflow and by the outgassing of methane. Results of the simulations of the carbon budget show that the peatland is now probably a sink for carbon, supported by field data showing peat is, in fact, accumulating at the rate of about 1 mm yr−1 [Glaser et al., 1997]. Excluding extreme values, Monte Carlo simulation results indicate that the Rapid River Peatland stores between −28.98 g C m−2 yr−1 (release) and 50.38 g C m−2 yr−1 (storage) with a mean accumulation of 12.74 g C m−2 yr−1 over the 1,506,200 m2 watershed. The peatland appears to be delicately poised with respect to net gain or loss of carbon.

https://agupubs.onlinelibrary.wiley.com/doi/pdf/10.1029/98GB02636

L. S. Chasar

Papers

Radiocarbon and stable carbon isotopic evidence for transport and transformation of dissolved organic carbon, dissolved inorganic carbon, and CH4 in a northern Minnesota peatland

Methane Concentration and Stable Isotope Distribution as Evidence of Rhizospheric Processes: Comparison of a Fen and Bog in the Glacial Lake Agassiz Peatland Complex

Estimating methane production rates in bogs and landfills by deuterium enrichment of pore water

A stochastic appraisal of the annual carbon budget of a large circumboreal peatland, Rapid River Watershed, northern Minnesota