Internal Carbon Cycling Research Articles

Dominance of net autotrophy in arid landscape low relief polar lakes, Nunavut, Canada.

The Arctic is the fastest warming biome on the planet, and environmental changes are having striking effects on freshwater ecosystems that may impact the regional carbon cycle. The metabolic state of Arctic lakes is often considered net heterotrophic, due to an assumed supply of allochthonous organic matter that supports ecosystem respiration and carbon mineralization in excess of rates of primary production. However, lake metabolic patterns vary according to regional climatic characteristics, hydrological connectivity, organic matter sources and intrinsic lake properties, and the metabolism of most Arctic lakes is unknown. We sampled 35 waterbodies along a connectivity gradient from headwater to downstream lakes, on southern Victoria Island, Nunavut, in an area characterized by low precipitation, organic-poor soils, and high evaporation rates. We evaluated whether lakes were net autotrophic or heterotrophic during the open water period using an oxygen isotopic mass balance approach. Most of the waterbodies were autotrophic and sites of net organic matter production or close to metabolic equilibrium. Autotrophy was associated with higher benthic primary production, as compared to its pelagic counterpart, due to the high irradiance reaching the bottom and efficient internal carbon and nutrient cycling. Highly connected midstream and downstream lakes showed efficient organic matter cycling, as evidenced by the strong coupling between gross primary production (GPP) and ecosystem respiration, while decoupling was observed in some headwater lakes with significantly higher GPP. The shallow nature of lakes in the flat, arid region of southern Victoria Island supports net autotrophy in most lakes during the open water season. Ongoing climate changes that lengthen the ice-free irradiance period and increase rates of nutrient evapoconcentration may further promote net autotrophy, with uncertain long-term effects for lake functioning.

Site-specific field management adaptation is key to feeding the world in the 21st century

Rapid climate change and growing population threaten global food security across the globe. Several studies have proposed early planting, increased irrigation, and increased fertilizer applications as climate adaptation strategies, yet none have considered combined and site-specific field management strategies as a comprehensive solution. Here, we analyzed non-irrigated wheat yield responses to climate change and field management adaptation using a mechanistic crop model evaluated against observed global non-irrigated wheat-yield over 3-year intervals spanning 13 years at 3749 sites (RMSE = 36 gC m−2). Early planting with later-maturing varieties provided the most benefit to future yields among the proposed field management adaptation strategies. Improved water use efficiency from increased CO2 led to relatively low benefits of additional irrigation. We estimated that spatially heterogeneous adaption strategies had the potential to improve global wheat yields by 91% by 2100 compared to the present day. The yield improvements from combined field adaptation strategies were larger than the sum of improvements from the individual strategies. These synergistic benefits were shown to result from complementary processes regulating nutrient and water uptake, physiological tolerance to heat stress, and internal carbon and nutrient cycling.

Drainage Ditch Cleaning Has No Impact on the Carbon and Greenhouse Gas Balances in a Recent Forest Clear-Cut in Boreal Sweden

Ditch cleaning (DC) is increasingly applied to facilitate forest regeneration following clear-cutting in Fennoscandinavia. However, its impact on the ecosystem carbon and greenhouse gas (GHG) balances is poorly understood. We conducted chamber measurements to assess the initial DC effects on carbon dioxide (CO2) and methane (CH4) fluxes in a recent forest clear-cut on wet mineral soil in boreal Sweden. Measurements were conducted in two adjacent areas over two pre-treatments (2018/19) and two years (2020/21) after conducting DC in one area. We further assessed the spatial variation of fluxes at three distances (4, 20, 40 m) from ditches. We found that DC lowered the water table level by 12 ± 2 cm (mean ± standard error) and topsoil moisture by 0.12 ± 0.01 m3 m−3. DC had a limited initial effect on the net CO2 exchange and its component fluxes. CH4 emissions were low during the dry pre-treatment years but increased particularly in the control area during the wet years of 2020/21. Distance to ditch had no consistent effects on CO2 and CH4 fluxes. Model extrapolations suggest that annual carbon emissions decreased over the four years from 6.7 ± 1.4 to 1.6 ± 1.6 t-C ha−1 year−1, without treatment differences. Annual CH4 emissions contributed <2.5% to the carbon balance but constituted 39% of the GHG balance in the control area during 2021. Overall, our study suggests that DC modified the internal carbon cycling but without significant impact on the carbon and GHG balances.

Genomic and metabolic adaptations of biofilms to ecological windows of opportunity in glacier-fed streams

In glacier-fed streams, ecological windows of opportunity allow complex microbial biofilms to develop and transiently form the basis of the food web, thereby controlling key ecosystem processes. Using metagenome-assembled genomes, we unravel strategies that allow biofilms to seize this opportunity in an ecosystem otherwise characterized by harsh environmental conditions. We observe a diverse microbiome spanning the entire tree of life including a rich virome. Various co-existing energy acquisition pathways point to diverse niches and the exploitation of available resources, likely fostering the establishment of complex biofilms during windows of opportunity. The wide occurrence of rhodopsins, besides chlorophyll, highlights the role of solar energy capture in these biofilms while internal carbon and nutrient cycling between photoautotrophs and heterotrophs may help overcome constraints imposed by oligotrophy in these habitats. Mechanisms potentially protecting bacteria against low temperatures and high UV-radiation are also revealed and the selective pressure of this environment is further highlighted by a phylogenomic analysis differentiating important components of the glacier-fed stream microbiome from other ecosystems. Our findings reveal key genomic underpinnings of adaptive traits contributing to the success of complex biofilms to exploit environmental opportunities in glacier-fed streams, which are now rapidly changing owing to global warming.

Sustained high CO

Many of the world’s rivers have been found to be sources of CO2 to the atmosphere, however, there has been limited assessment in arid regions. This analysis of a long-term (1979–2013) dataset (n = 3496) along Australia’s largest river system (River Murray) showed that there were sustained high pCO2 (carbon dioxide partial pressure) levels, ranging from 1210 ± 107 to 3066 ± 579 µatm along the main river channel, and 5114 ± 1221 µatm on the major tributaries. As a consequence, the River Murray is a significant source of CO2 to the atmosphere, with an estimated average annual (±s.d.) flux of 218 ± 98 g C m−2 year−1 and total emissions of 355 000 ± 29 000 t CO2 year−1 over a total river area of 386 km2 from below Lake Hume to Tailem Bend, although there is some uncertainty with gas transfer coefficients. Supersaturation with CO2 relative to the atmosphere was maintained even under drought conditions with minimal external carbon inputs, suggesting internal carbon cycling and respiration is important in driving net CO2 production. Supersaturation of the river water relative to calcium carbonate minerals was also observed under low flow conditions. Hydro-climatic changes could be having significant impacts on the CO2 system in the River Murray and other arid river systems.

Experimental evidence reveals impact of drought periods on dissolved organic matter quality and ecosystem metabolism in subalpine streams

AbstractSubalpine streams are predicted to experience lower summer discharge following climate change and water extractions. In this study, we aimed to understand how drought periods impact dissolved organic matter (DOM) processing and ecosystem metabolism of subalpine streams. We mimicked a gradient of drought conditions in stream‐side flumes and evaluated implications of drought on DOM composition, gross primary production, and ecosystem respiration. Our experiment demonstrated a production and release of DOM from biofilms and leaf litter decomposition at low discharges, increasing dissolved organic carbon concentrations in stream water by up to 50%. Absorbance and fluorescence properties suggested that the released DOM was labile for microbial degradation. Dissolved organic carbon mass balances revealed a high contribution of internal processes to the carbon budget during low flow conditions. The flumes with low discharge were transient sinks of atmospheric CO2 during the first 2 weeks of drought. After this autotrophic phase, the metabolic balance of these flumes turned heterotrophic, suggesting a nutrient limitation for primary production, while respiration remained high. Overall our experimental findings suggest that droughts in subalpine streams will enhance internal carbon cycling by transiently increasing primary production and more permanently respiration as the drought persists. We propose that the duration of a drought period combined with inorganic nutrient availability are key variables that determine if more carbon is respired in situ or exported downstream.

Novel Autotrophic Organisms Contribute Significantly to the Internal Carbon Cycling Potential of a Boreal Lake.

ABSTRACTOxygen-stratified lakes are typical for the boreal zone and also a major source of greenhouse gas emissions in the region. Due to shallow light penetration, restricting the growth of phototrophic organisms, and large allochthonous organic carbon inputs from the catchment area, the lake metabolism is expected to be dominated by heterotrophic organisms. In this study, we test this assumption and show that the potential for autotrophic carbon fixation and internal carbon cycling is high throughout the water column. Further, we show that during the summer stratification carbon fixation can exceed respiration in a boreal lake even below the euphotic zone. Metagenome-assembled genomes and 16S profiling of a vertical transect of the lake revealed multiple organisms in an oxygen-depleted compartment belonging to novel or poorly characterized phyla. Many of these organisms were chemolithotrophic, potentially deriving their energy from reactions related to sulfur, iron, and nitrogen transformations. The community, as well as the functions, was stratified along the redox gradient. The autotrophic potential in the lake metagenome below the oxygenic zone was high, pointing toward a need for revising our concepts of internal carbon cycling in boreal lakes. Further, the importance of chemolithoautotrophy for the internal carbon cycling suggests that many predicted climate change-associated fluctuations in the physical properties of the lake, such as altered mixing patterns, likely have consequences for the whole-lake metabolism even beyond the impact to the phototrophic community.

Changes in Carbon Cycling during Development of Successional Agroforestry

Successional agroforestry systems (SAFS) mimic the structure of natural forests while providing economical outputs. This study clarifies how carbon cycling and carbon sequestration change during successional development of SAFS. In Brazil, three successional stages of SAFS, 6, 12, and 34 years old, were compared in terms of carbon balance. Aboveground biomass, fruit harvest, litterfall, soil respiration, and soil organic carbon were measured for two years and analyzed. Carbon sequestration expressed by net primary productivity increased with age of SAFS from 9.8 Mg·C·ha−1·year−1 in 6-year-old system to 13.5 Mg·C·ha−1·year−1 in 34-year-old system. Accumulation of plant biomass and increased internal carbon cycling in SAFS led to an intensive sequestration of carbon. SAFS can be a sustainable way of agricultural production on vulnerable tropical soils.

Physical and biogeochemical mechanisms of internal carbon cycling in Lake Michigan

AbstractThe lakewide seasonal carbon cycle of Lake Michigan is poorly quantified and lacks the mechanistic links necessary to determine impacts upon it from eutrophication, invasive species, and climate change. A first step toward a full appreciation of Lake Michigan's carbon cycle is to quantify the dominant mechanisms of its internal carbon cycle. To achieve this, we use the MIT general circulation model configured to the bathymetry of Lake Michigan and coupled to an ecosystem model to simulate the seasonal cycle of productivity, temperature, circulation, and the partial pressure of CO2 in water (pCO2). This biogeochemistry is designed to be appropriate for the prequagga mussel state of the lake. The primary mechanism behind the seasonal cycle of primary productivity is lake physics. The offshore spring phytoplankton bloom begins following a reduction in deep vertical mixing and ends with the depletion of nutrients via thermal stratification. The exception is the western shoreline, where summer winds drive coastal upwelling, providing hypolimnetic nutrients and generating significant productivity. Surface pCO2 is controlled by the net effect from temperature on solubility, and is modulated by biological uptake of dissolved inorganic carbon (DIC) and isothermal mixing of DIC‐rich water in winter. Temperature tends to have the greatest seasonal impact in nearshore regions, while local DIC has the greatest impact in offshore regions. Lakewide, the model suggests that carbon is absorbed from the atmosphere during the spring bloom and released to the atmosphere during winter mixing and when summer surface temperatures are at their maximum.

Effects of nutrient enrichment on mangrove leaf litter decomposition

Nutrient enrichment of mangroves, a common phenomenon along densely populated coastlines, may negatively affect mangrove ecosystems by modifying internal carbon and nutrient cycling. The decomposition of litter exerts a strong influence on these processes and is potentially modified by eutrophication. This study describes effects of N and P enrichment on litter decomposition rate and mineralisation/immobilisation patterns.By making use of reciprocal litter transplantation experiments among fertiliser treatments, it was tested if nutrient addition primarily acts on the primary producers (i.e. changes in litter quantity and quality) or on the microbial decomposers (i.e. changes in nutrient limitation for decomposition). Measurements were done in two mangrove forests where primary production was either limited by N or by P, which had been subject to at least 5years of experimental N and P fertilisation.Results of this study indicated that decomposers were always N-limited regardless of the limitation of the primary producers. This leads to a differential nutrient limitation between decomposers and primary producers in sites where mangrove production was P-limited. In these sites, fertilisation with P caused litter quality to change, resulting in a higher decomposition rate. This study shows that direct effects of fertilisation on decomposition through an effect on decomposer nutrient availability might be non-significant, while the indirect effects through modifying litter quality might be quite substantial in mangroves. Our results show no indication that eutrophication increases decomposition without stimulating primary production. Therefore we do not expect a decline in carbon sequestration as a result of eutrophication of mangrove ecosystems.

A Realistic Role for Reference in Wetland Restoration

Wetland restoration projects are often evaluated for success or failure by comparing attributes of restored ecosystems to undisturbed reference ecosystems. This method of evaluation has several flaws, including site-specific responses of restored ecosystems to topographic and geomorphic changes during degradation and restoration, the substantial differences in structure and function of early (restored) and mature (reference) ecosystems, and the inherent variability that exists for ecosystems that may not be represented in reference systems. We need a better framework for evaluating restoration success. Success could be demonstrated by using ecosystem attributes that indicate ecosystem integrity and maturation over time, such as: stable landforms, native plant community development, internal carbon and nutrient storage and cycling, connectivity within the broader landscape, ecosystem heterogeneity, and increasing complexity in species and community diversity. A more realistic role for reference areas in wetland restoration is useful information on structural and functional attributes of ecosystems and an indication of the natural variability that exists in ecosystems that we hope to emulate. Ultimately though, success in wetland restoration depends on our ability to establish conditions that promote ecosystem maturation and integrity and reference systems may be of limited value to evaluate this success.

Combined effects of CO2 and light on the N2-fixing cyanobacterium Trichodesmium IMS101: physiological responses.

Recent studies on the diazotrophic cyanobacterium Trichodesmium erythraeum (IMS101) showed that increasing CO(2) partial pressure (pCO(2)) enhances N(2) fixation and growth. Significant uncertainties remain as to the degree of the sensitivity to pCO(2), its modification by other environmental factors, and underlying processes causing these responses. To address these questions, we examined the responses of Trichodesmium IMS101 grown under a matrix of low and high levels of pCO(2) (150 and 900 microatm) and irradiance (50 and 200 micromol photons m(-2) s(-1)). Growth rates as well as cellular carbon and nitrogen contents increased with increasing pCO(2) and light levels in the cultures. The pCO(2)-dependent stimulation in organic carbon and nitrogen production was highest under low light. High pCO(2) stimulated rates of N(2) fixation and prolonged the duration, while high light affected maximum rates only. Gross photosynthesis increased with light but did not change with pCO(2). HCO(3)(-) was identified as the predominant carbon source taken up in all treatments. Inorganic carbon uptake increased with light, but only gross CO(2) uptake was enhanced under high pCO(2). A comparison between carbon fluxes in vivo and those derived from (13)C fractionation indicates high internal carbon cycling, especially in the low-pCO(2) treatment under high light. Light-dependent oxygen uptake was only detected under low pCO(2) combined with high light or when low-light-acclimated cells were exposed to high light, indicating that the Mehler reaction functions also as a photoprotective mechanism in Trichodesmium. Our data confirm the pronounced pCO(2) effect on N(2) fixation and growth in Trichodesmium and further show a strong modulation of these effects by light intensity. We attribute these responses to changes in the allocation of photosynthetic energy between carbon acquisition and the assimilation of carbon and nitrogen under elevated pCO(2). These findings are supported by a complementary study looking at photosynthetic fluorescence parameters of photosystem II, photosynthetic unit stoichiometry (photosystem I:photosystem II), and pool sizes of key proteins in carbon and nitrogen acquisition.

Changes in morphology and diet of the coral Stylophora pistillata along a depth gradient

The light-limited environment of tropical coral reefs has not been intensively studied due to technical limitations. Studying this vast part of the coral reef is paramount to understanding the ecological and physiological significance of coral-algae symbiosis and defining the boundaries imposed on its bathymetric distribution by the underwater light field. In the present study we describe morphological changes in colonies of the coral Stylophora pistillata and track changes in its carbon sources (autotrophic/heterotrophic behavior) along its full bathymetric distribution. The growth form of hermatypic corals must compromise between an optimal light-trapping surface facil- itating photosynthesis and other structures and/or mechanisms that enhance exploitation of nutrient- rich sources such as zooplankton. That architectural modulation is constrained within the species- specific structural and biological characteristics. We found that the profusely branched S. pistillata colonies shift between subspherical morphology at high-light environments to a planar structure at depth. The stable carbon isotopic composition (δ 13 C) of the host coral tissue changed from a value of -15‰ in shallow water to -23‰ at the deep reef. The latter value indicates either a carbon source with a stable isotope composition equal or below -23‰ or, alternatively, internal carbon cycling between host and algae that involves isotopic fractionation (e). The δ 13 C values showed significant correlation to morphological traits, but contradicting trends were found within the traits. A clear shift to heterotrophy was not apparent, which, therefore, suggests that the internal cycling and Rubisco activity are the dominant processes determining isotopic composition.

Carbon isotopic composition of Trichodesmium spp. colonies off Bermuda: effects of colony mass and season

Colonies of Trichodesmium spp. are conspicuous, macroscopic components of the life in tropical and subtropical oceans. The large size and the morphology of the colony raise questions regarding the mechanism of carbon supply for photosynthesis. Constraints on these mechanisms may be indicated by the stable carbon isotopic composition (δ13C) that reflects the balance between carbon supply and speciation, as well as the growth rate and colony size. The δ13C of Trichodesmium off Bermuda measured here revealed a strong correlation between size of individual colonies and season. The smallest colonies, 2–7 μg C colony−1, showed the lightest δ13C composition (∼−19‰), increasing to asymptotic values of ∼−12‰ above 7 μg C colony−1. The average δ13C of the colonies was lightest immediately after the onset of stratification in the Sargasso Sea, gradually increasing by ∼4‰ to heavier values during the summer. We propose that the mass effect is due to increased use of HCO3− by the larger colonies, whereas the seasonal influence may be related to changes in irradiance and pCO2 affecting the internal carbon cycling.

Internal carbon and nutrient cycling in Lake Baikal: sedimentation, upwelling, and early diagenesis

The internal cycles of carbon, silica, nitrogen, and phosphorus in the South and North Basins of Lake Baikal were quantified in the frame of a multidisciplinary collaboration. Fluxes of particulate organic matter from the epilimnion to the deep water were quantified with integrating sediment traps deployed at 200- to 250-m water depth and compared with fluxes measured in near-bottom traps to reveal mineralization in the water column. Sedimentation rates were determined with dated sediment cores to calculate mass accumulation rates of elements in the sediment. Advective and turbulent transport of dissolved nutrients in the water column was based on a set of monitoring data, which included temperature and current data, as well as hydrochemical data of the water column. Diffusive fluxes from the sediment to the overlying water column were determined by applying different porewater sampling techniques. The combination of these data resulted in consistent internal budgets for carbon, nitrogen, and phosphorus in Lake Baikal: the new production in the South Basin was 1730 mmol C m −2 year −1 and the mass accumulation rate in the sediment 220 mmol C m −2 year −1, whereas in the more secluded North Basin, new production was only 1220 mmol C m −2 year −1 and mass accumulation rate 125 mmol C m −2 year −1. Fluxes of particle-bound nitrogen, phosphorus, and biogenic silica were by about 30% smaller in the North Basin than in the South Basin. Export fluxes of nitrogen from the surface zone to the deep water were 150 mmol N and 100 mmol N m −2 year −1. Denitrification rates in the sediment were estimated from mass-loss calculation to 38 and 53 mmol N m −2 year −1 for the South and North Basin, respectively, corresponding to 25% and 52% of the total nitrogen input to the hypolimnion. Nitrogen (19 and 13 mmol m −2 year −1) was finally buried in the sediments of the South and North Basins; 10.1 and 3.5 mmol P m −2 year −1, and 1830 and 1400 mmol Si m −2 year −1 were transferred to the deep water in the South and North Basin where 28% and 70% P, and 64% and 54% Si were retained in the sediments. A diatom bloom occurred during our sampling period of 2 years, which usually occurs only every 3 to 5 years. Accordingly, Si flux data from sediment traps were increased by an estimated 50% compared with the long-term average illustrating the necessity of several years of field measurements to compensate for the natural dynamics of Lake Baikal.

Biogeochemistry of an Iron-Rich Hypersaline Microbial Mat (Camargue, France)

In situ microsensor measurements were combined with biogeochemical methods to determine oxygen, sulfur, and carbon cycling in microbial mats growing in a solar saltern (Salin-de-Giraud, France). Sulfate reduction rates closely followed the daily temperature changes and were highest during the day at 25 degrees C and lowest during the night at 11 degrees C, most probably fueled by direct substrate interactions between cyanobacteria and sulfate-reducing bacteria. Sulfate reduction was the major mineralization process during the night and the contribution of aerobic respiration to nighttime DIC production decreased. This decrease of aerobic respiration led to an increasing contribution of sulfide (and iron) oxidation to nighttime O2 consumption. A peak of elemental sulfur in a layer of high sulfate reduction at low sulfide concentration underneath the oxic zone indicated anoxygenic photosynthesis and/or sulfide oxidation by iron, which strongly contributed to sulfide consumption. We found a significant internal carbon cycling in the mat, and sulfate reduction directly supplied DIC for photosynthesis. The mats were characterized by a high iron content of 56 micromol Fe cm(-3), and iron cycling strongly controlled the sulfur cycle in the mat. This included sulfide precipitation resulting in high FeS contents with depth, and reactions of iron oxides with sulfide, especially after sunset, leading to a pronounced gap between oxygen and sulfide gradients and an unusual persistence of a pH peak in the uppermost mat layer until midnight.

The mechanism of calcification and its relation to photosynthesis and respiration in the scleractinian coral Galaxea fascicularis

The mechanism of calcification and its relation to photosynthesis and respiration were studied with Ca2+, pH and O2 microsensors using the scleractinian coral Galaxea fascicularis. Gross photosynthesis (Pg), net photosynthesis (Pn) and dark respiration (DR) were measured on the surface of the coral. Light respiration (LR) was calculated from the difference between Pg and Pn. Pg was about seven times higher than Pn; thus, respiration consumes most of the O2 produced by the algal symbiont's photosynthesis. The respiration rate in light was ca. 12 times higher than in the dark. The coupled Pg and LR caused an intense internal carbon and O2 cycling. The resultant product of this cycle is metabolic energy (ATP). The measured ATP content was about 35% higher in light-incubated colonies than in dark-incubated ones. Direct measurements of Ca2+ and pH were made on the outer surface of the polyp, inside its coelenteron and under the calicoblastic layer. The effects on Ca2+ and pH dynamics of switching on and off the light were followed in these three compartments. Ca2+ concentrations decreased in light on the surface of the polyp and in the coelenteron. They increased when the light was switched off. The opposite effect was observed under the calicoblastic layer. In light, the level of Ca2+ was lower on the polyp surface than in the surrounding seawater, and even lower inside the coelenteron. The concentration of calcium under the calicoblastic layer was about 0.6 mM higher than in the surrounding seawater. Thus Ca2+ can diffuse from seawater to the coelenteron, but metabolic energy is needed for its transport across the calicoblastic layer to the skeleton. The pH under the calicoblastic layer was more alkaline compared with the polyp surface and inside the coelenteron. This rise in pH increased the supersaturation of aragonite from 3.2 in the dark to 25 in the light, and brought about more rapid precipitation of CaCO3. When ruthenium red was added, Ca2+ and pH dynamics were inhibited under the calicoblastic layer. Ruthenium red is a specific inhibitor of Ca-ATPase. The results indicated that Ca-ATPase transports Ca2+ across the calicoblastic layer to the skeleton in exchange for H+. Addition of dichlorophenyldimethylurea completely inhibited photosynthesis. The calcium dynamics under the calicoblastic layer continued; however, the process was less regular. Initial rates were maintained. We conclude that light and not energy generation triggers calcium uptake; however, energy is also needed.

The role of methane in internal carbon cycling in Lake Mendota during summer stratification

The rate of methane formation and evasion was estimated during summer stratification from methane profiles and the rate of methane oxidation measured with 14CH4. Organic carbon sedimentation was measured with sedimentation traps. For the deep‐water sediments of Lake Mendota, about 54% of the organic carbon sedimented returns as methane. Methane oxidation at the thermocline is a much more significant process in Lake Mendota than in Lake 227 during summer stratification, apparently because of the higher eddy diffusivity in Lake Mendota, with concomitant transfer of oxygen and nutrients to the zone where methane oxidation occurs.