Biogeochemical Cycles Of Carbon Research Articles

ПРОБЛЕМЫ МОДЕЛИРОВАНИЯ БИОГЕОХИМИЧЕСКОГО ЦИКЛА УГЛЕРОДА В АГРОЛАНДШАФТАХ

ПРОБЛЕМЫ МОДЕЛИРОВАНИЯ БИОГЕОХИМИЧЕСКОГО ЦИКЛА УГЛЕРОДА В АГРОЛАНДШАФТАХ

Molecular responses to freshwater limitation in the mangrove tree Avicennia germinans (Acanthaceae).

Environmental variation along the geographical space can shape populations by natural selection. In the context of global warming and changing precipitation regimes, it is crucial to understand the role of environmental heterogeneity in tropical trees adaptation, given their disproportional contribution to water and carbon biogeochemical cycles. Here, we investigated how heterogeneity in freshwater availability along tropical wetlands has influenced molecular variations of the black mangrove (Avicennia germinans). A total of 57 trees were sampled at seven sites differing markedly in precipitation regime and riverine freshwater inputs. Using 2,297 genome-wide single nucleotide polymorphic markers, we found signatures of natural selection by the association between variations in allele frequencies and environmental variables, including the precipitation of the warmest quarter and the annual precipitation. Additionally, we found candidate loci for selection based on statistical deviations from neutral expectations of interpopulation differentiation. Most candidate loci within transcribed sequences were functionally associated with central aspects of drought tolerance or plant response to drought. Moreover, our results suggest the occurrence of the rapid evolution of a population, probably in response to sudden and persistent limitations in plant access to soil water, following a road construction in 1974. Observations supporting rapid evolution included the reduction in tree size and changes in allele frequencies and in transcript expression associated with increased drought tolerance through the accumulation of osmoprotectants and antioxidants, biosynthesis of cuticles, protection against protein degradation, stomatal closure, photorespiration and photosynthesis. We describe a major role of spatial heterogeneity in freshwater availability in the specialization of this typically tropical tree.

Linking stream microbial community functional genes to dissolved organic matter and inorganic nutrients

AbstractThere is now increasing evidence for the importance of microbial regulation of biogeochemical cycling in streams. Resource availability shapes microbial community structure, but less is known about how landscape‐mediated availability of nutrients and carbon can control microbial functions in streams. Using comparative metagenomics, we examined the relationship between microbial functional genes and composition of dissolved organic matter (DOM), nutrients, and suspended microbial communities in 11 streams, divided into three groups based on the predominant land cover category (agriculture, forested, or wetland). Using weighted gene co‐occurrence network analysis, we identified clusters of functions related to DOM composition, agricultural land use, and/or wetland and forest land cover. Wetland‐dominated streams were characterized by functions related to nitrogen metabolism and processing of aromatic carbon compounds, with strong positive correlations with dissolved organic carbon concentration and DOM aromaticity. Forested streams were characterized by metabolic functions related to monomer uptake and carbohydrates, such as mannose and fructose metabolism. In agricultural streams, microbial functions were correlated with more labile, protein‐like DOM, PO4, and NO3, likely reflecting functional adaptation to labile DOM and higher nutrient concentrations. Distinct changes in the functional composition and loss of functional diversity of microorganisms became evident when comparing natural to agricultural catchments. Although all streams showed signs of functional redundancy, loss of species richness per function in agricultural catchments suggests that microbial functions in natural catchments may be more resilient to disturbance. Our results provide new insight into microbial community functions involved in nutrient and carbon biogeochemical cycles and their dependence on specific environmental settings.

Absorption and Fluorescence Spectra of Dissolved Organic Matter in Rivers of the Three Gorges Reservoir Area Under the Background of Urbanization

Studying the influences of urbanization on the composition, chemical structure, source and migration of dissolved organic matter (DOM) in rivers is of great significance for aquatic ecosystem protection and biogeochemical carbon (C) cycling. The DOM spectroscopic characteristics of the three typical rivers (Taohuaxi, Nanhe, and Puli) with different urbanization levels in the Three Gorges Reservoir area were investigated using the ultraviolet-visible (UV-Vis) absorption spectrum and three-dimensional fluorescence spectroscopy. The results demonstrated that the higher the degree of urbanization in rivers, the greater the concentrations of DOM and chromophoric dissolved organic matter (CDOM), the lower the humification degree and aromaticity of DOM, the smaller the proportion of hydrophobic components, the higher the relative level of protein-like substance concentration in DOM, and the greater the contribution of DOM from freshly autochthonous origins. The DOM of the three rivers was dominated by fulvic acids (E3/E4>3.5), and CDOM concentration was positively correlated with the concentration of DOM (P<0.01). There were significant positive correlations between the humification degree (SUVA254), aromaticity (SUVA280), and hydrophobic component (SUVA260) (P<0.01). The sources of humic substances in the DOM were mixed by terrestrial input and microbial or algae production (reflected by the fluorescence index averages of 1.7157-1.7571), and mainly from microbial sources.

Coordinated\xa0transformation of the gut microbiome and lipidome of bowhead whales provides novel insights into digestion

Whale digestion plays an integral role in many ocean ecosystems. By digesting enormous quantities of lipid-rich prey, whales support their energy intensive lifestyle, but also excrete nutrients important to ocean biogeochemical cycles. Nevertheless, whale digestion is poorly understood. Gastrointestinal microorganisms play a significant role in vertebrate digestion, but few studies have examined them in whales. To investigate digestion of lipids, and the potential contribution of microbes to lipid digestion in whales, we characterized lipid composition (lipidomes) and bacterial communities (microbiotas) in 126 digesta samples collected throughout the gastrointestinal tracts of 38 bowhead whales (Balaena mysticetus) harvested by Alaskan Eskimos. Lipidomes and microbiotas were strongly correlated throughout the gastrointestinal tract. Lipidomes and microbiotas were most variable in the small intestine and most similar in the large intestine, where microbiota richness was greatest. Our results suggest digestion of wax esters, the primary lipids in B. mysticetus prey representing more than 80% of total dietary lipids, occurred in the mid- to distal small intestine and was correlated with specific microorganisms. Because wax esters are difficult to digest by other marine vertebrates and constitute a large reservoir of carbon in the ocean, our results further elucidate the essential roles that whales and their gastrointestinal microbiotas play in the biogeochemical cycling of carbon and nutrients in high-latitude seas.

Pyrenoidal sequestration of cadmium impairs carbon dioxide fixation in a microalga.

Mixotrophic microorganisms are able to use organic carbon as well as inorganic carbon sources and thus, play an essential role in the biogeochemical carbon cycle. In aquatic ecosystems, the alteration of carbon dioxide (CO2 ) fixation by toxic metals such as cadmium - classified as a priority pollutant - could contribute to the unbalance of the carbon cycle. In consequence, the investigation of cadmium impact on carbon assimilation in mixotrophic microorganisms is of high interest. We exposed the mixotrophic microalga Chlamydomonas reinhardtii to cadmium in a growth medium containing both CO2 and labelled 13 C-[1,2] acetate as carbon sources. We showed that the accumulation of cadmium in the pyrenoid, where it was predominantly bound to sulphur ligands, impaired CO2 fixation to the benefit of acetate assimilation. Transmission electron microscopy (TEM)/X-ray energy dispersive spectroscopy (X-EDS) and micro X-ray fluorescence (μXRF)/micro X-ray absorption near-edge structure (μXANES) at Cd LIII- edge indicated the localization and the speciation of cadmium in the cellular structure. In addition, nanoscale secondary ion mass spectrometry (NanoSIMS) analysis of the 13 C/12 C ratio in pyrenoid and starch granules revealed the origin of carbon sources. The fraction of carbon in starch originating from CO2 decreased from 73 to 39% during cadmium stress. For the first time, the complementary use of high-resolution elemental and isotopic imaging techniques allowed relating the impact of cadmium at the subcellular level with carbon assimilation in a mixotrophic microalga.

Dynamics of atmospheric carbon dioxide over different land cover types in India.

This study presents an analysis of high-resolution space borne retrievals of the column-averaged dry-air mole fraction of carbon dioxide [Formula: see text] and the role of vegetation in controlling atmospheric CO2 dynamics over the Indian region. Nadir and glint mode [Formula: see text] retrievals from the Orbiting Carbon Observatory-2 (OCO-2) spectrometer for the period September 2014-July 2017 are studied with satellite-derived normalized difference vegetation index (NDVI) and rainfall over different land cover types. The atmospheric [Formula: see text] variability shows a strong negative correlation with satellite-derived NDVI. Higher rainfall favours the vegetative growth and photosynthetic activity, thus lowers atmospheric [Formula: see text] concentration. The mean monthly [Formula: see text] over terrestrial region of India is observed as 400.18 ± 3.85 ppm with seasonal variations over different land cover types. The correlation of [Formula: see text] with NDVI over mixed forest and deciduous broadleaf forest types was estimated as - 0.86, - 0.76 (p < 0.01), respectively. The study could also highlight strong seasonal [Formula: see text] variability at higher latitudinal zones of India. It is significant to note that space-based observations indicate that atmospheric carbon dioxide levels have surpassed a significant milestone of 400 ppm in recent times. The study provides significant inputs towards improving our understanding of terrestrial biogeochemical carbon cycle over India.

Afforestation driving long-term surface water browning.

Increase in surface water color (browning), caused by rising dissolved organic carbon (DOC) and iron concentrations, has been widely reported and studied in the last couple of decades. This phenomenon has implications to aquatic ecosystem function and biogeochemical carbon cycling. While recovery from acidification and changes in climate‐related variables, such as precipitation and length of growing season, are recognized as drivers behind browning, land‐use change has received less attention. In this study, we include all of the above factors and aim to discern their individual and combined contribution to water color variation in an unprecedentedly long (1940–2016) and highly resolved dataset (~20 times per month), from a river in southern Sweden. Water color showed high seasonal variability and a marked long‐term increase, particularly in the latter half of the dataset (~1980). Short‐term and seasonal variations were best explained by precipitation, with temperature playing a secondary role. All explanatory variables (precipitation, temperature, S deposition, and land‐use change) contributed significantly and together predicted 75% of the long‐term variation in water color. Long‐term change was best explained by a pronounced increase in Norway spruce (Picea abies Karst) volume—a measure of land‐use change and a proxy for buildup of organic soil layers—and by change in atmospheric S deposition. When modeling water color with a combination of explanatory variables, Norway spruce showed the highest contribution to explaining long‐term variability. This study highlights the importance of considering land‐use change as a factor behind browning and combining multiple factors when making predictions in water color and DOC.

Aggregation-dependent electron transfer via redox-active biochar particles stimulate microbial ferrihydrite reduction

Microbial Fe(III) reduction plays an important role for biogeochemical carbon and iron cycling in sediments and soils. Biochar is used as a soil amendment to increase fertility and lower N2O/CO2 emissions. It is redox-active and can stimulate microbial Fe(III) mineral reduction. It is currently unknown, however, how the aggregation of cells and Fe(III) minerals with biochar particles influence microbial Fe(III) reduction. Therefore, we determined rates and extent of ferrihydrite (Fh) reduction in S. oneidensis MR-1 cell suspensions with different particles sizes of wood-derived Swiss biochar and KonTiki biochar at different biochar/Fh ratios. We found that at small biochar particle size and high biochar/Fh ratios, the biochar, MR-1 cells and Fh closely aggregated, therefore addition of biochar stimulated electron transfer and microbial Fh reduction. In contrast, large biochar particles and low biochar/Fh ratios inhibited the electron transfer and Fe(III) reduction due to the lack of effective aggregation. These results suggest that for stimulating Fh reduction, a certain biochar particle size and biochar/Fh ratio is necessary leading to a close aggregation of all phases. This aggregation favors electron transfer from cells to Fh via redox cycling of the electron donating and accepting functional groups of biochar and via direct electron transfer through conductive biochar carbon matrices. These findings improve our understanding of electron transfer between microorganisms and Fe(III) minerals via redox-active biochar and help to evaluate the impact of biochar on electron transfer processes in the environment.

Inter‐annual simulation of global carbon cycle variations in a terrestrial–aquatic continuum

AbstractThe advanced process‐based model, National Integrated Catchment‐based Eco‐hydrology (NICE)‐BGC, which incorporates the whole process of carbon cycling in land, was modified to include the feedback between soil organic content and overland carbon fluxes. It is a crucial and difficult task to evaluate the balance of the terrestrial carbon budget including the effect of inland water robustly. To accomplish this purpose, NICE‐BGC was applied to quantify the global biogeochemical carbon cycle closely associated with the complex hydrological cycle during the 36 years between 1980 and 2015. The model demonstrated that the inter‐annual variations of carbon cycle have been greatly affected by the extreme weather patterns. In particular, spatial distribution of temporal trends in riverine carbon fluxes and their relation to soil organic carbon (SOC) were analysed between different biomes and major river basins. Although there was a positive relationship between SOC and riverine flux of dissolved organic carbon and particulate organic carbon in the northern boreal region, it is difficult to see this relation in other regions. Further, the evaluation of potential controlling factors of temporal trends in SOC and fluvial carbon exports was also helpful to quantify the inter‐annual variation or temporal trend caused by the various effects. SOC was more influenced by temperature variations, whereas riverine carbon exports were mainly determined by precipitation variations. Finally, net land flux including inland water (−1.49 ± 0.50 PgC/year) showed a slight decrease in the carbon sink in comparison with previous values (−2.33 ± 0.50 PgC/year). These results help to distinguish the carbon cycle in different river basins and to re‐evaluate carbon cycle change explicitly including the effect of inland water because this effect has been so far implicitly included within the range of uncertainty in the Earth's global carbon cycle comprising land, oceans, and atmosphere.

Data‐Driven Modeling of the Distribution of Diazotrophs in the Global Ocean

AbstractDiazotrophs play a critical role in the biogeochemical cycling of nitrogen, carbon, and other elements in the global ocean. Despite their well‐recognized role, the diversity, abundance, and distribution of diazotrophs in the world's ocean remain poorly characterized largely due to limited observations. Here we update the database of diazotroph nifH gene abundances and assess how environmental factors may regulate diazotrophs at the global scale. Our meta‐analysis more than doubles the number of observations in the previous database. Using linear and nonlinear regressions, we find that the abundances of Trichodesmium, UCYN‐A, UCYN‐B, and Richelia relate differently to temperature, light, and nutrients. We further apply a random forest algorithm to estimate the global distributions of these diazotrophic groups, identifying undersampled potential hot spots of diazotrophy in the South Atlantic and southern Indian Ocean, and in coastal waters. The distinct ecophysiologies of diazotrophs highlighted here argue for separate parameterizations of different diazotrophs in model simulations.

Trophic Status Is Associated With Community Structure and Metabolic Potential of Planktonic Microbiota in Plateau Lakes.

Microbes in various aquatic ecosystems play a key role in global energy fluxes and biogeochemical processes. However, the detailed patterns on the functional structure and the metabolic potential of microbial communities in freshwater lakes with different trophic status remain to be understood. We employed a metagenomics workflow to analyze the correlations between trophic status and planktonic microbiota in freshwater lakes on Yun-Gui Plateau, China. Our results revealed that microbial communities in the eutrophic and mesotrophic-oligotrophic lake ecosystems harbor distinct community structure and metabolic potential. Cyanobacteria were dominant in the eutrophic ecosystems, mainly driving the processes of aerobic respiration, fermentation, nitrogen assimilation, nitrogen mineralization, assimilatory sulfate reduction and sulfur mineralization in this ecosystem group. Actinobacteria, Proteobacteria (Alpha-, Beta-, and Gammaproteobacteria), Verrucomicrobia and Planctomycetes, occurred more often in the mesotrophic-oligotrophic ecosystems than those in the eutrophic ecosystems, and these taxa potentially mediate the above metabolic processes. In these two groups of ecosystems, a difference in the abundance of functional genes involved in carbohydrate metabolism, energy metabolism, glycan biosynthesis and metabolism, and metabolism of cofactors and vitamins significantly contribute to the distinct functional structure of microbiota from surface water. Furthermore, the microbe-mediated metabolic potentials for carbon, nitrogen and sulfur transformation showed differences in the two ecosystem groups. Compared with the mesotrophic-oligotrophic ecosystems, planktonic microbial communities in the eutrophic ecosystems showed higher potential for aerobic carbon fixation, fermentation, methanogenesis, anammox, denitrification, and sulfur mineralization, but they showed lower potential for aerobic respiration, CO oxidation, nitrogen fixation, and assimilatory sulfate reduction. This study offers insights into the relationships of trophic status to planktonic microbial community structure and its metabolic potential, and identifies the main taxa responsible for the biogeochemical cycles of carbon, nitrogen and sulfur in freshwater lake environments.

A review of the impact of hydropower reservoirs on global climate change

Three types of greenhouse gases (GHGs), namely carbon dioxide, methane and nitrous oxide emitted from reservoirs have an important but often ignored impact on the climate change. Based on the literature, this paper presents a comprehensive assessment of the long-term impact of GHGs generated by hydropower reservoirs, the systems for monitoring and assessing reservoir GHGs, the mechanism for their generation and key factors involved. Using limnology theory as a basis, the biogeochemical cycle of carbon in the environment is understood to be the main mechanism for generating reservoir based GHG emissions. A summary of relevant research in South American tropical rain forests and the Canada’s temperate zone, also shows the extent of current systematic analysis of GHG emission research. Proposals are made for more comprehensive data collection from a wider variety of sources and the key problems identified that need to be solved to provide a theoretical basis for future development policies and practice.

15N Natural Abundance, Nitrogen and Carbon Pools in Soil-Sorghum System Amended with Natural and NH4+-Enriched Zeolitites

The use of rocks containing high amounts of natural zeolites (zeolitites) as soil amendment has been found as a valuable method for increasing agriculture sustainability. However, the potentialities and the effects of zeolitites on the biogeochemical cycles of nitrogen (N) and carbon (C) have still not been clearly addressed in the literature. The objective of this study was therefore to investigate the N and C pools and 15N distribution in an agricultural soil amended with both natural and NH4+-enriched zeolitites with the aim of understanding their effects on the soil-plant system, during sorghum cultivation, under fertilization reductions. Zeolitites were applied to an agricultural soil both at natural state (5 and 15 kg m−2) and in an enriched state with NH4+ ions from pig slurry (7 kg m−2). Both zeolitites at natural and enriched state increased soil cation exchange capacity and affected microbial biomass, causing an initial decrease of microbial C and N and then a possible increase of fungal population. N-NO3− content was lower in natural zeolitite treatments, that lead to a lower NO3− availability for denitrifying bacteria. Zeolitites slightly affected the fixed N-NH4+ pool. δ15N turnover indicated that N from NH4+-enriched zeolitites remained in the soil until the growing season and that fertilizers partially substituted the fixed pool. Leaf δ15N content indicated that plants assimilated N from NH4+-enriched zeolitites and evidenced a higher fertilization recovery in natural zeolitite treatments. Organic C tended to be higher in all zeolitite treatment rhizospheres. In soils amended with zeolitites at natural state (at both application rates) sorghum yield was similar (+3.7%) to that obtained in the control while it was higher (+13.9%) in the plot amended with NH4+-enriched zeolitites.

The Influence of Large Bolide Impacts on Earth's Carbon Cycle

Human society's rapid release of vast quantities of CO2 into the atmosphere is a significant planetary experiment. An obvious natural process capable of similar emissions over geologically short time spans are very large bolide impacts. When striking a carbon-rich target, bolides significantly, and potentially catastrophically, disrupt the global biogeochemical carbon cycle. Independent factors, such as sulfur-rich targets, redox state of the oceans or encountering ecosystems already close to a tipping point, dictated the magnitude of further consequences and determined which large bolide strikes shaped Earth's evolution. On the early Earth, where carbon-rich sedimentary targets were rare, impacts may not have been purely destructive. Instead, enclosed subaqueous impact structures may have contributed to initiating Earth's unique carbon cycle.

A comparative study of total organic carbon-δ13C signatures in the Triassic–Jurassic transitional beds of the Central European Basin and western Tethys shelf seas

Stratigraphic studies are an integral component in understanding the chronology of events that led to the end-Triassic mass extinction, by resolving causal relationships between environmental upheavals and biotic response. Successful correlation of Triassic–Jurassic (Tr–J) successions is complicated by the disappearance of macro-fossils that are otherwise central components in stratigraphic studies. This problem is exacerbated in multiple Tr–J sections situated in Europe, where the so-called “Event Beds” – assumed to demarcate the extinction interval – are virtually devoid of fossils. An alternative stratigraphic approach entails the reconstruction of carbon isotope records, where stratigraphic fluctuations in carbon isotope composition are considered to track changes in the global biogeochemical carbon cycle. The predominance of carbonate-lean sections has prompted the reconstruction of total organic carbon (TOC)-based carbon isotope records. However, bulk rock derived TOC is the diagenetically stabilized remnant of organic components that accumulated on the sea floor, and which can originate from multiple sources. In this study, we assess long-term TOC-based carbon isotope trends at two sites: Bonenburg (Central European Basin) and Kuhjoch (the Tr–J Global Stratotype Section and Point; western Tethys shelf seas). We focus on the TOC 13C-enrichment of the Event Beds with the aim of deciphering stratigraphic fluctuations in relation to their main driver (the exogenic carbon pool versus organic matter source changes). By studying the systematic co-variance of several sedimentary parameters (TOC, total nitrogen [TN], and the palynomorph composition), we infer that the TOC composition is possibly characterized by insignificant organic matter source changes in terms of the marine and terrestrial organic carbon contributions. By contrast, a clay mineralogical shift to more K-depleted minerals as well as the elevated occurrence of wood fragments in the Event Beds suggest a terrestrial organic matter source shift from immature substrates to substrates predominated by “pre-aged” or “fossil organic matter” under a changing continental weathering regime. This outcome urges for reservations when interpreting TOC-based carbon isotope records in terms of global C-cycle perturbations, especially when coinciding with lithological and mineralogical changes. On a more positive note, the shift towards positive carbon isotope values appears to be a recurring feature, possibly testifying to a globally significant climate-controlled weathering regime shift.

Sedimentary Organic Carbon Budget Across the Slope to the Basin in the Southwestern Ulleung (Tsushima) Basin of the East (Japan) Sea

AbstractWith the total sediment oxygen uptake rates measured using an in situ benthic chamber, vertical distributions of organic carbon, and sedimentation rates estimated by excess 210Pb across the slope to the basin sediment of the southwestern region of the Ulleung (Tsushima) Basin (UB), the partitioning of organic carbon fluxes in the sediment was estimated to understand the biogeochemical cycles of organic carbon in the high productivity marginal sea. The results of depth attenuation of total oxygen uptake (TOU) demonstrate that the organic carbon oxidation of the UB sediment was 2.5 times higher than that obtained from the empirical relationship of the global's depth attenuation of TOU. Similar to TOU, the high mass accumulation rates observed in the slope region were 9.5 times higher than the rate in the basin, indicating that the slope may act as the depocenter of organic carbon. The organic carbon budget with water depth gradient implies that a significant fraction of the organic carbon deposited into sediment is supplied by lateral transport down the slope. Definite increasing C/N ratio with water depth indicates that the refractory organic carbon seems to be successively transported later from shelf to slope. The total burial flux in the sediment of southwestern UB was estimated to be 0.46±0.04 Tg C/year, which is similar to the megadepocenter of the Congo River fan. Our results imply that the UB sediment may be an important biogeochemical reaction place not only for organic carbon but also materials linked to primary production.

Benthic-pelagic coupling of carbon and nitrogen along a tropical estuarine gradient (Gulf of Nicoya, Costa Rica)

Tropical estuaries are very little studied compared to temperate ones, despite their generally high primary production rate and hence potential contribution to global carbon and nitrogen budgets. The Gulf of Nicoya (northwest Costa Rica Pacific coast) is one of the most productive estuaries worldwide. However, how changes in benthopelagic coupling affect biogeochemical cycling of carbon and nitrogen over seasons and along the estuarine gradient is not known for this and, in general, other tropical estuaries. For this purpose, we performed two sampling campaigns during the rainy and dry seasons to investigate the contribution of the pelagic and benthic compartments to the carbon and nutrient pools and dynamics of organic matter remineralization in the estuary. Areal concentrations of chlorophyll a (Chla), organic carbon (C) and total nitrogen (N) integrated along the water column photic layer and in the surface sediment generally reached maximum concentrations in the middle of the estuary. Although, integrated Chla in the water column and sediment were in the same range, integrated C and N in the surface sediment (5 cm) were 2–10 times higher than in the water column. The C:Chla and N:Chla ratios in the suspended particulate matter were much higher than those typical of phytoplankton indicating a strong contribution of allochthonous organic matter. However, the C:N ratio in the water column was frequently lower than the Redfield ratio suggesting an important contribution of particulate inorganic N, particularly in the rainy season. Diffusive O2 sediment uptake rate and the O2 penetration depth within the sediment correlated with the organic carbon content in the sediment. In contrast, total oxygen utilization and total CO2 fluxes showed higher variability along the estuary but no significant statistical correlations with any other sediment biogeochemical variables. Organic carbon flux (JC) to the sediment was highly variable along the estuary and between seasons. Despite differences in absolute terms, the fraction of JC accumulated within the sediment for the whole inner part of the Gulf of Nicoya was about 60% in both seasons. Total C burial in the inner part of the gulf was 390 and 813 Tn C d−1 in the rainy and dry seasons, respectively, representing 1.8 and 3.8% of the daily Costa Rica CO2 emissions.

Interactions between ultraviolet radiation exposure and phosphorus limitation in the marine nitrogen‐fixing cyanobacteria Trichodesmium and Crocosphaera

AbstractIncreased stratification and mixed layer shoaling of the surface ocean resulting from warming can lead to exposure of marine dinitrogen (N2)‐fixing cyanobacteria to higher levels of inhibitory ultraviolet (UV) radiation. These same processes also reduce vertically advected supplies of the potentially limiting nutrient phosphorus (P) to N2 fixers. It is currently unknown how UV inhibition and P limitation interact to affect the biogeochemical cycles of nitrogen and carbon in these biogeochemically critical microbes. We investigated the responses of the important and widespread marine N2‐fixing cyanobacteria Crocosphaera (strain WH0005) and Trichodesmium (strains IMS 101 and GBR) to UV‐A and UV‐B under P‐replete and P‐limited conditions. Growth, N2 fixation, and carbon dioxide (CO2) fixation rates of Trichodesmium IMS 101 and Crocosphaera were negatively affected by UV exposure. This inhibition was greater for Trichodesmium IMS 101 than for Crocosphaera, which fixes N2 only during the night and so avoids direct UV damage. Negative effects of UV on both IMS 101 and Crocosphaera were less in P‐limited cultures than in P‐replete cultures. In contrast, no UV inhibition was observed in GBR, regardless of P availability. UV inhibition was related to different amounts of UV‐absorbing compounds produced by these isolates. Responses to UV radiation and P availability interactions were taxon‐specific, but our results indicated that in general, UV radiation effects on Trichodesmium and Crocosphaera range from negative to neutral. UV inhibition and its interactions with P limitation may thus have a substantial influence on the present day and future nitrogen and carbon cycles of the ocean.

Non‐cyanobacterial diazotrophs dominate nitrogen‐fixing communities in permafrost thaw ponds

AbstractThere is a growing concern about the implications of accelerated thawing of permafrost for regional biogeochemical cycling of carbon and other bioreactive elements. One such element of concern is nitrogen, and in this study, we investigated the diversity and biogeography of potential diazotrophs within a series of thaw ponds representing different ontogenetic stages in pond development. Using metagenomic sequence data from subarctic thaw ponds, we identified an array of nitrogenase genes across the ponds. The iron‐only nitrogenase gene (anfH) was positively correlated with sulfate, while there was no correlation with methane despite previous findings that organisms carrying anfH can simultaneously participate in nitrogen fixation and methanogenesis. Sulfate is known to inhibit microbial uptake of molybdate, an element essential for the activity of the nifH (molybdenum‐iron) nitrogenase and this may explain the high potential for nitrogen fixation utilizing anfH in sulfate‐rich ponds. NifH was particularly abundant in the hypolimnion of the deeper and older ponds, with Deltaproteobacteria and Chlorobi as the putative dominant diazotrophs. In the epilimnetic waters, nifH composition was more variable, with various Gammaproteobacteria as abundant representatives, while cyanobacterial diazotrophs were scarce. Interestingly, nifH gene abundance was significantly positively correlated with in situ methane concentration. Based on genome‐resolved metagenomics, we hypothesize that diazotrophs and methanogens engage in syntrophic interactions in anoxic waters, possibly via propionate oxidation or (in Geobacter) by interspecies electron transfer. Our results also suggest that nitrogen fixers may supply bioreactive nitrogen compounds to the thaw pond communities, thereby enhancing growth and activity of methanogens.