Dissolved Organic Carbon Consumption Research Articles

PFOA and PFOS induces mineralization of soil organic carbon by accelerating the consumption of dissolved organic carbon

The wide spread of Perfluoroalkyl substances (e.g., PFOA and PFOS) in soil can alter native soil geochemical properties and microbial communities through various approaches. Nonetheless, it is unclear that how PFOA and PFOS in soil affect the mineralization of soil organic carbon (SOC). A laboratory-scale culture experiment (180 d) was performed to explore the contribution of PFOA and PFOS to SOC dynamics and soil geochemical processes. Results showed that positive priming effect (PE) occurred in the short term (i.e., 30 days after PFOA and PFOS addition) and subsequently shifted to a slightly negative PE (i.e., 90 days) and remained in a significant negative PE thereafter (i.e., > 180 days). The PEs caused by PFOA and PFOS both accelerated the consumption of dissolved organic carbon (DOC), resulting in a significant increase in SOC mineralization (10.45%–127.36%) under the short-term incubation (30 d). As for long-term exposure (> 90 d), the organic carbon mineralization rate was significantly reduced (58.30%–65.24%) due to the excessive DOC consumption in the initial stage. High throughput analysis indicated that both PFOA and PFOS changed soil bacterial and fungal community structures, altered the relevant metabolic pathways and resulted in the enrichment of specific taxa.Graphical

Impact of Water Mass Mixing on the Composition and Mineralization of Dissolved Organic Matter in the Deep Labrador Sea

AbstractThe Labrador Sea is a region of the North Atlantic known for its strong ocean currents and deep water formation, which contribute to the transport and mixing of water masses throughout the region. The absorbing and fluorescing properties of dissolved organic matter (DOM) were assessed to track the water masses and the in situ production in the Labrador Sea (>200 m). No significant differences in DOM composition were found in the mesopelagic waters (200–1,000 m). In the bathypelagic waters (1,000–4,000 m), the estimated dissolved organic carbon (DOC) and dissolved lignin concentrations, as well as humic‐like fluorescence intensities, allowed the discrimination of North East Atlantic Deep Water versus Denmark Strait overflow water. The humic‐like intensities were significantly different between upper Labrador Sea water (uLSW) and deep LSW (dLSW) suggesting their applications as tracers of deep winter mixing. The significant correlations with apparent oxygen utilization support the in situ production of humic‐like fluorescence and the net microbial consumption of DOC and lignin in the dark Labrador Sea. We also demonstrated that microbial activities play a role in the production of humic‐like compounds in the dLSW that experiences deep convection mixing.

Using 224Ra/228Th disequilibrium to quantify carbon transformation and export from intertidal sandy and muddy sediments

The intertidal zone is an important pathway for the transport of dissolved carbon across the land–ocean interface. Quantifying carbon transformation and export from the intertidal zone remains large uncertainties due to the natural heterogeneity and various driving mechanisms. Here, we used the 224Ra/228Th disequilibrium method to quantify porewater exchange and dissolved carbon export from intertidal sandy and muddy sediments, two main intertidal wetland types in the coast. Porewater exchange fluxes from sandy sediments ranged from 1.4 ± 0.1 to 1084 ± 53 L m−2 h−1, which gradually increased in the landward direction. Porewater exchange fluxes from mangrove muddy sediments ranged from 0 to 4.4 ± 0.7 L m−2 h−1. Advective porewater exchange was recognized as the dominant solute transport process in sandy sediments, whereas bioturbation may be the key process in mangrove muddy sediments. Furthermore, marine dissolved organic carbon (DOC) was removed in sandy sediments, indicating that the sandy sediment acted as a sink of DOC, likely via aerobic remineralization. DOC consumption fluxes ranged from 3.7 ± 0.2 to 206 ± 9 mmol C m−2 d−1. The total consumption flux of DOC in the entire sandy beach was one order of magnitude higher than the globally maximum DOC removal flux in the upper ocean. Porewater-derived dissolved inorganic carbon (DIC) export fluxes from sandy sediments ranged from 5.6 ± 0.3 to 883 ± 43 mmol C m−2 d−1. Nearly 50% of DIC export originated from the consumption of DOC. The rate constants for DOC transformation ranged from 0.009 to 0.155 h−1. In comparison, porewater-derived DOC and DIC export fluxes from muddy sediments were 25 ± 4 and 206 ± 30 mmol C m−2 d−1, respectively. Porewater-derived DIC export from sandy sediments was comparable or even higher than those from muddy sediments. These results showed that sandy sediments acted as a fast lane for marine DOC remineralization. Overall, both organic-poor sandy and organic-rich mangrove muddy sediments play important roles in the delivery of dissolved carbon to the ocean.

Effects of different amounts of straw return and nitrogen fertilizer application on soil CO2 emission from maize fields.

Understanding the effects of different amounts of straw returning and nitrogen fertilizer application on soil CO2 emission from maize field can provide theoretical support for carbon sequestration and CO2 emission reduction and the implementation of black soil region conservation plan. Three rates of straw returning were set up in the semi-arid area of northwest Liaoning Province, China, i.e. 3000 (S1), 6000 (S2) and 9000 kg·hm-2(S3, full amount of straw returned to the field); crossed with three nitrogen fertilizer application rates in the sub-region, respectively, i.e. 105 (N1), 210 (N2, conventional nitrogen application rate) and 420 kg N·hm-2(N3). In addition, there was a control treatment (CK) without nitrogen fertilizer and straw returning. Soil samples were collected after 4 years field experiment with maize plantation. The influence of different treatments on maize field soil CO2 emission and the relationship between CO2 emission and soil dissolved organic carbon (DOC) and microbial biomass carbon (MBC) were investigated in an incubation experiment. The results showed that both of straw returning and nitrogen fertilizer application promoted soil CO2 emission in maize field, which were increased significantly with the increases of straw returning amount and nitrogen application amount. Nitrogen fertilizer application was the most important factor promoting soil CO2 emission in maize field. Straw returning combined with nitrogen fertilizer promoted soil CO2 emission by increasing microbial biomass and increasing DOC consumption. MBC and DOC stimulated soil CO2 emission significantly in maize field, and were mainly affected by their contents in the early stage of incubation. From the perspective of ensuring the fertilization of straw return to the field while reducing CO2 emissions, results from our experiment showed that 210 kg N·hm-2 conventional nitrogen application in combination with 6000 kg N·hm-2 straw returning (N2S2) was the most promising mode in the semi-arid area of northwest Liaoning Province.

Partitioning the Export of Distinct Biogenic Carbon Pools in the Northeast Pacific Ocean Using a Biogeochemical Profiling Float

AbstractWe leverage observations from chemical and bio‐optical sensors mounted on a biogeochemical profiling float in the Northeast Pacific Ocean to quantify the cycling and export potential of distinct biogenic carbon pools, including particulate inorganic carbon (PIC), particulate organic carbon (POC), and dissolved organic carbon (DOC). Year‐round observations reveal complex carbon cycle dynamics among these carbon pools. Net DOC production peaked during bloom initiation, about 3 months prior to the summer peak in POC production. We validate the float estimates of DOC cycling with seasonal accumulation and removal rates derived from ship‐board DOC observations over the same period. By combining chemical and bio‐optical tracers of POC cycling, we estimate the instantaneous POC sinking flux (). The cooccurrence of DOC consumption and POC production and sinking during fall and winter resolves the regional conundrum of a persistent particle sinking flux observed by sediment traps during a season that is known to be heterotrophic. PIC production is small, and uncertainties are large. By combining float‐based estimates of instantaneous net primary production (NPP) and , we quantify a real‐time carbon export ratio ([/NPP] × 100%) for the euphotic zone. Elevated export ratios during summer are associated with an increase in the fraction of particles larger than 100 μm in size. Elevated export ratios during winter are associated with the physical redistribution of particles through seasonal deep mixing. Our study demonstrates how the combined use of multiple sensors on biogeochemical profiling floats can provide more nuanced information about upper ocean carbon cycle dynamics.

Differences and mechanism of dynamic changes of Cd activity regulated by polymorphous sulfur in paddy soil

Sulfur (S) can decrease the bioavailability of Cd in paddy soil, and therefore reduce Cd uptake by rice. However, the dynamic influence mechanism of different forms of sulfur on the bioavailability of cadmium in soil has not been systematically studied. In this study, we used Diffusive gradients in thin films (DGT) technology and soil pore water sampling technology to investigate the effects of different types of S application on the bioavailability of Cd. The three forms of S are elemental sulfur (S0), sodium sulfate (SO42−), and mercapto-grafted palygorskite (MP), which have been treated in six ways according to CK, SL, SH, SO42L, SO42H, MP (L and H are the low and high levels of treatments). The results showed that soluble and labile Cd concentration was quickly fixed after flooding but activated after rice transplantation. Both MP and SL treatments increased the content of dissolved organic carbon (DOC) and significantly reduced the accumulation of Cd in roots and grains. The Cd content of roots treated with MP was only 0.50 mg kg−1, which was 77.6% lower than the Cd content in CK (2.22 mg kg−1). S promotes the reduction and dissolution of Fe, and the formation of FeS/FeS2 is also one of the reasons for the low bioavailability of Cd, while the SH treatment has a greater effect on lowering pH, so the effect of fixing Cd is not obvious. In addition, SO42− treatment delays the soil reduction process, so it cannot effectively fix Cd. At the stage of rice maturity, dissolved Cd increased in S treatment, but MP treatment did not have this phenomenon. The possible reason was the consumption of DOC or the oxidation of CdS which caused Cd to enter the soil solution. Taken together, Fe–S play a critical role in controlling the mobilization of Cd in paddy systems.

Effects of temperature and microorganism densities on disinfection by-product formation

This study investigated the role of microorganisms on the correlation between temperature changes and disinfection by-product formation in natural waters. Climate changes have resulted in an increase in the global surface temperature. Studies have revealed that increases in temperature may change the composition of dissolved organic matter (DOM), which may contain major disinfection by-product (DBP) precursors. This change in the DOM composition may affect DBP formation after conventional water treatment processes. Understanding the role of microorganisms in DOM composition as well as DBP formation and speciation is critical for controlling DBP formation. In this study, laboratory stimulatory experiments were conducted on water samples from various sources, at various temperatures, and with various microbial concentrations. The results revealed a decreasing trend of dissolved organic carbon (DOC), trihalomethane formation potential (THMFP), and haloacetic acid formation potential (HAAFP) at high temperature incubations irrespective of microbial concentrates. This result may be attributed to the fact that microorganism activities or concentrations in water increase at higher temperatures, which may result in higher DOC consumption and lower DBP formation. Water samples spiked with bacteria concentrates exhibited higher THMFP or HAAFP reduction than did samples without bacteria concentrates. A higher biomass in water may contribute to a higher consumption of DOC and consequently lower DBP formation potentials, especially at high incubation temperatures.

Utilization of low-molecular-weight organic compounds by the filterable fraction of a lotic microbiome.

ABSTRACTFilterable microorganisms participate in dissolved organic carbon (DOC) cycling in freshwater systems, however their exact functional role remains unknown. We determined the taxonomic identity and community dynamics of prokaryotic microbiomes in the 0.22 µm-filtered fraction and unfiltered freshwater from the Conwy River (North Wales, UK) in microcosms and, using targeted metabolomics and 14C-labelling, examined their role in the utilization of amino acids, organic acids and sugars spiked at environmentally-relevant (nanomolar) concentrations. To identify changes in community structure, we used 16S rRNA amplicon and shotgun sequencing. Unlike the unfiltered water samples where the consumption of DOC was rapid, the filtered fraction showed a 3-day lag phase before the consumption started. Analysis of functional categories of clusters of orthologous groups of proteins (COGs) showed that COGs associated with energy production increased in number in both fractions with substrate addition. The filtered fraction utilized low-molecular-weight (LMW) DOC at much slower rates than the whole community. Addition of nanomolar concentrations of LMW DOC did not measurably influence the composition of the microbial community nor the rate of consumption across all substrate types in either fraction. We conclude that due to their low activity, filterable microorganisms play a minor role in LMW DOC processing within a short residence time of lotic freshwater systems.

Hotspot of Organic Carbon Export Driven by Mesoscale Eddies in the Slope Region of the Northern South China Sea

Mesoscale eddies frequently observed in the northern slope region of the South China Sea significantly modulate the biological and biogeochemical behavior of organic carbon. There have been few comparative studies on biological and biogeochemical processes in a pair of anticyclonic and cyclonic eddies (ACEs and CEs) in continental slope. In our research, an ACE-CE pair was observed in June 2015 on the northern slope of the SCS. The surface dissolved organic carbon (DOC) was approximately 3 μmol L-1 higher in the ACE than that of the CE, and particulate organic carbon (POC) was approximately 3.9 μmol L-1 higher in the CE than in the ACE. Along the transect across the ACE and CE, the concentrations of DOC and fluorescent dissolved organic matter (FDOM) were lower in ACE region than those of the CE region, which were coincident with the downwelling and upwelling in the eddies. In the euphotic layer, the total organic carbon (TOC) stock in the ACE was higher than that of the CE with a lower POC/TOC ratio. There was net consumption of both DOC and POC in the upper 120 m in the ACE; however, net POC production was observed within the CE. The results also indicate that the production of fresh organic carbon was higher in the CE while carbon export was higher in ACE. The vertical export rate of DOC and POC at 120 m in the ACE were approximately 70.2 and 1.69 mmol C m-2 d-1. Summarizing these measurements, the horizontal export of TOC across the slope to the SCS basin transported by dual eddies is estimated more than 22.1 × 109 g C. This estimate implies that mesoscale eddies can contribute significantly to carbon sequestration in the SCS.

Organic carbon in a seepage face of a subterranean estuary: Turnover and microbial interrelations

Subterranean estuaries, the mixing zone between terrestrial groundwater and coastal seawater, are important biogeochemical hotspots. In the present study, organic carbon cycling and related drivers, including the characterization of different organic carbon pools and sediment microbial community, were investigated in a subterranean estuary seepage face. Within the first 20 cm depth seepage face sediments, both production and removal of dissolved organic carbon (DOC) were observed, mainly driven by heterotrophic microbes. From spring to autumn, active DOC production occurred on the seepage face at the 15–20 cm depth, likely via aerobic degradation of sediment organic carbon (SOC) with subsequent release of dissolved fractions into the porewater. During winter, DOC production moved to a shallower depth of the seepage face due to increasing SOC content in the surface layer. DOC production rate depended on heterotrophic microbial biomass (e.g. Proteobacteria) and was enhanced by high microbial activity and porewater advection. DOC removal frequently occurred at the 0–5 cm depth layer except in winter. The seasonal shift in carbon source utilization (SOC to DOC) in this layer likely resulted from the decrease in SOC pool, especially the labile portion of SOC and the increased availability of DOC due to production in the deeper sediment (15–20 cm). Given the similarity in microbial community structure along the sediment profile, this shift suggests SOC as the preferential carbon source for benthic microbes as well as adaptive flexibility in microbial carbon source utilization. DOC removal was also significantly tied to microbial activity and advection rate. Because DOC production rates were higher compared to DOC consumption the seepage face acted as a net source of DOC to the coastal ecosystem.

Quantifying Dissolved Organic Carbon Dynamics Using a Three‐Dimensional Terrestrial Ecosystem Model at High Spatial‐Temporal Resolutions

AbstractArctic terrestrial ecosystems are very sensitive to the global climate change due to the large storage of soil organic carbon and the presence of snow, glacier, and permafrost, which respond directly to near surface air temperature that has warmed in the Arctic by almost twice as much as the global average. These ecosystems play a significant role in affecting regional and global carbon cycling, which have been traditionally quantified using biogeochemical models that have not explicitly considered the loss of carbon due to lateral flow of water from land to aquatic ecosystems. Building upon an extant spatially distributed hydrological model and a process‐based biogeochemical model, we have developed a three‐dimensional terrestrial ecosystem model to elucidate how lateral water flow has impacted the regional dissolved organic carbon (DOC) dynamics in the Tanana Flats Basin in central Alaska. The model explicitly simulates the production, consumption, and transport of DOC. Both in situ observational data and remote sensing‐based products were used to calibrate and validate the model. Our simulations show that (1) plant litter DOC leaching exerts significant controls on soil DOC concentration during precipitation and snowmelt events, (2) lateral transport plays an important role in affecting regional DOC dynamics, and (3) DOC export to the Tanana River is approximately 9.6 × 106 kg C year−1. This study provides a modeling framework to adequately quantify the Arctic land ecosystem carbon budget by considering the lateral transport of carbon affected by permafrost degradation. The quantification of the lateral carbon fluxes will also improve future carbon cycle modeling for Arctic aquatic ecosystems.

Non-conservative patterns of dissolved organic matter degradation when and where lake water mixes

In this study, we experimentally investigated the degradation of dissolved organic matter (DOM) during lateral and vertical mixing of different water masses in a peri-alpine lake. River intrusions and vertical winter turnover in Lake Geneva (Switzerland, France) were simulated through short-term laboratory incubations by mixing riverine and lacustrine waters or lacustrine waters collected at different depths in winter. Dissolved organic carbon (DOC) degradation was monitored by dissolved oxygen (DO) measurements and changes in DOM composition were tracked by fluorescence spectroscopy in pure and mixed treatments during 72 h. Initial DOC content and DOM composition were relatively similar between end-members. The amount of DOC respired at the end of the incubation was similar between treatments, but decay constants in mixed treatments derived from a first-order decay model were significantly higher than expected values calculated based on a simple mass balance model. Fluorescent dissolved organic matter (FDOM) in mixed treatments followed non-conservative patterns that could not be predicted based on observations made in pure treatments. Hence, one protein-like and one microbial-like fluorophore that were consumed in lake waters were continuously produced in mixed treatments although lake waters represented 90% of the mix. No relationships were observed between the rate of DOC consumption and the initial DOM composition, suggesting that other factors such as nutrients and/or interactions in microbial communities were involved. Moreover, no relationships were found between DOC and FDOM patterns during incubations, suggesting that these measurements targeted different facets of microbial metabolism of DOM, respectively microbial respiration (catabolism) and microbial production (anabolism). While additional investigations are required in order to identify the drivers of these changes, this study provides evidence of non-conservative behavior of DOM degradation in mixing zones in lakes.

Dissolved Organic Matter Composition in a Marsh‐Dominated Estuary: Response to Seasonal Forcing and to the Passage of a Hurricane

AbstractDissolved organic matter (DOM) is a large and complex mixture of compounds with source inputs that differ with location, season, and environmental conditions. Here, we investigated drivers of DOM composition changes in a marsh‐dominated estuary off the southeastern United States. Monthly water samples were collected at a riverine and estuarine site from September 2015 to September 2016, and bulk, optical, and molecular analyses were conducted on samples before and after dark incubations. Results showed that river discharge was the primary driver changing the DOM composition at the mouth of the Altamaha River. For discharge higher than ~150 m3/s, dissolved organic carbon (DOC) concentrations and the terrigenous character of the DOM increased approximately linearly with river flow. For low discharge conditions, a clear signature of salt marsh‐derived compounds was observed in the river. At the head of Sapelo Sound, changes in DOM composition were primarily driven by river discharge and possibly by summer algae blooms. Microbial consumption of DOC was larger during periods of high discharge at both sites, potentially due to the higher mobilization and influx of fresh material to the system. The Georgia coast was hit by Hurricane Matthew in October 2016, which resulted in a large input of carbon to the estuary. The DOC concentration was ~2 times higher and DOM composition was more aromatic with a stronger terrigenous signature compared to the seasonal maximum observed earlier in the year during peak river discharge conditions. This suggests that extreme events notably impact DOM quantity and quality in estuarine regions.

Terrestrial dissolved organic carbon consumption by heterotrophic bacterioplankton in the Huanghe River estuary during water and sediment regulation

Nearly 20%–50% of the annual terrestrial dissolved organic carbon (DOC) from the Huanghe (Yellow) River was transported to the estuary during the 5–14 d of water and sediment regulation. The concentration of DOC increased sharply during the period of water and sediment regulation, which may promote the terrestrial DOC consumption by heterotrophic bacterioplankton. Water and sediment regulation provides an ideal condition for the study of terrestrial DOC consumption by heterotrophic bacterioplankton when terrestrial DOC increases sharply in rainy season, which may help to seek the fates of terrestrial DOC in the estuaries and coasts. In this study, the concentration and stable isotope of DOC, the biomass, growth, and respiration of heterotrophic bacterioplankton were determined. By the study, we found both average percent contribution of terrestrial DOC to the DOC pool and Contribution of terrestrial DOC to the carbon composition of heterotrophic bacterioplankton decreased as distance from the river mouth increased offshore, which was deceased from (39.2±4.0)%, (37.5±4.3)% to (30.3±3.9)%, (28.2±3.9)% respectively. 255–184 μg C/(L d) terrestrial DOC was consumed by heterotrophic bacterioplankton. And 29%–45% terrestrial DOC consumed by heterotrophic bacterioplankton releasing as CO2 by respiration. Comparing with tropical estuary, terrestrial DOC consumed by heterotrophic bacterioplankton was lower in temperate estuary (this study). Temperature may limit the consumption of terrestrial DOC by heterotrophic bacterioplankton.

Effect of salinity on the decomposition of soil organic carbon in a tidal wetland

Climate warming and sea level rise have the potential to change the salt level of soil in tidal wetlands. And it is important to clarify the process and the mechanism of decomposition of soil organic carbon in a tidal wetland under varying salinities. The aim of this study was to evaluate the impacts of soil salinity on the decomposition rate of organic carbon (DROC) and dissolved organic carbon (DOC) in a tidal wetland. Two types of soil (surface soil in Suaeda salsa and bare tidal flat) were collected, air-dried, and homogenized. After adding different content of salt (0 g/L, 3 g/L, 6 g/L, 9 g/L, and 12 g/L), the soils were incubated for 28 days at stable room temperature (25 ± 2 °C) and added by deionized water to maintain the stability of soil moisture. The gases (CO2 and CH4) emission rates of each salt treatment were measured during 28-day incubation. DROC was determined by the sum of daily CO2-C emission rates and daily CH4-C emission rates in this study. Salt addition inhibited the process of gas emissions and DROC. Gases emission rates and DROC of two types of soil showed similar temporal trends, with distinctive drop in the beginning of experiment and no significant decrease followed. Significant difference of DOC among salt treatments was found in the bare tidal flat soil. Variations of partial correlation between DROC and soil salinity and DOC showed similar trends (e.g., in days 9–18, the positive effect of DOC on DROC was greatly promoted (R2 = 0.80, p < 0.001), and the negative effect of soil salinity was highly improved (R2 = 0.93, p < 0.001)). Soil properties, in particular DOC, may be primary factors accounting for the discrepancy of gases emission rates and DROC of two types of soil. Increased soil salinity had a negative effect on DROC during 28-day incubation. The impact of soil salinity and DOC on DROC were varied in different phases of laboratory experiment (soil salinity generally had increasingly negative relationship with DROC, but DOC showed most significantly positive relationship in the middle stage of incubation). Both the formation and consumption of DOC may be valuable for more detail research regarding to decomposition of soil organic carbon.

ORCHIDEE-SOM: modeling soil organic carbon (SOC) and dissolved organic carbon (DOC) dynamics along vertical soil profiles in Europe

Abstract. Current land surface models (LSMs) typically represent soils in a very simplistic way, assuming soil organic carbon (SOC) as a bulk, and thus impeding a correct representation of deep soil carbon dynamics. Moreover, LSMs generally neglect the production and export of dissolved organic carbon (DOC) from soils to rivers, leading to overestimations of the potential carbon sequestration on land. This common oversimplified processing of SOC in LSMs is partly responsible for the large uncertainty in the predictions of the soil carbon response to climate change. In this study, we present a new soil carbon module called ORCHIDEE-SOM, embedded within the land surface model ORCHIDEE, which is able to reproduce the DOC and SOC dynamics in a vertically discretized soil to 2 m. The model includes processes of biological production and consumption of SOC and DOC, DOC adsorption on and desorption from soil minerals, diffusion of SOC and DOC, and DOC transport with water through and out of the soils to rivers. We evaluated ORCHIDEE-SOM against observations of DOC concentrations and SOC stocks from four European sites with different vegetation covers: a coniferous forest, a deciduous forest, a grassland, and a cropland. The model was able to reproduce the SOC stocks along their vertical profiles at the four sites and the DOC concentrations within the range of measurements, with the exception of the DOC concentrations in the upper soil horizon at the coniferous forest. However, the model was not able to fully capture the temporal dynamics of DOC concentrations. Further model improvements should focus on a plant- and depth-dependent parameterization of the new input model parameters, such as the turnover times of DOC and the microbial carbon use efficiency. We suggest that this new soil module, when parameterized for global simulations, will improve the representation of the global carbon cycle in LSMs, thus helping to constrain the predictions of the future SOC response to global warming.

Methane- and dissolved organic carbon-fueled microbial loop supports a tropical subterranean estuary ecosystem

Subterranean estuaries extend inland into density-stratified coastal carbonate aquifers containing a surprising diversity of endemic animals (mostly crustaceans) within a highly oligotrophic habitat. How complex ecosystems (termed anchialine) thrive in this globally distributed, cryptic environment is poorly understood. Here, we demonstrate that a microbial loop shuttles methane and dissolved organic carbon (DOC) to higher trophic levels of the anchialine food web in the Yucatan Peninsula (Mexico). Methane and DOC production and consumption within the coastal groundwater correspond with a microbial community capable of methanotrophy, heterotrophy, and chemoautotrophy, based on characterization by 16S rRNA gene amplicon sequencing and respiratory quinone composition. Fatty acid and bulk stable carbon isotope values of cave-adapted shrimp suggest that carbon from methanotrophic bacteria comprises 21% of their diet, on average. These findings reveal a heretofore unrecognized subterranean methane sink and contribute to our understanding of the carbon cycle and ecosystem function of karst subterranean estuaries.

Consumption of dissolved organic carbon by Caribbean reef sponges

AbstractSponges are conspicuous and abundant within the benthic fauna on Caribbean reefs. The ability of these organisms to efficiently capture carbon from particulate sources is well known and the importance of dissolved organic carbon (DOC) uptake has been recognized for several species. We surveyed DOC ingestion by seven sponge species common to Florida Keys reefs using nondisruptive sampling methods on undisturbed individuals. Three of the seven species exhibited significant DOC removal ranging from 13% to 24% of ambient concentrations. The tested species that removed DOC host large microbial consortia within their tissues, while the converse was observed for those that did not. This divergent behavior may suggest an important role for sponge associated microbes in the utilization of DOC by these species. The feeding behaviors of individuals of Xestospongia muta were then monitored over time to investigate its respiratory consumption of particulate and DOC. The uptake rates of dissolved oxygen (DO) and organic carbon by two undisturbed individuals revealed that DOC represented 96% of removed C, and that the tested individuals removed approximately equal quantities of C and DO. This demonstrates that X. muta largely satisfies its respiration demands through DOC consumption, and that DOC likely represents the dominant C source for biomass production and cell overturn in this species. These results further illustrate the metabolic importance of DOC to sponges, and suggest that these organisms are an important pathway for remineralizing organic matter on Caribbean reefs.

Elevated pCO2 enhances bacterioplankton removal of organic carbon.

Factors that affect the removal of organic carbon by heterotrophic bacterioplankton can impact the rate and magnitude of organic carbon loss in the ocean through the conversion of a portion of consumed organic carbon to CO2. Through enhanced rates of consumption, surface bacterioplankton communities can also reduce the amount of dissolved organic carbon (DOC) available for export from the surface ocean. The present study investigated the direct effects of elevated pCO2 on bacterioplankton removal of several forms of DOC ranging from glucose to complex phytoplankton exudate and lysate, and naturally occurring DOC. Elevated pCO2 (1000–1500 ppm) enhanced both the rate and magnitude of organic carbon removal by bacterioplankton communities compared to low (pre-industrial and ambient) pCO2 (250 –~400 ppm). The increased removal was largely due to enhanced respiration, rather than enhanced production of bacterioplankton biomass. The results suggest that elevated pCO2 can increase DOC consumption and decrease bacterioplankton growth efficiency, ultimately decreasing the amount of DOC available for vertical export and increasing the production of CO2 in the surface ocean.

Influence of hydrological pathways on dissolved organic carbon fluxes in tropical streams.

Water flow pathways and water balance are fundamental components for understanding the dynamics of C in the soil/water interface of small basins. The objective of this study was to describe the seasonal variations and estimate the annual balance of dissolved organic carbon (DOC) by comparing two tropical microbasins (preserved forest—PF and cacao plantation—CP). Twenty‐one weekly collections were conducted from September to December 2012 and from April to June 2013. The calculation of the partial balance considered precipitation (P) as inflow and the stream as outflow. The samples were filtered and analyzed using a TOC analyzer. Overall, the DOC was higher CP compared with FP. The behavior of both venues showed that rainy season caused an increase in concentrations in the overland flow (OF) and in the stream, and a decrease in the precipitation (P) and in the throughfall (T). In the CP, the outflow and the soil were chiefly responsible for the high DOC concentrations in the stream, when compared to the PF, which is the result of constant OM decomposition. Soil composition contributes to the control of DOC consumption in each type of soil. The balances were negative in both microbasins, although losses were higher in the AFS (agroforestry systems) when compared to the PF, especially during rainy seasons (−8.98 and −3.05 kg ha−1 year−1, CP and FP, respectively). Thus, the high annual loss of DOC in the CP of the microbasins during the rainy season indicates changes in ecosystem metabolism due to the vegetation cover and to the interactions with the soil.