Low Molecular Weight Organic Carbon Research Articles

Estimating the global root exudate carbon flux

Root exudation, the export of low-molecular weight organic carbon (C) from living plant roots to soil, influences microbial activity, nutrient availability, and ecosystem feedbacks to climate change, but the magnitude of this C flux at ecosystem and global scales is largely unknown. Here, we synthesize in situ measurements of root exudation rates and couple those to estimates of fine root biomass to estimate global and biome-level root exudate C fluxes. We estimate a global root exudate flux of 13.4 (10.1–20.2) Pg C y−1, or about 9% (7–14%) of global annual gross primary productivity. We did not find differences in root mass-specific exudation rates among biomes, though total exudate fluxes are estimated to be greatest in grasslands owing to their high density of absorptive root biomass. Our synthesis highlights the global importance of root exudates in the terrestrial C cycle and identifies regions where more in situ measurements are needed to improve future estimates of root exudate C fluxes.

Fate of low molecular weight organics in paddy vs. upland soil: A microbial biomarker approach

Low-molecular-weight organic carbon (LMWOC) from root exudate influences soil organic carbon cycling via priming of microbial activity. However, the mechanisms underlying the uptake and utilization of specific exudates by microorganisms in soils remain unclear. To address this gap in knowledge, a one-month 13C (0.1 mg C﹒g soil) tracer incubation study was conducted to investigate the fate of the most abundant root exudate groups (using 13C-labeled glucose, acetic acid, and oxalic acid) in paddy vs. upland soil. After 2 days of incubation, the microbial substrate use efficiency (SUE) was >80% in paddy soil, which was approximately 1.9, 2.9, and 1.3 times that in uplands with glucose, acetic acid, and oxalic acid addition, respectively. The SUE in paddy soil with glucose or acetic acid addition was always higher than that in uplands over time (P < 0.05). In both soils, the SUE of glucose was 1–4 times that of carboxylic acids (P < 0.05). The recovery of 13C-labeled total phospholipid fatty acids (PLFAs) in paddy soils was 1.5–2 times that in uplands (P < 0.05). In both soils, bacteria preferred to utilize glucose and acetic acid to synthesize cellular components. Throughout the incubation period, bacteria dominated over fungi in terms of LMWOC consumption. Gram-positive and -negative bacteria were dominant in upland and paddy soils, respectively. From days 11–30, the contribution of fungi and actinomycetes to LMWOC utilization began to appear. Temperature positively regulated 13C distribution in microbial groups (P < 0.05), and increased dissolved organic carbon in upland soil accelerated microbial SUE. The results of this study clarify microbial effects on the high soil carbon sequestration capacity of paddy soil as compared with upland in subtropical areas.

Mechanisms for retention of low molecular weight organic carbon varies with soil depth at a coastal prairie ecosystem

Though primary sources of carbon (C) to soil are plant inputs (e.g., rhizodeposits), the role of microorganisms as mediators of soil organic carbon (SOC) retention is increasingly recognized. Yet, insufficient knowledge of sub-soil processes complicates attempts to describe microbial-driven C cycling at depth as most studies of microbial-mineral-C interactions focus on surface horizons. We leveraged a well-studied paleo-marine terrace (90 ka) located near Santa Cruz, CA, to characterize the short-term (days to weeks) and intermediate-term (months to years) fate of two low molecular weight organic carbon. compounds at three depths in the soil profile (∼25 cm, A horizon; ∼75 cm A/B transition; and ∼125 cm, B horizon). We employed isotopically-labeled glucose (GLU) and oxalic acid (OXA) to represent two common classes of rhizodeposits: carbohydrates and organic acids. Using a combination of laboratory (9 d) and field (490 d) incubations, we traced the fate of GLU-C and OXA-C through dissolved-, metal-associated-, and microbially-respired CO2 and bulk SOC pools. Our results suggest new SOC retention (i.e., defined as 13C label identified in solid or aqueous fractions) over intermediate time frames (490 d) is correlated with patterns in short-term (9 d) cycling dynamics, which in turn is related to the theoretical efficiency by which microorganisms process each substrate. For all horizons (A, A/B, and B) GLU-C was converted to CO2 more quickly than OXA-C with modeled decomposition rates ∼2–4 times faster for GLU depending on microbial density (higher in A than B horizon). The faster decomposition rates of GLU-C increased fractional recovery (0.399 ± 0.026 to 0.504 ± 0.030 for GLU-C) compared to OXA-C (0.035 ± 0.003 to 0.127 ± 0.010) among all horizons in our field experiment (490 d). Though the overall proportion of GLU-C recovered in solid fractions did not vary significantly with horizon, based on 13C recovered in aqueous fractions the apparent mechanism for retention did. After the 9-d laboratory incubation, fractional recovery for GLU-C among C pools associated with microbial biomass was almost 20× higher than OXA-C (0.192 versus 0.010, respectively across all horizons). More than a year later, 43–46% of GLU-C retained in the field incubation was extractable with a neutral salt (representing a pool of soil C residing within or available to microbial biomass) among A and A/B horizons, while only 6% of retained GLU-C was similarly extractable in the B horizon. Thus, it appears among depths with higher microbial density (A, A/B horizons), anabolic recycling is the most likely process contributing to the persistence of glucose C, whereas abiotic sinks contributed more to intermediate-term stability for GLU-C in the B horizon. By contrast, most OXA-C was lost, presumably as CO2, over the short-term from the A and A/B horizons (fractional recovery: 0.136 ± 0.011 and 0.091 ± 0.002, respectively). However, though substantially lower than GLU-C recovered at the conclusion of our field experiment, the fraction of oxalic acid C retained in the B horizon over both short- (0.72 ± 0.037) and intermediate-time (0.127 ± 0.010) frames was several-fold higher than for overlying horizons. The specific process(es) (e.g., more efficient microbial utilization, metal-organic complexation, direct adsorption to the mineral matrix, etc.) contributing to higher retention for OXA-C at depth are discussed but remain unresolved.

Mechanisms for Retention of Low Molecular Weight Organic Carbon Varies with Soil Depth at a Coastal Prairie Ecosystem

Mechanisms for Retention of Low Molecular Weight Organic Carbon Varies with Soil Depth at a Coastal Prairie Ecosystem

Microbial utilization of low molecular weight organic carbon substrates in cultivated peats in response to warming and soil degradation

Peatlands store vast amounts of carbon (C) within the global terrestrial biosphere. Drainage and cultivation of peat soils lead to rapid soil degradation and C losses, and this may worsen under warming as the soils are no longer protected by anaerobic conditions. To predict the rates of soil C loss and design effective mitigation strategies, it is important to understand what controls organic matter mineralization in these soils. Using the 0–10 cm soil depth of thick and thin (degraded) agricultural peat soils, we investigated the fate of low molecular weight organic substrates (LMWOS) and how the microbial biomass consuming these substrates responded to temperature. We incubated the soils under increasing temperatures (4, 10, 20, and 30 °C) for 72 h. Either 14C-labelled glucose or amino acids were added to the soils and their speed of breakdown, partitioning into anabolic/catabolic processes and microbial C use efficiency (CUE) were determined. The total 14CO2 loss from soil increased significantly with increasing temperature during 72-h incubation, regardless of peat layer thickness. Warming altered the dynamics of LMWOS mineralization by changing C allocation and the turnover rate of different microbial C pools. The half-life of LMWOS decreased more than 50% when temperature increased from 4 to 30 °C for both substrates. CUE was always higher for thin than thick peat soil and both declined by 0.002–0.005 °C-1 with increasing temperature. Thin peat decreased substrate C allocation into the fast cycling C pool compared to the thick peat, but had no overall effect on pool turnover rate. Our work suggests that climate warming will accelerate C mineralization and soil loss in drained peat soils, with larger effects expected in thick peat soil. This study provides an important initial step in characterizing the response of the microbial utilization of labile C to temperature change and soil degradation in cultivated peatlands.

Stabilization of exogenous carbon in soil density fractions is affected by its chemical composition and soil management

Low molecular weight organic carbon (LMWOC) can be physiochemically protected by soil particles via sorption and enfolding. Soil management usually changed the soil particle distribution (e.g. upland vs paddy soil). Higher organic carbon stocks in paddy soil than upland soil could partially be because of the greater immobilization and inaccessibility of LMWOC in paddy soil. To assess the effects of soil management and types of LMWOC on its carbon stabilization in soil density fractions, a simulated study was conducted to investigate the distribution of LMWOC (i.e., glucose, acetic acid, and oxalic acid) in soil density fractions based on 13C-labeling experiments in both soils. The results showed that the total 13C recovery was determined by LMWOC types, following the order of glucose> acetic acid > oxalic acid (p < 0.05). Both soil management and types of LMWOC significantly affect carbon distribution in soil density fractions. The 13C from LMWOC was predominant in the 1.85–2.4 g.cm−3 fraction of upland soil, while the highest concentration of LMWOC 13C was found in mineral grains (2.4–2.65 g.cm−3) in paddy soil, suggesting that more LMWOC was preserved in high density fractions in paddy soil than in upland soil. Our results highlighted that the LMWOC was mainly associated with aggregates in upland soil while subjected to mineral adsorption in paddy soil.

Pathways of mineral-associated soil organic matter formation: Integrating the role of plant carbon source, chemistry, and point of entry.

To predict the behavior of the terrestrial carbon cycle, it is critical to understand the source, formation pathway, and chemical composition of soil organic matter (SOM). There is emerging consensus that slow-cycling SOM generally consists of relatively low molecular weight organic carbon substrates that enter the mineral soil as dissolved organic matter and associate with mineral surfaces (referred to as "mineral-associated OM," or MAOM). However, much debate and contradictory evidence persist around: (a) whether the organic C substrates within the MAOM pool primarily originate from aboveground vs. belowground plant sources and (b) whether C substrates directly sorb to mineral surfaces or undergo microbial transformation prior to their incorporation into MAOM. Here, we attempt to reconcile disparate views on the formation of MAOM by proposing a spatially explicit set of processes that link plant C source with MAOM formation pathway. Specifically, because belowground vs. aboveground sources of plant C enter spatially distinct regions of the mineral soil, we propose that fine-scale differences in microbial abundance should determine the probability of substrate-microbe vs. substrate-mineral interaction. Thus, formation of MAOM in areas of high microbial density (e.g., the rhizosphere and other microbial hotspots) should primarily occur through an in vivo microbial turnover pathway and favor C substrates that are first biosynthesized with high microbial carbon-use efficiency prior to incorporation in the MAOM pool. In contrast, in areas of low microbial density (e.g., certain regions of the bulk soil), MAOM formation should primarily occur through the direct sorption of intact or partially oxidized plant compounds to uncolonized mineral surfaces, minimizing the importance of carbon-use efficiency, and favoring C substrates with strong "sorptive affinity." Through this framework, we thus describe how the primacy of biotic vs. abiotic controls on MAOM dynamics is not mutually exclusive, but rather spatially dictated. Such an understanding may be integral to more accurately modeling soil organic matter dynamics across different spatial scales.

Seasonal variation of potential denitrification rate and enhanced denitrification performance via water-lifting aeration technology in a stratified reservoir—A case study of Zhoucun reservoir

Zhoucun reservoir is one of the major water resources in Zao Zhuang city, northern China. The seasonal distribution of surface sediment denitrification rate and enhanced performance of denitrification via water-lifting aeration technology were explored using the acetylene-inhibition technique. Surface sediment denitrification rates ranged from 2.57 ± 1.32 to 923.90 ± 86.81 nmol N2/(g dw·h), with the highest rates in November (ANOVA, p < 0.05) and significantly low rates in June, July, and August (ANOVA, p < 0.05), mainly because of the seasonal differences in nitrate concentration, water temperature, and sediment total nitrogen (STN). Meanwhile, the N2/(N2+N2O) ratio (83.44–91.70% for the highest sediment denitrification period) indicated that N2 accounted for a majority of denitrification. Correlation analysis between various environmental factors and denitrification was conducted, and nitrate concentration, STN, low molecular weight organic carbon, the number of aerobic denitrifying bacteria, and the environmental parameters of oxidation-reduction potential (ORP), pH, electrical conductivity (EC), and chlorophyll a (Chl-a) presented significant relationships during the entire study period. On the basis of the multiple regression model, nitrate and low molecular weight organic carbon concentration were the most influential factors on denitrification variability. Moreover, the denitrification rates of the surface sediment clearly increased, from 5.28 to 13.22 nmol N2/(g dw·h) to 1117.02–3129.47 nmol N2/(g dw·h), which were higher than those in the non-operating year. This suggests that the denitrification in the sediment system could be enhanced in situ by water-lifting and aeration technology in the reservoir ecosystem.

The Role of Low-Molecular-Weight Organic Carbons in Facilitating the Mobilization and Biotransformation of As(V)/Fe(III) from a Realgar Tailing Mine Soil

ABSTRACTSeveral low-molecular-weight organic carbon (LMWOC) compounds (acetate, propionate, butyrate, lactate, and glucose) were added to flooded arsenic-rich tailing mine soil to investigate their effect to the mobilization of As/Fe and potential shift of microbial community. A promoting effect to the mobilization and biotransformation of As(V)/Fe(III) in the soils resulting from the supplementation with LMWOCs substrate was indicated compared to the biotic microcosm amended with deionized water alone. During 38-day biotic incubation, more than 2100 μg/L of As(III) and 4.2 mg/L of Fe(II) levels were released from the soils amended with LMWOCs substrates, compared to the levels of As(III) and Fe(II) (less 35 μg/L and 1.82 mg/L) derived from the biotic supplementation with deionized water alone. PCR-DGGE indicated that several LMWOCs-responded bacteria were mostly related to Firmicutes and Proteobacteria. Moreover, a negligible impact on the abundance of Fe(III)-reducing family Geobacteraceae was indicated in the LMWOCs-amended soils. However, an increased abundance of sulfate-reducing bacteria but a decreased abundance of arsenate-respiring bacteria were indicated upon the soils supplemented with acetate alone, compared with other LMWOC amendments. DNA-stable isotope probing analysis demonstrated that the dual roles of acetate was not only served as an electron donor for biotransformation of As(V)/Fe(III) in soil, but also assimilated as a powerful energy source to promote the growth of sulfate-reducing bacteria. The findings suggest that there are specific bacteria that preferentially respond to the additions of LMWOC for controlling the biochemical cycle process of As/Fe in soils.

Assimilable Organic Carbon (AOC) in Soil Water Extracts Using Vibrio harveyi BB721 and Its Implication for Microbial Biomass

Assimilable organic carbon (AOC) is commonly used to measure the growth potential of microorganisms in water, but has not yet been investigated for measuring microbial growth potential in soils. In this study, a simple, rapid, and non-growth based assay to determine AOC in soil was developed using a naturally occurring luminous strain Vibrio harveyi BB721 to determine the fraction of low molecular weight organic carbon in soil water extract. Calibration of the assay was achieved by measuring the luminescence intensity of starved V. harveyi BB721 cells in the late exponential phase with a concentration range from 0 to 800 µg l−1 glucose (equivalent to 0–16.0 mg glucose C kg−1 soil) with the detection limit of 10 µg l−1 equivalent to 0.20 mg glucose C kg−1 soil. Results showed that bioluminescence was proportional to the concentration of glucose added to soil. The luminescence intensity of the cells was highly pH dependent and the optimal pH was about 7.0. The average AOC concentration in 32 soils tested was 2.9±2.2 mg glucose C kg−1. Our data showed that AOC levels in soil water extracts were significantly correlated (P<0.05) with microbial biomass determined as microbial biomass carbon, indicating that the AOC concentrations determined by the method developed might be a good indicator of soil microbial biomass. Our findings provide a new approach that may be used to determine AOC in environmental samples using a non-growth bioluminescence based assay. Understanding the levels of AOC in soil water extract provides new insights into our ability to estimate the most available carbon pool to bacteria in soil that may be easily assimilated into cells for many metabolic processes and suggest possible the links between AOC, microbial regrowth potential, and microbial biomass in soils.

Microbially mediated carbonate precipitation in a hypersaline lake, Big Pond (Eleuthera, Bahamas)

AbstractMicrobial metabolism impacts the degree of carbonate saturation by changing the total alkalinity and calcium availability; this can result in the precipitation of carbonate minerals and thus the formation of microbialites. Here, the microbial metabolic activity, the characteristics and turnover of the extracellular polymeric substances and the physicochemical conditions in the water column and sediments of a hypersaline lake, Big Pond, Bahamas, were determined to identify the driving forces in microbialite formation. A conceptual model for organomineralization within the active part of the microbial mats that cover the lake sediments is presented. Geochemical modelling indicated an oversaturation with respect to carbonates (including calcite, aragonite and dolomite), but these minerals were never observed to precipitate at the mat–water interface. This failure is attributed to the capacity of the water column and upper layers of the microbial mat to bind calcium. A layer of high Mg‐calcite was present 4 to 6 mm below the surface of the mat, just beneath the horizons of maximum photosynthesis and aerobic respiration. This carbonate layer was associated with the zone of maximum sulphate reduction. It is postulated that extracellular polymeric substances and low molecular weight organic carbon produced at the surface (i.e. the cyanobacterial layer) of the mat bind calcium. Both aerobic and anaerobic heterotrophic microbes consume extracellular polymeric substances (each process accounting for approximately half of the total consumption) and low molecular weight organic carbon, liberating calcium and producing inorganic carbon. The combination of these geochemical changes can increase the carbonate saturation index, which may result in carbonate precipitation. In conclusion, the formation and degradation of extracellular polymeric substances, as well as sulphate reduction, may play a pivotal role in the formation of microbialites both in marine and hypersaline environments.

Lake secondary production fueled by rapid transfer of low molecular weight organic carbon from terrestrial sources to aquatic consumers

Carbon of terrestrial origin often makes up a significant share of consumer biomass in unproductive lake ecosystems. However, the mechanisms for terrestrial support of lake secondary production are largely unclear. By using a modelling approach, we show that terrestrial export of dissolved labile low molecular weight carbon (LMWC) compounds supported 80% (34-95%), 54% (19-90%) and 23% (7-45%) of the secondary production by bacteria, protozoa and metazoa, respectively, in a 7-km(2) boreal lake (conservative to liberal estimates in brackets). Bacterial growth on LMWC was of similar magnitude as that of primary production (PP), and grazing on bacteria effectively channelled the LMWC carbon to higher trophic levels. We suggest that rapid turnover of forest LMWC pools enables continuous export of fresh photosynthates and other labile metabolites to aquatic systems, and that substantial transfer of LMWC from terrestrial sources to lake consumers can occur within a few days. Sequestration of LMWC of terrestrial origin, thus, helps explain high shares of terrestrial carbon in lake organisms and implies that lake food webs can be closely dependent on recent terrestrial PP.

Composition and variations in the occurrence of dissolved free simple organic compounds of an unproductive lake ecosystem in northern Sweden

Low molecular weight organic carbon compounds are potentially important carbon and energy substrates to heterotrophic production in the aquatic environment. We studied the occurrence of dissolved free amino acids (AA), monosaccharides (CHO), and carboxylic acids (CA) in the subarctic Lake Diktar-Erik. The lake is unproductive with slightly humic water, and receives water via one major inlet stream draining a birch forest environment. The concentration of dissolved organic carbon (DOC) in the inlet stream was strongly correlated with the discharge. This relationship changed from season to season, indicating changes in the sources of the DOC entering the stream. AA and CHO each accounted for an average of less than 0.5% of the DOC. After high discharge events during the ice-free period, AA and CHO occurred in especially high concentrations. CA occurred in higher concentrations during the ice-free period, when it generally accounted for 20–30% of the DOC pool. The CA content relative to the total DOC pool was strongly inversely correlated with overall DOC concentration, and at low DOC levels the relative content of CA was high and vice versa. This followed a seasonal trend, with CA accounting for a smaller proportion of the DOC in winter and a larger part in spring/early summer. A conservative estimate suggested that the studied simple organic carbon compounds potentially could cover 30% of the bacterial gross production in the lake and therefore potentially also was an important source of CO2 that occur in supersaturated concentrations in the lake.

Differential uptake of dissolved and particulate organic carbon by the marine musselPerna viridis

We used radiotracers in laboratory experiments to determine the organic carbon uptake by the marine mussel Perna viridis from different diets (phytoplankton and detritus) and from the dissolved phase (colloidal and low molecular weight organic carbon). Standard compounds (glucose, arginine, and leucine, and the carbohydrate macromolecular dextran with sizes ranging from 3–2,000 kDa) were used to study organic matter flux into the green mussels. Carbon absorption was significantly affected by the food type and quality. The highest absorption efficiency (AE, 80%) was from the diatom Thalassiosira pseudonana, while the AE from Chlorella autotrophica was comparable with those from three types of algal detritus (20–30%). Mussels were able to directly ingest macromolecular dextran colloidal materials. Significant accumulation of these compounds was found both in the gills and the digestive glands of the mussels within the 12–17–h exposure period. Similarly, a significant accumulation of biogenic colloidal organic carbon (COC) and low molecular weight ultrafiltered organic carbon (UOC) was found. Uptake of UOC and COC resulting from the decomposition of different algal sources was comparable, with an uptake rate constant of 6.08 × 10–−4 L g−1 dry wt h−1 and an absorption efficiency of 0.0053%. A kinetic model was subsequently applied to quantitatively evaluate the carbon contribution from dissolved and particulate sources to general organic carbon uptake by the mussels. Mussels accumulated organic carbon predominantly from the particulate phase, with very little (&lt;0.2%) coming from the DOC. However, the apparent ingestion and accumulation of colloids suggest that they can actively participate in the food chain dynamics in marine systems.

Dimethyl sulphide and methanethiol formation in microbial mats: potential pathways for biogenic signatures

Mechanisms of dimethyl sulphide (DMS) and methanethiol (MT) production and consumption were determined in moderately hypersaline mats, Guerrero Negro, Mexico. Biological pathways regulated the net flux of DMS and MT as revealed by increases in flux resulting from decreased salinity, increased temperature and the removal of oxygen. Dimethylsulphoniopropionate (DMSP) was not present in these microbial mats and DMS and MT are probably formed by the reaction of photosynthetically produced low-molecular weight organic carbon and biogenic hydrogen sulphide derived from sulphate reduction. These observations provide an alternative to the notion that DMSP or S-containing amino acids are the dominant precursors of DMS in intertidal sediment systems. The major sink for DMS in the microbial mats was biological consumption, whereas photochemical oxidation to dimethylsulphoxide was the major sink for DMS in the overlying water column. Diel flux measurements demonstrated that significantly more DMS is released from the system during the night than during the day. The major consumers of DMS in the presence of oxygen were monooxygenase-utilizing bacteria, whereas under anoxic conditions, DMS was predominantly consumed by sulphate-reducing bacteria and methanethiol was consumed by methanogenic bacteria. Aerobic and anaerobic consumption rates of DMS were nearly identical. Mass balance estimates suggest that the consumption in the water column is likely to be smaller than net the flux from the mats. Volatile organic sulphur compounds are thus indicators of high rates of carbon fixation and sulphate reduction in these laminated sediment ecosystems, and atmospheric sulphur can be generated as a biogenic signature of the microbial mat community.

Ecological implications of surficial marine gas hydrates for the associated small-sized benthic biota at the Hydrate Ridge (Cascadia Convergent Margin, NE Pacific)

The effect of methane released from decomposing surficial gas hydrates (SGH) on stand- ing stocks and activities of the small-sized benthic biota (SSBB; i.e. bacteria, fungi, protozoa, and meiobenthic organisms) was studied at about 790 m water depth, at the Hydrate Ridge, Cascadia sub- duction zone. Presence of SGH and elevated sulfide concentrations in the sediment were indicated by extensive bacterial mats of Beggiatoa sp. and clam fields of the bivalve mollusc Calyptogena sp. Verti- cal and horizontal distribution patterns of the SSBB biomass were derived from DNA and total adeny- late (TA) sediment assays. Potential bacterial exoenzymatic hydrolytic activity was measured using fluorescein-di-acetate (FDA) as substrate. Estimates of chemoautotrophic production of particulate or- ganic carbon (POC.) were determined by 14 CO2 uptake incubations. Inventories of chl a and pheopig- ments were determined as parameters of surface water primary produced POC input. Average SSBB bio- mass in clam field sediments integrated over the upper 10 cm (765.2 gC m -2 , SD 190.1) was 3.6 times higher than in the adjacent control sites (213 gC m -2 , SD 125). Average SSBB biomass in bacterial mat sediments, which were almost devoid of eukaryotic organisms > 31 µm, was 209 gC m -2 (SD 65). Sig- nificant correlations between FDA, DNA and plant pigments imply that productivity of the SSBB at SGH sites is only partially uncoupled from the primary production of the surface water. Areal estimates of autotrophic Corg production at control sites, bacterial mat sites and in clam field sites were 5.7, 59.7 and 190.0 mgC m -2 d -1 , respectively. Based on different models predicting vertical POC fluxes from surface water primary production and water depth, these autotrophic POC productions account for 5 to 17% (controls), 35 to 68% (bacterial mats), and 63 to 87% (clam fields) of the bulk POC (sum of allochthonous POC input through the water column and sedimentary autochthonous autotrophic POC production) pro- vided at the various sites. At SGH sites inventories of chl a and pheopigments, integrated over the upper 10 cm of the sediment, were half of that found at the control sites. This might be due to enhanced degra- dation of phytodetritally associated organic matter. The resulting low molecular weight organic carbon compounds might stimulate and fuel sulfate reduction, which is conducted in a microbial consortium with anaerobic methane consuming archaea. This syntrophic consortium might represent a prominent interface between gas hydrate derived carbon and allochthonous Corg flow. We infer that degradation kinetics of SGH is affected by, e.g., seasonally varying input of allochthonous organic matter.

Colloidal organic carbon and phytoplankton speciation during a coastal bloom

During the 1995 spring bloom in Bedford Basin, Nova Scotia, dissolved organic carbon (DOC) in the photic zone was separated into low-molecular-weight and colloidal size fractions by cross-flow ultrafiltration. DOC, colloidal organic carbon (COC) and low-molecular-weight organic carbon (LOC) were then analyzed by high-temperature catalytic oxidation. COC (associated with the production of phytoplankton exudates) did not follow the concentration of bulk chlorophyll a (chi a) or the total number of phytoplankton cells. Instead, surface-active COC (that adhered to the ultrafiltration system) was produced early during the bloom when the diatom Skeletonema costatum was at a maximum. Later on, as the bloom began to decline, less surface-active COC (that remained largely free of the ultrafiltration surfaces) was produced and was associated more with variations in cell number of Chaetoceros socialis, the predominant diatom for most of the bloom. These results suggest that chl a or the total number of phytoplankton cells may not be reliable indices of the production of COC. On the contrary, the results suggest that relatively high COC concentrations were associated with specific diatom species. In addition, the maximum amount of COC was associated during the early stages of the bloom with a diatom (S.costatum) that was a small fraction (<5.6%) of total phytoplankton cell number. This finding, that relatively large amounts of colloids were associated with exopolymer production during the onset and development of a bloom, does not agree with reports suggesting that the production of exopolymers by diatoms is primarily an end-of-bloom occurrence.