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Hansman, RL, Griffin S, Watson JT, Druffel ERM, Ingalls AE, Pearson A, Aluwihare LI.  2009.  The radiocarbon signature of microorganisms in the mesopelagic ocean. Proceedings of the National Academy of Sciences of the United States of America. 106:6513-6518.   10.1073/pnas.0810871106   AbstractWebsite

Several lines of evidence indicate that microorganisms in the meso-and bathypelagic ocean are metabolically active and respiring carbon. In addition, growing evidence suggests that archaea are fixing inorganic carbon in this environment. However, direct quantification of the contribution from deep ocean carbon sources to community production in the dark ocean remains a challenge. In this study, carbon flow through the microbial community at 2 depths in the mesopelagic zone of the North Pacific Subtropical Gyre was examined by exploiting the unique radiocarbon signatures (Delta(14)C) of the 3 major carbon sources in this environment. The radiocarbon content of nucleic acids, a biomarker for viable cells, isolated from size-fractionated particles (0.2-0.5 mu m and >0.5 mu m) showed the direct incorporation of carbon delivered by rapidly sinking particles. Most significantly, at the 2 mesopelagic depths examined (670 m and 915 m), carbon derived from in situ autotrophic fixation supported a significant fraction of the free-living microbial community (0.2-0.5 mu m size fraction), but the contribution of chemoautotrophy varied markedly between the 2 depths. Results further showed that utilization of the ocean's largest reduced carbon reservoir, (14)C-depleted, dissolved organic carbon, was negligible in this environment. This isotopic portrait of carbon assimilation by the in situ, free-living microbial community, integrated over > 50,000 L of seawater, implies that recent, photosynthetic carbon is not always the major carbon source supporting microbial community production in the mesopelagic realm.

Hansman, RL, Thurber AR, Levin LA, Aluwihare LI.  2017.  Methane fates in the benthos and water column at cold seep sites along the continental margin of Central and North America. Deep-Sea Research Part I-Oceanographic Research Papers. 120:122-131.   10.1016/j.dsr.2016.12.016   AbstractWebsite

The potential influence of methane seeps on carbon cycling is a key question for global assessments, but the study of carbon cycling in surface sediments and the water column of cold seep environments is complicated by the high temporal and spatial variability of fluid and gas fluxes at these sites. In this study we directly examined carbon sources supporting benthic and planktonic food webs at venting methane seeps using isotopic and molecular approaches that integrate this variability. At four seep environments located along North and Central America, microorganisms from two size fractions were collected over several days from 2800 to 90501 of seawater to provide a time-integrated measure of key microbial groups and the carbon sources supporting the overall planktonic microbial community. In addition to water column measurements, the extent of seafloor methane release was estimated at two of the sites by examining the stable carbon isotopic signature (delta C-13) of benthic metazoan infauna. This signature reveals carbon sources fueling the base of the food chain and thus provides a metric that represents a time-integrated view of the dominant microbial processes within the sediment. The stable carbon isotopic composition of microbial DNA (delta C-13-DNA), which had values between -17.0 and -19.5%(0), indicated that bulk planktonic microbial production was not ultimately linked to methane or other C-13-depleted seep-derived carbon sources. Instead these data support the importance of organic carbon derived from either photo- or chemoautotrophic CO2 fixation to the planktonic food web. Results of qPCR of microbial DNA sequences coding for a subunit of the particulate methane monooxygenase gene (pmoA) showed that only a small percentage of the planktonic microbial community were potential methane oxidizers possessing pmoA (< 5% of 16S rRNA gene copies). There was an overall decrease of C-13-depleted carbon fueling the benthic metazoan community from 3 to 5 cm below the seafloor to the sediment surface, reflecting limited use of isotopically depleted carbon at the sediment surface. Rare methane emission as indicated by limited aerobic methane oxidation acts to corroborate our findings for the planktonic microbial community.

Haskell, WZ, Prokopenko MG, Hammond DE, Stanley RHR, Berelson WM, Baronas JJ, Fleming JC, Aluwihare L.  2016.  An organic carbon budget for coastal Southern California determined by estimates of vertical nutrient flux, net community production and export. Deep-Sea Research Part I-Oceanographic Research Papers. 116:49-76.   10.1016/j.dsr.2016.07.003   AbstractWebsite

Organic carbon export and burial in coastal upwelling regions is an important mechanism for oceanic uptake of atmospheric CO2. In order to understand how these complex systems will respond to future climate forcing, further studies of nutrient input, biological production and export are needed. Using a Be-7-based approach, we produced an 18-month record of upwelling velocity estimates at the San Pedro Ocean Time-series (SPOT), Southern California Bight. These upwelling rates and vertical nutrient distributions have been combined to make estimates of potential new production (PNP), which are compared to estimates of net community oxygen production (NOP) made using a one-dimensional, two-box non-steady state model of euphotic zone biological oxygen supersaturation. NOP agrees within uncertainty with PNP, suggesting that upwelling is the dominant mechanism for supplying the ecosystem with new nutrients in the spring season, but negligible in the fall and winter. Combining this data set with estimates of sinking particulate organic carbon (POC) flux from water column Th-234:U-238 disequilibrium and sediment trap deployments, and an estimate of the ratio of dissolved organic carbon (DOC):POC consumption rates, we construct a simple box model of organic carbon in the upper 200 m of our study site. This box model (with uncertainties of +/- 50%) suggests that in spring, 28% of net production leaves the euphotic zone as DOC, of this, similar to 12% as horizontal export and 16% via downward mixing. The remaining similar to 72% of net organic carbon export exits as sinking POC, with only 10% of euphotic zone export reaching 200 m. We find the metabolic requirement for the local heterotrophic community below the euphotic zone, but above 200 m, is similar to 105 +/- 50 mmol C m(-2) d(-1), or similar to 80% of net euphotic zone production in spring. (C) 2016 Elsevier Ltd. All rights reserved.