Publications

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2017
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.

2016
Kharbush, JJ, Allen AE, Moustafa A, Dorrestein PC, Aluwihare LI.  2016.  Intact polar diacylglycerol biomarker lipids isolated from suspended particulate organic matter accumulating in an ultraoligotrophic water column. Organic Geochemistry. 100:29-41.   10.1016/j.orggeochem.2016.07.008   Abstract

Intact polar diacylglycerols (IP-DAGs) are essential components of cell membranes. Because they are structurally diverse and hypothesized to represent primarily living cells, they are potential molecular markers for a recent contribution by microbial communities to various carbon reservoirs. This study employed a novel molecular networking approach to investigate the evolution of IP-DAG structural diversity with depth in an ultraoligotrophic environment of the western South Pacific Ocean to test the hypothesis that particle transport to depth is rapid enough to preserve the IP-DAG biomarker signature of the photosynthetic community. IP-DAG profiles of several cultured cyanobacteria and photosynthetic picoeukaryotes were used as templates for constructing molecular networks to compare and interpret IP-DAG signatures of suspended particles isolated from a water column depth profile. Analysis of corresponding genetic community composition data for the same field samples was used to connect IP-DAG structures with their likely biological sources. Our data show that, although most IP-DAG classes associated with photosynthetic organisms were not observed below the euphotic zone, several other IP-DAG classes in deep samples might provide interesting targets for future studies seeking to examine the in situ contribution of deep sea microbes to suspended particulate organic matter (POM). Overall, the results represent the deepest water column IP-DAG dataset to date and demonstrate the utility of molecular networking for analyzing and visualizing complex environmental datasets.