Probing aerobic methanotrophs associated with seepages in offshore southwestern Taiwan
Abstract
Marine sediments are considered to be the largest reservoir of methane on Earth with methane leakage enabled through seepage or mud volcano into seawater column. Decadal measurements, however, yield dissolved methane concentrations of surface seawater often nearly in equilibrium with the atmospheric methane, suggesting a strong sink in either near-seafloor sediments or seawater. While anaerobic methanotrophy could account for most methane removal in sediments, the exact role of aerobic methanotrophy in near-seafloor sediments versus seawater remains unclear. In this study, measurements of methane concentration combined with stable isotope probing were carried out to characterize the capacity, activity and population composition of aerobic methanotrophs in seafloor sediments and seawater associated with seepage offshore southwestern Taiwan. The results showed that methane concentrations in surface seawater above two seeps ranged from 1 to 8 nM and in near-seafloor sediments ranged up to several hundreds of nM. This range of sea-surface methane concentration is close to that in equilibrium with the atmospheric methane, suggesting that the net methane emission is limited even in such high methane environments. Incubations of seawater alone or the mixture of seafloor sediments and seawater with 13C-labeled methane, however, yielded consistent production of 13C-CO2 only in the mixture of sediment and seawater. Analyses of 16S rRNA genes on environmental and incubated samples revealed that Proteobacteria, Bacteroidetes, Planctomycetes, Chloroflexi, and Acidobacteria were among the most abundant phyla. Potential aerobic methanotrophs and methylotrophs were prevalent and disproportionally more abundant in heavy DNA fractions for incubated sediment-seawater mixtures when compared with light DNA fractions, or original seawater and sediments. These methanotrophs and methylotrophs were affiliated with Methylococcaceae and Methylophaga within Gamma-Proteobacteria, respectively. Analyses of pmoA genes yielded comparable results. Overall, methanotrophs residing in seafloor sediments offshore southwestern Taiwan appear to outcompete those in seawater column, effectively consuming residue methane originating from deep source and maintaining limited methane emission from sea surface.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFMOS53C1228W
- Keywords:
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- 3004 Gas and hydrate systems;
- MARINE GEOLOGY AND GEOPHYSICS;
- 4805 Biogeochemical cycles;
- processes;
- and modeling;
- OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL;
- 4815 Ecosystems;
- structure;
- dynamics;
- and modeling;
- OCEANOGRAPHY: BIOLOGICAL AND CHEMICAL;
- 4219 Continental shelf and slope processes;
- OCEANOGRAPHY: GENERAL