Abstract
The marine subsurface is a reservoir of the greenhouse gas methane. While microorganisms living in water column and seafloor ecosystems are known to be a major sink limiting net methane transport from the marine subsurface to the atmosphere, few studies have assessed the flow of methane-derived carbon through the benthic mat communities that line the seafloor on the continental shelf where methane is emitted. We analyzed the abundance and isotope composition of fatty acids in microbial mats grown in the shallow Coal Oil Point seep field off Santa Barbara, CA, USA, where seep gas is a mixture of methane and CO2. We further used stable isotope probing (SIP) to track methane incorporation into mat biomass. We found evidence that multiple allochthonous substrates supported the rich growth of these mats, with notable contributions from bacterial methanotrophs and sulfur-oxidizers as well as eukaryotic phototrophs. Fatty acids characteristic of methanotrophs were shown to be abundant and 13C-enriched in SIP samples, and DNA-SIP identified members of the methanotrophic family Methylococcaceae as major 13CH4 consumers. Members of Sulfuricurvaceae, Sulfurospirillaceae, and Sulfurovumaceae are implicated in fixation of seep CO2. The mats’ autotrophs support a diverse assemblage of co-occurring bacteria and protozoa, with Methylophaga as key consumers of methane-derived organic matter. This study identifies the taxa contributing to the flow of seep-derived carbon through microbial mat biomass, revealing the bacterial and eukaryotic diversity of these remarkable ecosystems.
Highlights
The oxidation of methane by marine microorganisms is a key control in the flux of this potent greenhouse gas from the ocean to the atmosphere (Reeburgh, 2007)
Taken together with the betaine lipids captured by our intact polar lipid (IPL) analysis, these findings suggest a role for phototrophy and photomixotrophy in Shane Seep mats, perhaps supplying oxygen to aerobic processes within these microbial mats
We used parallel lipid and DNA analysis to explore the microbial mats that line the seafloor at Shane Seep, part of one of the world’s most vigorous methane seep systems
Summary
The oxidation of methane by marine microorganisms is a key control in the flux of this potent greenhouse gas from the ocean to the atmosphere (Reeburgh, 2007). In the presence of hydrocarbons, methanotrophic activity can support heterotrophic bacteria and eukaryotes in a range of marine communities (Childress et al, 1986; Cavanaugh et al, 1987; Hanson and Hanson, 1996). Marine microbial mats are typically filamentous and dominated by sulfideoxidizing bacteria, including Beggiatoa, Thioploca, Sulfurovum, and Sulfurimonas (Nelson et al, 1989; Zhang C.L. et al, 2005; Moussard et al, 2006; Gilhooly et al, 2007). These communities may support thriving populations of benthic grazers and other fauna (Van Dover and Fry, 1994; Al-Zaidan et al, 2006)
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