Methanotrophic bacteria occupy benthic microbial mats in shallow marine hydrocarbon seeps, Coal Oil Point, California

Abstract

[1] Microbial mats composed of giant sulfur bacteria are observed throughout the benthos along continental margins. These communities serve to oxidize dissolved sulfides to sulfate, and are typically associated with the recent exposure of sulfide-rich sediments. Such mats are also ubiquitous in areas of hydrocarbon seepage, where they are thought to consume sulfide generated in underlying sediment. Despite the high abundance of dissolved methane in hydrocarbon seeps, few studies have considered the importance of methanotrophy in mat communities. To assess the importance of methanotrophs in microbial mats from hydrocarbon seeps, an approach involving lipid biomarkers, stable isotopes and enrichment culturing was applied. Microbial mat samples were collected from benthic surfaces at two hydrocarbon seeps located in the Coal Oil Point seep field, offshore from Goleta, California. Both samples display a high abundance of 16:1 fatty acids, including two isomers specific to type I methanotrophic bacteria, 16:1(w8) and 16:1(w6). Depleted values of d 13 C found in 16:1 fatty acids suggests methane assimilation into biomass, whereas three separate investigations of sulfide-oxidizing bacteria yield fractionation factors too small to account for these values. On the basis of these observations and experiments, an isotope mass balance was applied to fatty acids present in the microbial mat samples which indicates methanotrophs contribute up to 46% of total fatty acids. These results implicate methanotrophy as an important function for microbial mats in seep areas, despite the visual appearance of these mats as being composed of giant sulfur bacteria.

Colonial Tube-Dwelling Ciliates Influence Methane Cycling and Microbial Diversity within Methane Seep Ecosystems

“…This finding is consistent with microscopic analyses of Thioploca sp. sheaths from cold seeps in Monterey Bay and Santa Barbara which also host a diverse bacterial and protistan assemblage associated with interstitial spaces between trichomes (Buck et al, 2014), including methanotrophs (Ding and Valentine, 2008). Together, these findings support the notion that small-scale heterogeneity created by lorica, sheaths and the folliculinids themselves can provide habitat niches for other organisms and may influence seep community structure.…”

A hydrothermal seep on the Costa Rica margin: middle ground in a continuum of reducing ecosystems

“…exhibit the isotopically light d 13 C indicating methane-derived carbon (d 13 C 256‰ to 246‰; figure 4a); these species may consume the aerobic methanotrophs detected in venting fluids. Notably, the attached filamentous bacteria bathed by hydrothermal fluids are also light (d 13 C ¼ 240‰), potentially reflecting a light DIC pool and methane influence, or an association with methylotrophs [59]. Similar trophic pathways have been reported from seeps in the Gulf of Mexico [60] and Anya's Garden, a sedimented vent on the Mid-Atlantic Ridge [61].…”

Effects of natural and human-induced hypoxia on coastal benthos

“…Although this review does not treat these interactions (see Middelburg and Levin, 2009), we consider mat-forming microbes to be a fundamental macroscopic feature of benthic ecosystems subject to severe hypoxia. These mats are formed largely of filamentous sulfide oxidizing bacteria in the genera Beggiatoa, Thioploca, or Thiomargarita, although sulfate reducing and methane oxidizing bacteria are often intermixed (Graco et al, 2004;Ding and Valentine, 2008). Primary differences among the dominant genera involve the presence of a sheath and multiple filaments (Thioploca) or bead-like construction (Thiomargarita), and their ability to function in the presence or absence of oxygen (Jørgensen and Gallardo, 1999;Brüchert et al, 2003Brüchert et al, , 2006.…”