Distribution of the Dominant Microbial Communities in Marine Sediments Containing High Concentrations of Gas Hydrates

Methanogens are implicated in the production of methane that accumulates in marine sediments. However, the factors that control the distribution of the microbial communities that influence the presence of methane in these sediments are not well understood. Our objective is to determine the quantity, diversity, and distribution of microbial communities in the context of abiotic (e.g., grain size, presence/absence of hydrates) and geochemical (redox state, organic carbon content) properties in gas-rich marine sediments. To this aim, DNA was extracted from deep marine sediments (25-175 mbsf) cored from continental slope locations including offshore India and the Cascadia Margin. The sediments yielded low levels of DNA (0.3-1.5 ng/g of sediment), and bacterial DNA appeared to be more readily amplified than archaeal DNA. Preliminary analysis of a subset of these samples from India using PhyloChip technology indicated 200-800 distinct archaeal or bacterial taxa in each sample. The PhyloChip detected the presence of methanogens, sulfate reducers, sulfur oxidizers, and other metal reducers, as more prevalent taxa. Infrequently, cores from relatively shallow sediments from central Hydrate Ridge and northern Cascadia (offshore Vancouver Island), and from India’s eastern margin contained macroscopically visible, pigmented biofilms. The Hydrate Ridge biofilm consisted of high concentrations (ca. 1-2 μm) cocci in a tetrad when viewed under a microscope. Preliminary terminal-restriction fragment length polymorphism (t-RFLP) characterization of the 16S rDNA amplified from these samples suggested the presence of as many as 7-18 distinct taxa. These discrete biofilm communities are anomalous compared to the normally sparse distribution of cells in the sediments and based on their considerable biomass may factor prominently in the cycling of carbon in the shallow sediments. Understanding the fine-scale distribution and factors that control the presence of sediment communities will provide better parameters for computational models that describe carbon cycling in these systems.