Sediment fatty acids associated with seafloor methane seepage in the Nankai and Sagami Troughs, off central Japan

Abstract

Microbial abundance and composition represented by sediment fatty acids serve as the biomarkers for seafloor methane seeps even at ‘juvenile’ stages, while macroscopic animals serve as a visible marker for some ‘aged’ seeps. Sediment samples were collected using remotely operated vehicles from the bathyal to abyssal seafloors of Nankai Trough and Sagami Trough. A piston core sample from the West Philippine Basin seafloor was used as a non-seep reference. Sediment fatty acids were extracted as either total fatty acids or phospholipid fatty acids. Phosopholipid fatty acids represent the abundance and composition of ‘living’ microflorae. Compositions of total fatty acids and phospholipid fatty acids were cluster-analyzed to construct dendrograms. As a general result, high presence of fatty acid markers for sulfate-reducing bacteria and methane-oxidizing bacteria was associated with known methane seepage. The fatty acid profiles of a tested (suspected) seep site were closely similar to those of known seeps, which strongly suggests that the tested site is an incipient seep yet-to-be colonized. As methane acts as a greenhouse gas or a biospheric poison, it should be important to probe, i.e., detect and localize, seafloor methane seeps associated with sub-seafloor gas hydrates.