H2 concentrations in highly active deep-sea methane seep sediments in Hydrate Ridge and Astoria Canyon
Molecular hydrogen is an essential intermediate in the degradation of organic matter and is vital for chemical processes like sulfate reduction and methanogenesis. Therefore, it is important to understand the relationship of hydrogen in these chemical processes, as well as the microbes that inhabit deep-sea methane seeps, where reduced chemicals like methane and hydrogen rise to the seafloor. The competition for hydrogen between sulfate-reducing bacteria (SRB) and methanogenic archaea (MA) affects the overall hydrogen concentrations in anoxic sediment. When SRB are functioning at their thermodynamic limit, they generally keep hydrogen concentrations low while reducing sulfate to sulfide, preventing methanogens from accessing it. But in high organic matter sites, there may be increased hydrogen concentrations due to higher production from fermenters, which may decrease competition between methanogens and sulfate reducers. Because hydrogen is challenging to measure, little is known about its concentration in methane seep sediments. To understand how organic matter deposits and hydrocarbon-rich fluids at deep-sea methane seeps impact hydrogen dynamics in anoxic methane seeps, we measured triplicate hydrogen concentrations in Astoria Canyon and Hydrate Ridge offshore of Oregon, USA. To understand the impact on microbial communities, we extracted DNA and prepared 16S rRNA gene amplicon surveys. Our gravity core 1 (Gc1) had a hydrogen concentration of ~1.7 nM, with very low variations (SD ~ 0.1 0.3) at < 35 cm depth but increased in variability below this depth. In addition, at depths of <50 cm in Gc1, Methanomicrobia are absent, whereas they are present in the sediments below 50 cm, an area with more hydrogen variability. Our results suggest that whether hydrogen is stable and controlled or variable and robust impacts microbial methane cycling.