Since studying microbes living within the rocky seafloor is challenging, researchers sample hydrothermal fluid seeping out of the seafloor as a window into subseafloor communities. The microbes at such sites thrive on energy sources such as hydrogen sulfide and methane derived from water-rock reactions, and are therefore directly linked to the chemistry and geology of the hydrothermal vent.
In a recent publication, DCO’s Julie Huber (Marine Biological Laboratory, Woods Hole, USA), Jeffrey Seewald (Woods Hole Oceanographic Institution, USA), and Jill McDermott (University of Toronto, Canada) and colleagues provide a comprehensive examination of microbial life in the subseafloor at two newly discovered deep-sea hydrothermal systems at the world’s deepest spreading center in the Caribbean Sea, the mafic-hosted Piccard at 4960m and the ultramafic-influenced Von Damm at 2350m. Despite large differences in depth, geologic setting, and vent fluid chemistry, the sites are located only 20 km apart and both host fluids highly enriched in hydrogen, an important energy source for microbes. As this was the first exploration of these two hydrothermal systems, the researchers used a combination of chemical measurements, thermodynamic modeling, total cell and domain-specific enumeration, targeted stable isotope tracing experiments, and 16S rRNA gene amplicon and shotgun metagenomic sequencing to determine and compare the chemistry, energy landscape, abundance, community composition, diversity, and function of microbes in vent fluids from both sites.
Results indicate the Von Damm field hosts a wider diversity of lineages and metabolisms in comparison to Piccard, consistent with thermodynamic models that predict more numerous energy sources at ultramafic systems. There was little overlap in the organisms found at each site, although similar and dominant hydrogen-utilizing genera were present at both. Despite the differences in diversity, community structure, depth, geology, and fluid chemistry, energetic modeling and metagenomic analysis indicate near functional equivalence between Von Damm and Piccard, likely driven by the high hydrogen concentrations and elevated temperatures at both sites. These results, when compared to hydrothermal sites worldwide, provide a global perspective on the distinctiveness of these newly discovered sites and the interplay among rocks, fluid composition, and life in the subseafloor at deep-sea hydrothermal vents.
"The sites discovered at the Mid-Cayman Rise are ideal for examining how hydrogen and sunlight-independent organic carbon fuel microbial activities in deep subsurface ecosystems,” said Huber. “We are especially interested in understanding how abiotic synthesis associated with hydrothermal alteration of the oceanic lithosphere can support subseafloor microbial communities, and the vents at the Mid-Cayman Rise are ideal for this given the unique rock types and chemistry of the systems over a range of pressure and temperature conditions"
Piccard vent field, Mid-Cayman Rise. Credit: WHOI Deep Submergence and Chris German.