Since the origin of life, microbial activity has regulated energy and elemental flows on Earth. Microorganisms inhabit and influence every niche imaginable on Earth, including surface soils and waters, surficial rocks and ice, hydrothermal springs on the sea floor, and even the deep biosphere hundreds of meters beneath the Earth's surface. Despite the fact that microorganisms represent a significant fraction of the living biomass on Earth and are important mediators of global biogeochemical cycles, our understanding of the interactions and feedbacks between microorganisms and the geosphere they inhabit remains limited.
Studying the microbiology of hypersaline environments may provide clues to the potential distributions and metabolisms of microorganisms on early Earth and on extraterrestrial bodies (e.g., Mars, Europa). The metabolic adaptations required for surviving in a hypersaline niche almost assure discovery of unique microorganisms of potential interest to biotechnology. Hypersaline habitats are abundant on the Earth’s surface, where saline lakes account for a substantial portion of inland water bodies, and on the seafloor, where brine seeps are widespread, occurring in the Black, Red, and Mediterranean Seas and in the Gulf of Mexico. In the Gulf of Mexico, brines laden with energy-rich compounds flow from the deep subsurface to the seafloor along salt diaper–associated faults. High-flow seafloor brines, known as mud volcanoes, are characterized by explosive discharges of fluidized mud, brine, gas, and oil, often at temperatures =50 °C. Low-flow brine seeps occur when brine percolates through sediments and pools in seafloor depressions, generating brine pools. Because the microbiology of seafloor brines is largely unexplored, such areas are the sites of an NSF-funded Microbial Observatory project.
Hypersaline brine pools are features of the Gulf of Mexico petroleum basin. They form when warm, salty fluids migrate through fissures in the sediment. The brine is more dense than sea water, so it pools on the seafloor surface after cooling to ambient temperature. To date, we have examined two brine pools. One stable brine pool with apparently lower rates of fluid flow (GC233 brine) has been stable long enough for a dense community of methanotrophic mussels to develop around the pool's edge (A). Such chemoautotrophic symbiotic associations are common at sites of fluid and gas seepage, as seen in the Gulf of Mexico, along the Florida Escarpment, and along the Cascadia margin. The other brine site is an active mud volcano that is known for high rates of fluid flow (GB425, B).
The GB425 brine has frequent eruptions of warmer (10 or more ºC warmer than bottom waters) fluid (B), and macrofaunal communities are not common here. We sample the brines using a novel device called the “brine trapper” (C, 3-m long gray PVC device in lower part of figure), which is deployed from the side of the submersible. A photo of the brine trapper deployed in the GB425 brine pool is shown in image (D). The brine in sediment is particle rich (E), as noted by the change in color deeper in the brine (from L to R in the image). A unique feature of the mud volcano site is the abundance of barite (Barium-Sulfate) chimneys (F). Barium originating from the brine precipitates when it comes into contact with sulfate-rich seawater.