Although little is known regarding microbial life within our planet’s rock-hosted deep subseafloor biosphere, boreholes drilled through deep ocean sediment and into the underlying basaltic crust provide invaluable windows of access that have been used previously to document the presence of microorganisms within fluids percolating through the deep ocean crust. In this study, the analysis of 1.7 million small subunit ribosomal RNA genes amplified and sequenced from marine sediment, bottom seawater and basalt-hosted deep subseafloor fluids that span multiple years and locations on the Juan de Fuca Ridge flank was used to quantitatively delineate a subseafloor microbiome comprised of distinct bacteria and archaea. Hot, anoxic crustal fluids tapped by newly installed seafloor sampling observatories at boreholes U1362A and U1362B contained abundant bacterial lineages of phylogenetically unique Nitrospirae, Aminicenantes, Calescamantes and Chloroflexi. Although less abundant, the domain Archaea was dominated by unique, uncultivated lineages of marine benthic group E, the Terrestrial Hot Spring Crenarchaeotic Group, the Bathyarchaeota and relatives of cultivated, sulfate-reducing Archaeoglobi. Consistent with recent geochemical measurements and bioenergetic predictions, the potential importance of methane cycling and sulfate reduction were imprinted within the basalt-hosted deep subseafloor crustal fluid microbial community. This unique window of access to the deep ocean subsurface basement reveals a microbial landscape that exhibits previously undetected spatial heterogeneity.
The basaltic ocean crust is the largest aquifer system on Earth, yet the rates of biological activity in this environment are unknown. Low-temperature (<100°C) fluid samples were investigated from two borehole observatories in the Juan de Fuca Ridge (JFR) flank, representing a range of upper oceanic basement thermal and geochemical properties. Microbial sulfate reduction rates (SRR) were measured in laboratory incubations with 35S-sulfate over a range of temperatures and the identity of the corresponding sulfate-reducing microorganisms (SRM) was studied by analyzing the sequence diversity of the functional marker dissimilatory (bi)sulfite reductase (dsrAB) gene. We found that microbial sulfate reduction was limited by the decreasing availability of organic electron donors in higher temperature, more altered fluids. Thermodynamic calculations indicate energetic constraints for metabolism, which together with relatively higher cell-specific SRR reveal increased maintenance requirements, consistent with novel species-level dsrAB phylotypes of thermophilic SRM. Our estimates suggest that microbially-mediated sulfate reduction may account for the removal of organic matter in fluids within the upper oceanic crust and underscore the potential quantitative impact of microbial processes in deep subsurface marine crustal fluids on marine and global biogeochemical carbon cycling.
Our C-DEBI science team will share the excitement of their deep biosphere research to promote secondary STEM education by weaving their story into astrobiology, the search for life in the universe as a joint effort with the University of Hawaii NASA Astrobiology team. They are working with secondary science teachers at their ALI’I summer program as well as Hawaii teachers to develop activities aligned to the Next Generation Science Standards. Since our teachers work with our target groups of students with special needs, Native Hawaiian and Pacific Island, and those at-risk, they provide expertise on how best to serve these students. We also are partnering with C-MORE, Center for Microbiology Oceanography: Research and Education, here at University of Hawaii and are using their field-tested kits for the workshops. The technology of underwater robotics promoted by the MATE competition and other instruments as essential tools for studying science will be emphasized. The teachers will pilot test activities and then conduct secondary workshops in Hawaii and other states. Our science team will also visit schools in Hawai’i to give presentations and participate in the secondary workshops. Evaluation methodology and instruments will be developed with our educational consultant to ensure validity and reliability as we assess our outcomes. Assessments will be coordinated with the C-DEBI and C-MORE outreach community. This collaborative effort will bring the excitement of deep biosphere research into various science courses serving different student populations across the nation.