Igneous oceanic crust encompasses ~60% of Earth’s surface and is composed of basalt glass and mafic, ultramafic, and felsic minerals. A vast marine aquifer lies within the crust, exchanging geochemically altered fluids with seawater from the overlying ocean at ridge crests, flanks, seamounts, and outcrops where permeable crust is exposed. Correlation studies of crustal surface rocks have shown that mineralogy is linked to microbiology; however, the influence of individual mineral phases or their compositions on microbial communities has yet to be empirically demonstrated. In addition, the habitable zone of oceanic crust can extend to depths of several kilometers and communities deeper within this zone may be more representative of the whole suboceanic aquifer ecosystem than those communities found just at the surface where the environment is influenced by infiltration of cold, oxic seawater. The focus of this work was to explore how deep subsurface biofilm communities in the suboceanic aquifer of the Juan de Fuca Ridge (JdFR) are influenced by igneous mineral phases and their compositions. We expect that crustal mineralogy will affect the microbial community structure and metabolism of these aquifer communities. Exploring the metabolisms of these communities will also lead to a greater understanding of the functioning of the suboceanic aquifer ecosystem and its role in the global carbon cycle.
Microbial communities that colonized a variety of in situ-incubated igneous minerals and glasses were investigated. We used International Ocean Drilling Program (IODP) borehole 1301A as a subseafloor observatory to incubate these mineral substrates for a four-year period. After retrieval, we found that taxa related to thermophilic and hyperthermophilic chemolithotrophs and heterotrophs were present. Archaeal taxa included three genera of Archaeoglobaceae, including members of the
sulfate-reducing genus Archaeoglobus. Bacterial taxa were overwhelmingly dominated by Clostridia and other deep-branching Bacteria. Most taxa were not closely related to known organisms so their metabolic capabilities could not be predicted by taxonomic association. Microbial communities were also influenced by the mineralogical properties of attachment surfaces; particularly with respect to iron rich phases. Communities attached to rocks, minerals, and glasses in this environment were more similar to each other than they were to aquifer fluid communities, bottom seawater, and other marine or deep crustal communities, and thus represented a distinct mineral-colonizing “attached” community. Metabolic reconstruction of metagenome-derived genomes from olivine also showed that sulfate reduction, carbon fixation, and hydrogenotrophic pathways dominated that community. These
results suggest there is a potential for hydrogen-based chemolithoautotrophy in the deep oceanic crust, that these microbial communities do not fully rely on photosynthetically-derived organic carbon for energy and carbon, and that the suboceanic aquifer biosphere may play a role in global carbon cycling and productivity.