Serpentinization is the process in which ultramafic rocks, characteristic of the upper mantle, react with water liberating mantle carbon and reducing power to potenially support chemosynthetic microbial communities. These communities may be important mediators of carbon and energy exchange between the deep Earth and the surface biosphere. Our work focuses on the Coast Range Ophiolite Microbial Observatory (CROMO) in Northern California where subsurface fluids are accessible through a series of wells. Preliminary analyses indicate that the highly basic fluids (pH 9-12) have low microbial diversity, but there is limited knowledge about the metabolic capabilities of these communties. Metagenomic data from similar serpentine environments  have identified Betaproteobacteria belonging to the order Burkholderiales and Gram-positive bacteria from the phylum Clostridiales, as key components of the serpentine microbiome. In an effort to better characterize the microbial community, metabolism, and geochemistry at CROMO, fluids from two representative wells (N08B and CSWold) were sampled during a recent field campaign. The wells selected can be differentiated in that N08B had cell counts ranging from 105 -106 cells mL-1 of fluid, and abundance of the Betaproteobacterium Hydrogenophaga. In contrast, fluids from CSWold have lower cell counts (~103 cells mL-1 ) and an abundance of Dethiobacter, a taxon within the phylum Clostridiales. Geochemical characterization of the fluids includes measurements of dissolved gases (H2, CO, CH4), dissolved inorganic and organic carbon, volatile fatty acids, and nutrients. Microcosm experiments were conducted with the purpose of monitoring carbon fixation and metabolism of small organic compounds, such as acetate, while tracing changes in fluid chemistry and microbial community composition. These experiments are expected to provide insight into the biogeochemical dynamics of the serpentinite subsurface at CROMO and represent a first step for developing RNA based Stable Isotope Probing (RNA-SIP) experiments to trace microbial activity at this site.
The nature of the energy yielding mechanisms in the lowenergy organic-poor sedimentary environment underlying the South Pacific Gyre (SPG) is not fully constrained. We used the approach of Wang et al. (2008) to quantify rates of organic-fuelled metabolic activities at most IODP Expedition 329 Sites (U1365 through U1370). At Site U1366 and U1370 net rates of oxygen-reducing organic oxidation averaged 1.77E-2 and 1.64E-3 fmol O2 cell-1 yr-1, respectively, representing a tremendously low cellular metabolism. At Site U1370, we observe net oxygen reduction throughout the entire sediment column. At Site U1366, statistically significant net oxygen reduction is not detected at depths greater than 11 meters below seafloor. Despite these low rates of organic oxidation, most cell counts are above the minimum detection limit throughout the entire sequence at both sites. Hydrogen from natural radioactive splitting of water has been hypothesized to be a significant electron donor in organic-poor sediment of the SPG. Becauses water radiolysis produces H2 and - O2 simultaneously, oxidation of this H2 does not contribute to net O2 reduction in the sediment. Our calculation of radiolytic H2 production, based on radioactive element content and sediment physical properties, indicate that on average 5.63E-1 and 9.79E-2 fmol H2 yr-1 cell-1 is available throughout the sequence at Sites U1366 and U1370, respectively. Despite these relatively high production rates, dissolved H2 abundances are below detection at both sites. These results suggest that H2 from in situ water radiolysis fuels the predominant energy-yielding pathway for microbes in SPG sediment.
In recent years, multiple research groups have tremendously advanced understanding of subseafloor sedimentary life. Microbes in subseafloor sediment are now known to be abundant, diverse and characterized by extraordinarily low mean rates of activity. Some discoveries challenge our sense of what is possible. For example, per-cell energy fluxes are far below the rates believed necessary for reproduction. What mechanisms might allow cells to reproduce at such low rates? Or do many of them live for millions of years without reproducing? Bulk population studies show that a very large fraction of these cells is active. However, we know essentially nothing about cell-to-cell variation in respiration, biomass turnover or reproduction. Furthermore, we do not clearly understand how organic-fueled respiration can persist for tens of myrs at very slow rates. Subseafloor community structure is largely unexplored. We have very limited understanding of the ways in which subseafloor microbes compete and almost no understanding of how they cooperate. Roles of viruses, eukaryotes, resting stages and bacterial spores in subseafloor ecosystems are largely unknown. The proximate causes and ultimate consequences of natural selection in subseafloor communities remain unknown. For the most part, we do not yet know the genetic potential of subseafloor microbes, the extent to which their potential is expressed, or the conditions under which they are expressed. The actual limits to subseafloor life are not yet known. Advancing understanding of these issues will yield fundamental insight into the nature of life.
Silicate minerals represent an important reservoir of essential nutrients at Earth's surface. Due to the slow kinetics of primary silicate mineral dissolution and the potential for nutrient sequestration by secondary mineral precipitation, the bioavailability of many silicate-bound nutrients may be limited by the ability of microorganisms to actively scavenge these nutrients via organic ligand production. In this study, the effectiveness of ligand production as a means to scavenge Fe from Fe-silicates is addressed through targeted laboratory experiments using olivine as a model mineral.
Preliminary results show that microbial Fe-binding ligands (i.e. siderophores) can accelerate olivine dissolution rates stoichiometrically by almost an order of magnitude in experiments buffered at circumneutral pH. In addition to higher reaction rates, organic Fe-binding ligands fostered the accumulation of dissolved Fe in solution, which was below detection in the abiotic experiments due to the precipitation of secondary Fe minerals in the presence of O2. Accelerated olivine dissolution rates in the presence of microbial Fe-binding ligands is somewhat unexpected because these ligands are known to be highly selective towards Fe3+ whereas olivine dominantly contains Fe2+. Spectrophotometric analysis of the ligand complexes produced during reaction with olivine reveals the dominance for Fe3+ -ligand complexes in solution.