Elucidating the Mechanisms of Microbial Weathering of Submarine Basalts

In recent years there has been as increasing interest in microbe-mineral interactions, specifically the molecular mechanisms of mineral formation and dissolution. While not a true mineral, submarine basaltic glass represents an important rock surface and one of the most reactive components of the ocean crust. The high solubility of reduced glasses and the large disequilibrium with oxygenated seawater leads to large scale chemical exchange of Ca, Mg, Si, Al, Mn, Sr, as well as the pervasive oxidation of Fe(II). A variety of different mechanisms can be envisioned to contribute to the weathering of basalt, yet our basic understanding of what mechanisms actually occur and which are the most important is exceedingly small. To gain a comprehensive understanding of the mechanisms of basalt weathering it is necessary to be able to measure weathering rates, distinguish between biotic and abiotic components of weathering, and relate these rates to the various microbial processes that may be occurring. This requires an integration of geochemical, microbiological, molecular biological and mineralogical approaches. In addition, comparative studies between laboratory and field experiments and between different environments are necessary to assess the dominant pathways for basalt weathering. Given the chemical abundance and availability of reduced Fe and to a lesser extent, reduced Mn in basalts which may serve as energy sources, our group is focusing on bacteria that carry out redox transformations of these metals or produce compounds that complex these metals. Our approach includes cultivation and characterization of bacteria from natural basalt surfaces of various ages and from different environments, and using these isolates for laboratory studies of basalt colonization and weathering. Natural basaltic glass as well as synthetic basaltic substrates amended with enhanced concentrations of Mn, phosphate and varying Fe oxidation states have been placed back in the environment for exposure and retrieval after months to years for subsequent analysis of microbial populations and rates of weathering. Our primary study sites, the seamounts Loihi in Hawai'i and Vailulu'u in American Samoa, provide access to a wide range of environments characterized by different temperatures and chemistry and will allow us to assess the variety mechanisms of basalt weathering and their universality.