Laboratory Investigation of the Role of Bacteria in the Weathering of Basalt Near Deep Sea Hydrothermal Vents

The principal goal of this study was to assess the potential role of bacteria on the weathering of basalts near deep-sea hydrothermal vents (DSHVs). Natural basalt samples were collected from the vicinity of Axial Seamount on the Juan de Fuca plate during the New Millennium Observatory (NeMO) 2000 cruise and characterized by scanning electron microscopy (SEM) and X-ray diffractometry. Bacteria were isolated from the naturally weathered basalt samples and used in a laboratory batch experiment. The bacteria (identified as marine gamma proteobacteria SWAT4) and unweathered basalt fragments were placed in artificial seawater in the presence and absence of an organic carbon supplement (analogous abiotic control systems were also included). These systems were incubated in the dark (4°C, ambient pressure) for four months, and optical density, pH, EH and concentrations of dissolved organic carbon, cations and anions were measured over time. Despite the limited data available (i.e., one system per set of chemical conditions) and the fact that contamination occurred in one abiotic system, we were still able to draw conclusions with respect to the role of bacteria in basalt weathering. The presence of the bacteria induced the release of Fe and Mn from the basalt relative to the abiotic controls, especially with the addition of the organic carbon supplement. At the conclusion of the experiment, the basalt fragments from the batch experiment were examined by SEM. The surfaces of the basalt fragments from the biotic systems, which were colonized by bacteria and showed evidence of secondary Fe-mineral precipitation, were very similar to the naturally weathered basalts. The results of this study therefore suggest that chemoorganotrophic bacteria are involved in the cycling of Fe and Mn and the weathering of basalt near DSHVs.