Utilization of substrate components during basaltic glass colonization by Pseudomonas and Shewanella Isolates

Many recent studies have shown that submarine basaltic rocks can host a diverse, well-developed microbial community and yet the ocean crust has been shown to be extremely oligotrophic, especially below its surface. This study demonstrates that iron-oxidizing and -reducing bacterial strains, isolated from Loihi Seamount off the southeast coast of the Big Island of Hawai'i, are able to utilize different nutrients (phosphate), electron donors (reduced iron as Fe(II)) and electron acceptors (oxidized iron as Fe(III)) found within basaltic glasses. To test whether microbial life is able to acquire specific required nutrients and energy sources directly from basaltic substrates under nutrient-limiting conditions, we prepared three different basaltic glass substrates: One amended with increased levels of phosphate (apatite), one with predominantly Fe(III) and one with predominantly Fe(II) and exposed these glasses in an annular reactor to a suite of metal-oxidizing and reducing isolates and a microbial mat consortium. Lithoautotrophic growth of Pseudomonas LOB-7, an obligate Fe(II)-oxidizing bacterium, was found on all basaltic substrates in excess of that found on a background borosilicate glass, while enhanced growth was observed on the apatite infused glass over other basaltic substrates when phosphate was absent in the growth medium. Anaerobic, heterotrophic growth of Shewanella 601R-1 with lactate revealed an ~2x increase in cell growth on the Fe(III)-enriched basalt. A parallel experiment performed using a natural inoculum from a Fe(III)-rich microbial mat revealed enhanced growth on all basalt surfaces over the background borosilicate glass. These results indicate that the chemical composition of basaltic substrates likely plays an important role in microbial colonization and enhanced growth under minimal nutrient conditions.