Ocean crust microbial biomass is more substantial than that of microbial life in the deep sea. The most likely source of energy for sustaining sub-seafloor microbial populations comes from redox-active iron that is slowly released from basalt glasses and minerals, including olivine and fayalite. Fluid flow creates a transport mechanism for microbes to colonize new crust as it forms, and it has been shown that older crusts have the highest biomass and greatest alteration. The main purpose of this study was to determine which minerals in basalt are preferentially colonized by bacteria, and why.

A variety of igneous minerals and glasses were incubated in microbial flow cells for four years in IODP borehole site 1301A on the eastern flank of the Juan de Fuca ridge. The total number of cells on each mineral were stained with DAPI and counted using epifluorescent microscopy. Oligotrophic mesophiles and thermophiles were isolated on marine R2A plates and reported as colony forming units (CFUs) per gram of mineral. Isolate 3d.6 from fayalite was incubated in olivine cultures to determine iron oxidation and reduction capabilities.

Total cell counts reflect a preference among bacteria for iron-rich minerals from basalt. Thermophilic bacteria were isolated from olivine and fayalite only, and mesophilic bacteria were present in larger numbers on the iron-rich minerals. Our results also indicate iron oxidation of olivine minerals by isolate 3d.6. This bacteria is was not found to be capable of iron reduction in olivine cultures with either lactate, acetate, succinate, or pyruvate.

Our results suggest iron is an important source of redox energy in the ocean crust which helps support a greater biomass. Our results also indicate that olivine is capable of supporting iron-oxidizing populations of bacteria.