Potential for Iron-reduction and Iron-cycling in Iron Oxyhydroxide-rich Microbial Mats at Loihi Seamount

Abstract

The summit of Loihi Seamount (∼1000 mbsl) is host to active hydrothermalism with vent fluids that are rich in Fe(II), resulting in copious deposition of microbial iron mats. A variety of freshly collected mat samples were assessed for their potential to support Fe-reduction, by incubating them under anaerobic conditions with 5 mM acetate. Ten of eleven samples showed active Fe-reduction within 5 to 10 days. Most probable number (MPN) analysis for Fe-reducing bacteria indicated they were present, typically around 103 cells . ml−1, but composed well under 1% of the total bacterial population. A novel, obligately anaerobic Fe-reducing bacterium, Geothermobacter, strain HR-1, was isolated from one of the mats, and was used in experiments to test Fe-reduction rates under different conditions. Strain HR-1 initiated growth more rapidly on biogenic oxides than synthetically prepared ferrihydrite, furthermore magnetite production was not observed in cells grown on biogenic oxides, but was present when cells were grown on synthetic ferrihydrite. When Fe-mats were inoculated with HR-1 and 5 mM acetate rapid iron reduction occurred; in mat samples not amended with acetate the rates and amounts of Fe-reduction were significantly less, suggesting that the system was limited for C. Similar results, with even more pronounced differences between acetate-amended and unamended samples, were found when HR-1 was inoculated into biogenic oxides formed by a pure culture of Mariprofundus ferrooxydans, a common Fe-oxidizing bacterium in the Loihi Fe mats. Together these results suggest that Fe-reducing bacteria (FeRB) are present in the iron mats at Loihi, albeit at low numbers, and that in situ they contribute only in a minor way to the iron budget at Loihi. The results are discussed in the context of bacterial iron cycling and the implications for understanding the genesis of banded iron formations.