Insights into the Global Microbial Community Structure Associated with Iron Oxyhydroxide Minerals Deposited in the Aerobic Biogeosphere

Over the last decade or so, several studies have investigated the formation, mineralogy and microbial composition of iron oxyhydroxides. Here, we focus on the exploration of the microbial community structure of iron oxyhydroxide minerals that often occur in tight association with microbial life, which are referred to as Biogenic Iron Oxyhydroxides (BIOS). BIOS deposits host a wide range of microbial taxa usually found living in the oxic, suboxic and anaerobic interfaces of the biogeosphere. These organisms are responsible for the global cycling of Fe, N, C and S. Near the surface of BIOS mats, O2 at ambient atmospheric levels rapidly reacts with dissolved Fe(II) to form insoluble Fe(III) minerals. Due to the activity of highly efficient heterotrophic O2 respirers, BIOS biofilms usually transit from the top to the bottom (within a mm range) into a suboxic zone where microaerophilic iron-oxidizing bacteria thrive, transforming Fe(II) to Fe(III), while generating organic carbon. The bulk of the Fe(III), produced by both biotic and abiotic processes, is precipitated and fixed into a variety of cellular matrices. The resulting iron oxyhydroxides provide highly reactive surfaces responsible for the secondary sequestration of a vast range of trace elements. Given the fact that the mineralogical composition of BIOS is fairly well known, we focus here on the diversity of the microorganisms associated with BIOS deposited over wide geographical scales. This manuscript also presents condensed information that demonstrates that the biogeochemistry of BIOS appears to provide niche-specific conditions that select for similar microbial taxa across different spatial scales. We suggest that globally, these lineages play an important and similar role in BIOS development in the aerobic biogeosphere regardless of the location.