In situ analysis of oxygen consumption and diffusive transport in high‐temperature acidic iron‐oxide microbial mats

Summary The role of dissolved oxygen as a principal electron acceptor for microbial metabolism was investigated within Fe ( III )‐oxide microbial mats that form in acidic geothermal springs of Y ellowstone N ational P ark ( USA ). Specific goals of the study were to measure and model dissolved oxygen profiles within high‐temperature (65–75°C) acidic (pH = 2.7–3.8) Fe ( III )‐oxide microbial mats, and correlate the abundance of aerobic, iron‐oxidizing M etallosphaera yellowstonensis organisms and mRNA gene expression levels to Fe ( II )‐oxidizing habitats shown to consume oxygen. In situ oxygen microprofiles were obtained perpendicular to the direction of convective flow across the aqueous phase/ Fe ( III )‐oxide microbial mat interface using oxygen microsensors. Dissolved oxygen concentrations dropped from ∼ 50–60 μM in the bulk‐fluid/mat surface to below detection (< 0.3 μM) at a depth of ∼ 700 μm (∼ 10% of the total mat depth). Net areal oxygen fluxes into the microbial mats were estimated to range from 1.4–1.6 × 10 −4  μmol cm −2  s −1 . Dimensionless parameters were used to model dissolved oxygen profiles and establish that mass transfer rates limit the oxygen consumption. A zone of higher dissolved oxygen at the mat surface promotes Fe ( III )‐oxide biomineralization, which was supported using molecular analysis of M etallosphaera yellowstonensis 16 S rRNA gene copy numbers and mRNA expression of haem Cu oxidases ( FoxA ) associated with Fe ( II )‐oxidation.