Hydrogen Peroxide Cycling in Acidic Geothermal Environments and Potential Implications for Oxidative Stress

Abstract:

Hydrogen peroxide (H₂O₂) may be produced in natural waters via photochemical reactions between dissolved oxygen, organic carbon and light. Other reactive oxygen species (ROS) such as superoxide and hydroxyl radicals are potentially formed in environments with high concentrations of ferrous iron (Fe(II), ~10-100 µM) by reaction between H₂O₂ and Fe(II) (i.e., Fenton chemistry). Thermophilic archaea and bacteria inhabiting acidic iron-oxide mats have defense mechanisms against both extracellular and intracellular peroxide, such as peroxiredoxins (which can degrade H₂O₂) and against other ROS, such as superoxide dismutases. Biological cycling of H₂O₂ is not well understood in geothermal ecosystems, and geochemical measurements combined with molecular investigations will contribute to our understanding of microbial response to oxidative stress. We measured H₂O₂ and other dissolved compounds (Fe(II), Fe(III), H₂S, O₂), as well as photon flux, pH and temperature, over time in surface geothermal waters of several acidic springs in Norris Geyser Basin, Yellowstone National Park, WY (Beowulf Spring and One Hundred Spring Plain). Iron-oxide mats were sampled in Beowulf Spring for on-going analysis of metatranscriptomes and RT-qPCR assays of specific stress-response gene transcription (e.g., superoxide dismutases, peroxiredoxins, thioredoxins, and peroxidases). In situ analyses show that H₂O₂ concentrations are lowest in the source waters of sulfidic systems (ca. 1 μM), and increase by two-fold in oxygenated waters corresponding to Fe(III)-oxide mat formation (ca. 2 - 3 μM). Channel transects confirm increases in H₂O₂ as a function of oxygenation (distance). The temporal dynamics of H₂O₂, O₂, Fe(II), and H₂S in Beowulf geothermal waters were also measured during a diel cycle, and increases in H₂O₂ were observed during peak photon flux. These results suggest that photochemical reactions may contribute to changes in H₂O₂. We hypothesize that increases in H₂O₂ and O₂ concentrations in iron-oxidizing habitats will induce higher transcription rates of genes responsible for H₂O₂ degradation and O₂ respiration. Subsequent measurements of additional ROS and analysis of transcript data will provide broader insight on the interactions and metabolic response within iron mat communities under oxidative stress.