Bacteria have been implicated as both a source and sink of hydrogen peroxide (H 2 O 2 ), a reactive oxygen species which can both impact microbial growth and participate in the geochemical cycling of trace metals and carbon in natural waters. In this study, simultaneous H 2 O 2 production and decay by twelve species of heterotrophic bacteria were evaluated in both batch and flow-through incubations. While wide speciesto-species variability of cell-normalized H 2 O 2 decay rate coefficients [2 × 10 −8 to 5 × 10 −6 hr −1 (cell mL −1 ) −1 ] was observed, these rate coefficients were relatively consistent for a given bacterial species. By contrast, observed production rates (below detection limit to 3 × 10 2 amol cell −1 hr −1 ) were more variable even for the same species. Variations based on incubation conditions in some bacterial strains suggest that external conditions may impact extracellular H 2 O 2 levels either through increased extracellular production or leakage of intracellular H 2 O 2 . Comparison of H 2 O 2 production rates to previously determined superoxide (O 2 − ) production rates suggests that O 2 − and H 2 O 2 production are not necessarily linked. Rates measured in this study indicate that bacteria could account for a majority of H 2 O 2 decay observed in aqueous systems but likely only make a modest contribution to dark H 2 O 2 production.

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Hypersaline paleolake sediments are understudied ecosystems whose microbial ecology is largely unknown. Here we present mineralogical, geochemical, and small-subunit 16S rRNA gene sequence data on one such environment, the Pilot Valley Basin (PVB), a sub-basin of ancient Lake Bonneville located in northwest Utah. PVB exhibits a variety of aqueous minerals including phyllosilicates, carbonates, and sulfates, as well as microbially-induced sedimentary structures. As perchlorate occurs naturally (up to 6.5 ppb) in Pilot Valley sediments, and because recent evidence suggests that it is subject to biotic reduction, PVB has been proposed as a Mars analog site for astrobiological studies. 16S rRNA gene sequencing was used to investigate microbial diversity and community structure along horizontal and vertical transects within the upper basin sediments and beta diversity analyses indicate that the microbial communities in Pilot Valley are structured into three discrete groups. Operational taxonomic units (OTUs) belonging to the main archaeal phylum, Euryarchaeota, make up ~23% of the sequences, while OTUs belonging to three bacterial phyla, Proteobacteria, Bacteroides and Gemmatimonadetes, constitute ~60-70% of the sequences recovered at all sites. Diversity analyses indicate that the specific composition of each community correlates with sediment grain size, and with biogeochemical parameters such as nitrate and sulfate concentrations. Interestingly, OTUs belonging to the phylum Gemmatimonadetes are co-located with extreme halophilic archaeal and bacterial taxa, which suggests a potential new attribute, halophilicity, of this newly-recognized phylum. Altogether, results of this first comprehensive geomicrobial study of Pilot Valley reveal that basin sediments harbor a complex and diverse ecosystem.

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