A case study for late Archean and Proterozoic biogeochemical iron- and sulphur cycling in a modern habitatthe Arvadi Spring

As a consequence of Earth's surface oxygenation, ocean geochemistry changed from ferruginous (iron(II)-rich) into more complex ferro-euxinic (iron(II)-sulphide-rich) conditions during the Paleoproterozoic. This transition must have had profound implications for the Proterozoic microbial community that existed within the ocean water and bottom sediment; in particular, iron-oxidizing bacteria likely had to compete with emerging sulphur-metabolizers. However, the nature of their coexistence and interaction remains speculative. Here, we present geochemical and microbiological data from the Arvadi Spring in the eastern Swiss Alps, a modern model habitat for ferro-euxinic transition zones in late Archean and Proterozoic oceans during high-oxygen intervals, which enables us to reconstruct the microbial community structure in respective settings for this geological era. The spring water is oxygen-saturated but still contains relatively elevated concentrations of dissolved iron(II) (17.2 ± 2.8 μM) and sulphide (2.5 ± 0.2 μM) with simultaneously high concentrations of sulphate (8.3 ± 0.04 mM). Solids consisting of quartz, calcite, dolomite and iron(III) oxyhydroxide minerals as well as sulphur-containing particles, presumably elemental S ⁰, cover the spring sediment. Cultivation-based most probable number counts revealed microaerophilic iron(II)-oxidizers and sulphide-oxidizers to represent the largest fraction of iron- and sulphur-metabolizers in the spring, coexisting with less abundant iron(III)-reducers, sulphate-reducers and phototrophic and nitrate-reducing iron(II)-oxidizers. 16S rRNA gene 454 pyrosequencing showed sulphide-oxidizing Thiothrix species to be the dominating genus, supporting the results from our cultivation-based assessment. Collectively, our results suggest that anaerobic and microaerophilic iron- and sulphur-metabolizers could have coexisted in oxygenated ferro-sulphidic transition zones of late Archean and Proterozoic oceans, where they would have sustained continuous cycling of iron and sulphur compounds.