A case study for late Archean and Proterozoic biogeochemical iron‐ and sulphur cycling in a modern habitat—the Arvadi Spring

Abstract

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 , 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.

Distinguishing Biotic and Abiotic Iron Oxidation at Low Temperatures

“…It is possible that FeOB occur in microenvironments or oxidize iron at rates below the detection limits of the experiments. Indeed, Koeksoy et al 63 used most probably number counts to determine that a small community of FeOB inhabit Arvadi spring, a location featuring rapid abiotic Fe(II) oxidation where our method did not detect a biological signal. In most experiments, the difference in the rate of biological and abiotic oxidation must be greater than 0.1 μM/min for the standard protocol or 0.05 μM/min for the modified protocol to determine that biology is enhancing the rate.…”

Microbial community response to hydrocarbon exposure in iron oxide mats: an environmental study

“…These result in total represent possible connections between hydrocarbons and the nitrogen, iron, and sulfur cycles and suggest that the community within the iron mat could potentially contribute to bioremediation of benzene or other hydrocarbons. Freshwater iron mats have been previously hypothesized to connect the iron and sulfur cycles (Koeksoy et al, 2018;Brooks and Field, 2021); however, connections to the nitrogen cycle have not been previously proposed. Of interest, future studies would be whether microbial activity rates in the iron mat are the same when exposed to hydrocarbons, given the previously discussed loss of diversity.…”

Orange leads to black: evaluating the efficacy of co‐culturing iron‐oxidizing and sulfate‐reducing bacteria to discern ecological relationships

“…FeOB have been hypothesized to interact, either directly or indirectly, with a variety of functional guilds including iron‐reducing, sulfur‐oxidizing and methanogenic bacteria (Melton et al ., 2014). Potential symbioses have also been proposed between FeOB and oxygenic phototrophs (Field et al ., 2016) as well as between FeOB and sulfate‐reducing bacteria (SRB) (Li et al ., 2006; Bruun et al ., 2010; McBeth et al ., 2013; Mumford et al ., 2016; Koeksoy et al ., 2018). A symbiotic relationship between FeOB and other functional guilds could have important implications for biogeochemical cycling in coastal environments.…”