The Great Oxidation Event occurred ~2.5 Ga, when the concentration of oxygen in the atmosphere increased from &lt;0.001% of the present atmospheric level (PAL) to within 1-10%. Following the initial rise, concentrations of O2 in the atmosphere and oceans remained well below present-day atmospheric levels through the Proterozoic until a second rise ~ 0.6 Ga to levels around those observed today. Thus, for much of Earth’s history, deep oceans probably remained oxygen-poor until the most recent increase in atmospheric O2. In addition to low levels of O2, at least portions of the oceans were euxinic (sulfide-rich) with H2S often reaching the photic zone. Oxygenic photosynthesis is the largest source of O2 in the atmosphere. Primary productivity and the remineralization of organic matter are intimately linked to planetary redox and thus to levels of O2. Using a Proterozoic-like environment, we aim to build a model of oxygen production and organic carbon burial in a sulfidic environment where Cyanobacteria contribute to primary productivity in the absence of oxygen accumulation. Here we report on the isolation and characterization of a cyanobacterium and a green sulfur bacterium that are the dominant members of pinnacle mats in Little Salt Spring—a karst sinkhole in Florida with perennially low levels of dissolved oxygen and sulfide. The red pinnacle mats bloom in the anoxic basin of the sinkhole and receive low levels of light to support photosynthesis. Characterization of the isolates is consistent with observations of oxygenic and anoxygenic photosynthesis in situ—both organisms perform anoxygenic photosynthesis under conditions of very low light. Despite the energetic favorably of oxygenic photosynthesis, this activity in the cyanobacterium isolate is inhibited by the presence of sulfide and under optimal light conditions, rates of anoxygenic photosynthesis by the cyanobacterium are nearly double that of oxygenic photosynthesis. The green sulfur bacterium is tolerant of oxygen and has a very low affinity for sulfide. Characterization of sulfide oxidation and primary productivity in the isolates coupled to 13C isotope fractionation will provide information critical for developing a model of primary productivity and oxygen levels on early Earth. Collectively, isotopes signatures, biosignatures, in situ data and pure culture studies have the potential to inform our understanding of biogeochemical cycling in the low oxygen Archean and Proterozoic.

Living on the edge: Model photoautotrophs from a Proterozoic ocean analog

253rd National Meeting (2017)

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