Microbes in a bottle: Where model organisms and analog systems meet

Abstract:

Understanding the evolution of the Earth's surface chemistry is one of the most exciting challenges in modern geoscience. The Great Oxidation Event occurred ~2.5 Ga, when the concentration of oxygen in the atmosphere increased from <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. As a result, biologic carbon isotope fractionation and other biomarkers (i.e. hopanoids) facilitate our interpretation of biogeochemical cycling during the Proterozoic Eon.

Here, we describe the isolation and characterization of two photoautotrophs—the dominant primary producers—from a Proterozoic Ocean analog. We examined the ¹³C fractionation in the microbial mat and employed in situ microcosms to estimate primary productivity. In addition, we deployed diver-operated microsensors to determine oxygen production and sulfide consumption over a 24-hour cycle and sequenced total RNA from 4 time points. Using these data, we examined primary production in pure cultures of the dominant Cyanobacteria and green sulfur bacteria from the mat under conditions that mimic those observed in situ. We use this information to begin to build a model of oxygen production and organic carbon burial in a Proterozoic-like environment where Cyanobacteria can contribute to primary productivity in the absence of oxygen production. Furthermore, we examined the differences between ¹³C fractionation in cultures maintained under “ideal” conditions compared to those observed in situ. Collectively, the RNA sequencing data, the in situ primary productivity data and pure culture information were necessary to interpret the ¹³C signal from the mats.