Figure 5.

The possible role of photothiotrophy in Earth's history. (A) Redox potentials of reductants for primary production vs time. Midpoint potentials are taken from sources listed in the legend of Fig. 1. GOE: great oxidation event. MIFs: mass-independent sulfur fractionations (a proxy for O₂ absence or presence under 10⁻⁵ present atmospheric levels in the atmosphere). Mass-dependent sulfur isotope effects are also pronounced prior to the GOE (not shown, see the text). Whiffs indicate the presence of oxidants in the water column or slight O₂ presence prior to the GOE. BIFs: banded iron formations. The column ‘primary production’ at right underscores the point that before the origin of chlorophototrophy, primary production was H₂-dependent and mechanistically dependent on flavin-based electron bifurcation (indicated by gray shading). The figure suggests that photothiotrophy preceded photoferrotrophy and water splitting in evolution. (B) A possible photothiotrophic sulfur cycle in Archean oceans before the GOE. See the text.