Archean Photoautotrophy: Some Alternatives and Limits

Abstract

From the Archean geological record, one can infer that photoautotrophy evolved early in earth history; however, the nature of this photosynthesis - whether it was predominantly bacterial or cyanobacterlal - is tess clearly understood. General agreement that the earth's atmosphere did not become oxygen rich before tile Early Proterozoic era places constraints on theories concerning more ancient biotas. Accommodating this limitation in various ways, different workers have hypothesized (1) that blue-green algae first evolved in the Early Proterozoic; (2) that oxygen producing proto-cyanobacteria existed in the Archean, but had no biochemical mechanism for coping with ambient O2; and (3) that true cyanobacteria flourished in the Archean, but did not oxygenate the atmosphere because of high rates of oxygen consumption caused, in part, by the emanation of reduced gases from widespread Archean volcanoes. Inversion of hypothesis three leads to another, as yet unexplored, alternative. It is possible that physiologically modern blue-green algae existed in Archean times, but had low productivity. Increased rates of primary production in the Early Proterozoic era resulted in the atmospheric transition documented in strata of this age. An answer to the question of why productivity should have changed from the Archean to the Proterozoic may fie in the differing tectonic frameworks of the two areas. The earliest evidence of widespread, stable, shallow marine platforms is found in Lower Proterozoic sedimentary sequnces. In such environments, productivity was, and is, high. In contrast, Archean shallow water environments axe often characterized by rapid rates of clastic and pyroclastic influx - conditions that reduce rates of benthonic primary production. This hypothesis suggests that the temporal correlation of major shifts in tectonic mode and atmospheric composition may not be fortuitous. It also suggests that sedimentary environments may have constituted a significant limit to the abundance and diversity of early life.