SACCAMMINID FORAMINIFERS FROM ANOXIC SEDIMENTS: IMPLICATIONS FOR PROTEROZOIC ECOSYSTEM DYNAMICS

Eukaryote origin and earliest diversification occurred in the Proterozoic when Earth’s atmosphere was undoubtedly different from that of today. Atmospheric oxygen levels were increasing from the primordial anoxic atmosphere due, for example, to cyanobacterial oxygenic photosynthesis. Late Proterozoic (~0.6-0.9 Gya) deep-ocean oxygen concentrations are less certain, but geochemical evidence suggests anoxia and hydrogen-sulfide enrichment. It can, therefore, be postulated that initial eukaryotic diversification occurred in oxygen-depleted, sulfide-enriched environments. Foraminifera are aerobes and, thus, not expected in anoxic settings. Recently, however, we found a saccamminid allogromian in a deep-water anoxic, sulfidic setting. Samples were collected from Santa Barbara Basin (California) when bottom-water oxygen was undetectable and sediments smelled strongly of hydrogen sulfide. Foraminiferal SSU rDNA sequences recovered from sediments included one from a previously-uncharacterized saccamminid. Ultrastructural analysis indicated presence of intact Golgi, mitochondria, and prokaryotic endobionts. Saccamminid occurrence in environmental conditions known to exist during the Proterozoic supports the possibility of their origin early in eukaryotic evolution. Extant saccamminids could have competed well in the prokaryote-dominated Proterozoic benthic ecosystem given their diet includes bacteria, bacterial biofilms and unicellular algae. Thus, Proterozoic foraminifers may have been top carnivores. Funded by NASA NRA-01-01-EXB-057, the Geological Society of America’s W. Storrs Cole Memorial Research Award, and NSF OPP0003639.