WERE PROTISTS THE BEGINNING OF THE END FOR STROMATOLITES?

The oldest fossil stromatolites are >3.4 billion years old and are the most visible manifestations of pervasive microbial life on the early Earth. The changes in stromatolite abundance and morphology over time document complex interplays between biological and geological processes. We focus on one of the greatest geological enigmas: the possible connection between stromatolite decline and the rise of complex life. While much is known about microbialite autotrophs, little is known about their heterotrophic eukaryotes. Details of stromatolite formation and preservation are poorly understood, and a drastic decline in stromatolite occurrence and diversity in the late Precambrian has long been a conundrum. A popular hypothesis to explain this decline at ~1 billion years ago is that eukaryotic organisms evolved to become predators on stromatolites. To date, the most commonly proposed predatory culprit is an unidentified metazoan, although evidence of such an organism is lacking from the fossil record. Protists, most of which are not expected to leave an obvious fossil record, are additional possible stromatolitic predators, but they have been largely ignored in this context. We have been testing the hypotheses that (1) Heterotrophic protist activity caused the textural change from stromatolites (layered sediment fabric) to thrombolites (clotted sediment fabric) and (2) Heterotrophic protists caused the decimation of Neoproterozoic stromatolites. It is impossible to recreate the Neoproterozoic, so experimental, ecological, microscopic, molecular, and chemical studies of modern analogs must serve to indirectly test our hypotheses. Highlighted in this talk will be our initial results on Highborne Cay (Bahamas) and Hamelin Pool (Australia) microbialites. For example, Laser Scanning Confocal Micrographs of Fluorescently Labeled Embedded Cores (FLEC, Bernhard et al. 2003 L&O) will show life positions of eukaryotes in different microbialites from both sites. Comparative CAT scans of the same cores will show sediment fabrics. Also presented will be initial results, using the same methods, from an experiment where allogromiid foraminifera were seeded onto unlithified smooth stromatolites to determine the effect of their pseudopodial activities on stromatolite fabric. Supported by NSF OCE-0926421.