MICROBIAL BIOTA FROM FRACTURED STROMATOLITE AND BIOFILM SAMPLES: BIOMARKERS FROM A HYPERSALINE LAKE

A variety of biota is found in biofilm and on and within stromatolite samples from the inland hypersaline Storr’s Lake on San Salvador Island in the Bahamas. Studies by Brigmon, using 16S rRNA analysis, indicate five genera of sulfate-reducing bacteria. Morphological observations with a field emission scanning electron microscope (FESEM) and an environmental electron microscope (ESEM) indicate diatoms, filamentous, rod-, ovoid- and spheroid-shaped features. Energy dispersive x-ray spectroscopy (EDS) indicates that the features contain magnesium, sulfur, silicon and calcium. The matrix material is primarily calcium carbonate.

Examination of fresh fracture surfaces of the stromatolite samples reveals abundant spheroids composed of four distinct size populations: large (m=5.3 mm), medium (m=2.0 mm), small (m=0.55 mm), and tiny (m=0.13 mm). Surface textures vary from smooth to rough. The two larger classes of spheroids are likely to be fossilized bacteria coccoids; they occur in clusters, are closely associated with the biofilm, have a limited size range, and are primarily carbonate composition.

The small and tiny spheres are frequently embedded in smooth, filmy biofilms, which vary in appearance from thick to filamentous to web-like to vesiculated. Averaging at 0.13 microns each, the tiny spheres are unlikely to be fossilized bacteria because of extremely small size and unusual composition: they are made of primarily magnesium and silicon, with no definitive carbon. We propose that they may be precipitates analogous to features described by Allen et al. (2000).

In addition to the coccoidal forms, the biofilm samples also contain abundant rod and dumbbell structures, varying in size from 2-4 microns lengthwise and similar to those produced by Chafetz et al. (1991). EDS analyses indicate calcite composition. We interpret these rods, hollow filaments, and dumbbell-shaped forms to be calcite precipitates nucleated by microbes.

If there was water once present on the surface of Mars or Europa, it is possible that subtidal pools formed that were similar to Storr’s Lake. Stromatolites could be the universal first step in life’s mass aggregation, in any environment where bacteria-like organisms form. Future work on stromatolites may further indicate their usefulness as biomarkers.