Microbial Community Composition in Modern Thrombolites: A Comparison of Open Marine and Freshwater Systems Using Molecular Techniques

Thrombolites are found in the fossil record as far back as 1.92 billion years ago and are still actively growing at localities around the world. While fossil thrombolites are found primarily in open marine paleoenvironments, modern thrombolites form in open marine, sheltered marine, freshwater, and hypersaline waters. We examined thrombolites in two of these environments (open marine in Highborne Cay, Exumas, Bahamas and freshwater in Green Lake, Fayetteville, NY) to see if the structures were formed in the same manner by similar microorganisms. At Highborne, the top layers of the thrombolite heads are composed of four different microbial mats. Each mat type had a unique microbial community that appeared to represent different colonization stages. At Green Lake, the top layer of the thrombolites on the main platform was characterized by one of two different microbial mat types, though smaller thrombolitic heads and the sides of the platform lacked these mats. Consumption and production of oxygen and dissolved inorganic carbon were measured for each of the thrombolite mat types (if present) and internal structure (if mats were not present) from both locations over three consecutive diel cycles. While the overall pattern was consistent with expectations, each mat type responded differently to environmental conditions, specifically changes in light. Denatured Gradient Gel Electrophoresis (DGGE) was run for each of the mat types and the internal structure from both locations. Each mat type had a slightly different microbial assemblage. The variation in community composition may help to explain how microbialites with broadly similar morphologies are adapted to very different environments.