“What are microbialites?”
Microbialites are organosedimentary structures that are the direct result of the interactions between microbes and their surrounding environment. Although once dominant on the ancient Earth, today microbialites are rare, found only in unique and sometimes extreme locations around the globe.
These complex microbial ecosystems have a propensity for the trapping, binding, and precipitation of organosedimentary structures such as calcium carbonates. Microbialite structures can range from the well-structured, laminated stromatolites to the unlayered, clotted thrombolites. Although once dominant on ancient Earth, today modern analogues of these microbial ecosystems are relegated to a few locations around the globe.
One such location is the margin of Exuma Sound, Bahamas, the only known site of actively forming stromatolites in the open ocean. Within the Exuma Sound, the stromatolites of the islands of Highborne Cay and Little Darby represent well-laminated microbialites.
“What organisms make microbialites?”
Modern microbialites are the product of the metabolisms and activities of microbial mats.
Five groups of microbes found in the mats are key to microbialite development in the Bahamas and include: cyanobacteria, aerobic heterotrophic bacteria, sulfate-reducing bacteria, sulfide-oxidizing bacteria, and fermentative bacteria. Together these groups of organisms generate steep vertical chemical gradients within the stromatolites (i.e., oxygen, sulfide) with large diel fluctuations. These gradients can abruptly change throughout the vertical stratification. The dominant organisms responsible for this oxygen-rich layer are photoautotrophic cyanobacteria.
For more specific detail on the microbial diversity in microbialites please refer to these publications.
Microbial-mediated carbon cycling in a stromatolitic mat is coupled with the elemental cycles of O, S, N, Fe which serve as e- donors/acceptors. Differential use of one cycle over another can increase the precipitation potential (e.g. S>O) and influence the saturation index (e.g. excess anaerobic respiration; modified from Dupraz et al., 2008).
“Why are microbialites important ecosystems to study?”
Microbialites represent modern analogues to ancient Earth ecosystems. By studying these communities we can gain an understanding of the origins and evolution of life on Earth.
Microbialites undergo extensive sequestration of carbon through the process of organomineralization. The microbial mats precipitate the carbon as calcium carbonate (limestone). Microbialites can increase the sink of carbon in the water through microbial metabolisms such as photosynthesis. These ecosystems, while rare on the modern Earth, may provide novel insights into the microbes and molecular pathways that enhance and promote the precipitation of calcium carbonates.
As previously mentioned the microbial mats that make these lithified carbonate structures are dominated by cyanobacteria. These cyanobacteria also have demonstrated high levels of tolerance to environmental stress, specifically UV radiation and oxidative stress. Understanding the molecular mechanisms by which these organisms tolerate high UV and reactive oxygen species may elucidate novel pathways to protect cells from the damaging cellular effects of these
Key functional groups of microbes associated with microbial mats and their metabolisms that result either in carbonate precipitation or dissolution. Equations represent combined metabolic and geochemical reactions. Those reactions that occur during the day are labeled with yellow arrows, those reaction that occur at night are noted in blue. Modified from Dupraz et al., 2009
Little Darby Research Station, Exumas, Bahamas
The Little Darby Research Station is a new marine research lab that opened in September 2009. The lab is directed by Dr. Pamela Reid of the University of Miami’s Rosenstiel School of Marine and Atmospheric Science and The Danguillecourt Project (http://tropicbirds.org). The station is dedicated to the study and preservation of the Bahamian environment through scientific research and educational outreach. A particular focus is modern marine stromatolites.