Microbial mats as bioreactors: populations, processes, and products

Microbial mats are dynamic and complex ecosystems exhibiting spatial and temporal heterogeneity. The physical/chemical environment is typified by steep gradients and distinct microenvironments. These microenvironments support a great diversity of species with a wide range of metabolic processes. These processes often result in coupled reactions and biogeochemical cycles, and produce important end products such as trace gases and mineral precipitates. The latter can impact the composition and character of the sediment, imparting a “biosignature.” These biosignatures can be preserved in the rock record and are useful in the interpretation of fossil record on Earth and possibly as an indication of life on other planetary bodies. The modern marine stromatolites of the Exuma Cays, Bahamas, provide an ideal system for studying the populations, processes, and products in a microbial ecosystem using a multidisciplinary approach. In order to acquire redox energy, microbial populations need to carry out metabolic reactions at rates faster than the equivalent chemical (abiotic) reactions. As such, microbes can be viewed as bioreactors that preferably oxidize carbon to CO2 to maximize the energy yield. The study of the microbial role in carbonate sedimentation and lithification in these stromatolites provides a picture of microbial mats as bioreactors producing a biosignature.