Production and cycling of natural microbial exopolymers (EPS) within a marine stromatolite

Extracellular polymeric secretions (EPS) that are produced by cyanobacteria represent potential structuring agents in the fori-nation of marine stromatolites. The abundance, production, and degradation of EPS in the upper layers of a microbial mat forming shallow subtidal stromatolites at Highborne Cay, Bahamas, were determined using C-14 tracer experiments and were integrated with measurements of other microbial community parameters. The upper regions of a Type 2 [Reid, R.P., Visscher, P.T., Decho, A.W., Stolz, J., Bebout, B., MacIntyre, I.G., Dupraz, C., Pinckney, J., Paerl, H., Prufert-Bebout, L., Steppe, T., Des Marais, D., 2000. The role of microbes in accretion, lamination and early lithification of modem marine stromatolites. Nature (London) 406, 989-992] stromatolite mat exhibited a distinct layering of alternating "green" cyanobacteria-rich layers (Layers 1 and 3) and "white" layers (Layers 2 and 4), and the natural abundance of EPS varied significantly depending on the mat layer. The highest EPS abundance occurred in Layer 2. The production of new EPS, as estimated by the incorporation of 14 C-bicarbonate into EPS, occurred in all layers examined, with the highest production in Layer I and during periods of photosynthesis (i.e., daylight hours). A large pool (i.e., up to 49%) of the total C-14- bicarbonate uptake was released as low molecular-weight (MW) dissolved organic carbon (DOC). This DOC was rapidly mineralized to CO2 by heterotrophic bacteria. EPS degradation, as determined by the conversion of C-14-EPS to (CO2)-C-14, was slowest in Layer 2. Results of slurry experiments, examining O-2 uptake following additions of organic substrates, including EPS, supported this degradation trend and further demonstrated selective utilization by heterotrophs of specific monomers, such as acetate, ethanol, and uronic acids. Results indicated that natural EPS may be rapidly transformed post-secretion by heterotrophic degradation, specifically by sulfate-reducing bacteria, to a more-refractory remnant polymer that is relatively slow to accumulate. A mass balance analysis suggested that a layer-specific pattern in EPS and low-MW DOC turnover may contribute to major carbonate precipitation events within stromatolites. Our findings represent the first estimate of EPS turnover in stromatolites and support an emerging idea that stromatolite formation is limited by a delicate balance between evolving microbial activities and environmental factors. (c) 2004 Elsevier B.V. All rights reserved.