Benthic microbial mats: important sources of fixed nitrogen and carbon to the Twin Cays, Belize ecosystem

We collected surface microbial mats at sites on Twin Cays, Belize to determine rates of primary production and nitrogen transformations. A diverse array of cyanobacteria including filamentous, coccoidal and heterocystous cyanobacteria, as well as purple sulfur bacteria and heterotrophic bacteria, were important components of microbial mat communities. Sediment chlorophyll a concentrations illustrated a high photosynthetic biomass in surface sediments. Rates of primary carbon fixation, measured as gross oxygenic photosynthesis, and nitrogen fixation and denitrification, measured using specific metabolic inhibitors, were determined during day-and-night incubations. Primary production rates were similarly high across different mat types. Nitrogen fixation rates were substantial under in situ conditions and nighttime activity frequently exceeded daytime activity. In situ denitrification rates were very low in all incubations. In the presence of added nitrate, however, denitrification rates increased significantly during daytime and nighttime incubations so that they equaled or exceeded nitrogen fixation rates. Collectively, our data show that microbial mats are a significant source of fixed carbon and nitrogen to the Twin Cays ecosystem and suggest that mats may serve as an important component of the ecosystem's food web.