Diazotrophy in modern marine Bahamian stromatolites

N-2 fixation (nitrogenase activity), primary production, and diazotrophic c ommunity composition of stromatolite mats from Highborne Cay, Exuma, Bahama s, were examined over a 2-year period (1997-1998). The purpose of the study was to characterize the ecophysiology of N-2 fixation in modern marine str omatolites. Microbial mats are an integral surface component of these strom atolites and are hypothesized to have a major role in stromatolite formatio n and growth. The stromatolite mats contained active photosynthetic and dia zotrophic assemblages that exhibited temporal separation of nitrogenase act ivity (NA) and photosynthesis. Maximal NA was detected at night. Seasonal d ifferences in NA and net O-2 production were observed, photosynthetic activ ity and the availability of reduced organic carbon appear to be the key det erminants of NA. Additions of the de novo protein synthesis inhibitor chlor amphenicol did not inhibit NA in March 1998, but greatly inhibited NA in Au gust 1998. Partial sequence analysis of the nifH gene indicates that a broa d diversity of diazotrophs may be responsible for NA in the stromatolites.