NITROGENASE ACTIVITY OF MICROCOLEUS-LYNGBYACEUS MAT COMMUNITIES IN A EUTROPHIC, TROPICAL MARINE-ENVIRONMENT

Floating and attached cyanobacterial mats consisting primarily of Microcoleus lyngbyaceus, a nonheterocystous, filamentous cyanobacterium, are a permanent feature of the nearshore marine environment at La Parguera, Puerto Rico. Nitrogenase activity in these mat communities, as determined by acetylene reduction, exhibited a diel cycle with daytime inhibition and enhanced nocturnal activity. Contrary to reports regarding benthic cyanobacterial mats adapted to oligotrophic conditions, nitrogenase activity in M. lyngbyaceus mats is closely regulated by concentrations of NH4+ and NO3- within the range of values common to their environment. The opportunistic strategy of N utilization used by M. lyngbyaceus, whereby molecular N is primarily consumed only in the absence of significant concentrations of alternate inorganic N sources, assures the success of these cyanobacterial mat communities.