Genetic variability and nitrogenase activity of cyanobacterial communities associated with tropical seagrass meadows (western Indian Ocean)

Abstract: Tropical seagrass ecosystems are highly productive and important for sustaining marine life and associated coastal societies. In this study, the diversity and role of nitrogen-fixing cyanobacteria associated with five common seagrass genera in coastal regions of the western Indian Ocean (WIO; Tanzania) were examined, as well as the impact of anthropogenic activities. Cyanobacteria were characterized morphologically and genetically (16S rRNA and nifH gene phylogeny), as were diel variations in nifH gene expression, NifH protein levels and nitrogenase activity. The results revealed that WIO seagrass beds supported rich cyanobacterial diversity and that these represented approx. 83% of total clones obtained (DNA and RNA nifH clone libraries). Non-heterocystous genera, such as Oscillatoria, Lyngbya, Leptolyngbya, Phormidium and Microcoleus dominated, while heterocystous morphotypes such as Calothrix were less frequent and unicellular morphotypes (e.g. Gloeocapsa, Chroococcus and Chroococcidiopsis) were few. Additionally, the phylogenetic analysis revealed several novel uncharacterized cyanobacterial clades. Cyanobacterial composition and nitrogenase activity varied over seasons and between the seagrass species. Day time nitrogenase activity originated primarily from heterocystous phylotypes, while non-heterocystous filamentous phylotypes fixed nitrogen at night. The highest activity in the diel cycle was 358 ± 232 nmol C2H4 g-1 h-1at 09.00 associated with epiphytes of the seagrass Cymodocea. Nitrogenase activity was consistently lower in anthropogenically disturbed (eutrophication) seagrass sites. Such data suggest that diazotrophic cyanobacteria may be a significant source of ‘new’ nitrogen in the often oligotrophic coastal regions of tropical oceans. It is also proposed that the rapid shifts in the cyanobacterial population and function found may also be used as early disturbance indicator in coastal management practices.