Diazotrophy and diversity of benthic cyanobacteria in tropical coastal zones

Discoveries in recent years have disclosed the importance of marine cyanobacteria in the context of primary production and global nitrogen cycling. It is hypothesized here that microbial mats in tropical coastal habitats harbour a rich diversity of previously uncharacterized cyanobacteria and that benthic marine nitrogen fixation in coastal zones is substantial. A polyphasic approach was used to investigate cyanobacterial diversity in three tropical benthic marine habitats of different characters; an intertidal sand flat and a mangrove forest floor in the Indian Ocean, and a beach rock in the Pacific Ocean. In addition, nitrogenase activity was measured over diel cycles at all sites. The results revealed high cyanobacterial diversity, both morphologically and genetically. Substantial nitrogenase activity was observed, with highest rates at daytime where heterocystous species were present. However, the three habitats were dominated by non-heterocystous and unicellular genera such as Microcoleus, Lyngbya, Cyanothece and a large group of thin filamentous species, identified as members of the Pseudanabaenaceae family. In these consortia nocturnal nitrogenase activities were highest and nifH sequencing also revealed presence of noncyanobacterial potential diazotrophs. A conclusive phylogenetic analysis of partial nifH sequences from the three sites and sequences from geographically distant microbial mats revealed new clusters of benthic potentially nitrogen-fixing cyanobacteria. Further, the non-heterocystous cyanobacterium Lyngbya majuscula was subjected to a physiological characterization to gain insights into regulatory aspects of its nitrogen fixation. The data demonstrated that nitrogenase activity is restricted to darkness, which called upon a re-evaluation of its diazotrophic behaviour.