Epiphytic cyanobacteria of the seagrass Cymodocearotundata: diversity, diel nifH expression andnitrogenase activityMariam Hamisi,1* Beatriz Díez,2†Thomas Lyimo,3Karolina Ininbergs2and Birgitta Bergman21College of Natural & Mathematical Sciences, TheUniversity of Dodoma, P.O. Box 338, Dodoma,Tanzania.2Department of Ecology, Environment and PlantSciences, Stockholm University, Lilla Frescati, SE-10691 Stockholm, Sweden.3Department of Molecular Biology & Biotechnology, Dares Salaam University, P.O. Box 35179, Dar es Salaam,Tanzania.SummarySeagrasses are photoautotrophic, ecologically im-portant components of many globally widespreadcoastal ecosystems, in which combined nitrogen maylimit their production. We examined the biodiversityand diazotrophic capacity of microbial epiphytesassociated with the phyllosphere of the seagrassCymodocea rotundata of the Western Indian Ocean.Light microscopy, 16S rRNA and nifH gene analysisrevealed the dominance of cyanobacteria in the epi-phytic microbial community. Most phylotypes wererelated to free-living uncultured benthic cyanobacte-ria, while some to cyanobacterial endosymbionts ofmarine diatoms. Novel and potentially diazotrophicspecies, some of known pantropical distribution,were also discovered. Significant diel nitrogenaseactivities (acetylene reduction assay) were recorded(up to 358 ⫾ 232 nmol C2H4g-1of seagrass FW h-1).The nifH gene expression patterns showed that het-erocystous phylotypes may be the dominant diazo-trophs during the day and non-heterocystous atnight. These data show that C. rotundata is colonizedby diverse diazotrophic cyanobacteria species andsuggest that these may be beneficial partners of