Nitrogen fixation (acetylene reduction) associated with communities of heterocystous and non-heterocystous blue-green algae in mangrove forests of Sinai

Summary High rates of nitrogen fixation (acetylene reduction) are associated with communities of heterocystous and non-heterocystous blue-green algae, which are widespread and abundant in the coastal mangrove forests of the Sinai Peninsula.Heterocystous forms, particularly representatives of the Rivulariaceae, grow in aerobic environments, where nitrogenase activity may be limited by the availability of nutrients such as Fe and PO₄–P. Desiccated communities of Scytonema sp. reduce acetylene within ten minutes of wetting by tidal sea water. Communities dominated by the non-heterocystous Hydrocoleus sp., Hyella balani, Lyngbya aestuarii, Phormidium sp. and Schizothrix sp., occur in close contact with anaerobic sediments and reduce acetylene in the dark as well as in the light.Nitrogen fixation in all these communities is light dependant and may be supplemented by an alternative source of reductant in the dark. The indications are that nitrogen fixation by these communities of blue-green algae, makes a significant contribution to the overall nitrogen input of the mangrove ecosystem.