Nitrogen fixation by blue-green algal communities in the intertidal zone of the lagoon of Aldabra Atoll.

All eight types of blue-green algal community sampled from the intertidal sediments of the lagoon of Aldabra showed detectable rates of acetylene reduction, and six of them showed at least some examples of rates considered by previous authors to be rapid. When the reduction rates are related to the chlorophyll a contents of the communities, the maximum rates recorded come in the following order: Hyella balani, Scytonema sp., Hyella balani-Schizothrix sp., Calothrix crustacea, Rivularia sp., Microcoleus chthonoplastes, Hyella balani-purple sulphur bacteria, Pleurocapsa-Chroococcus. With the exception of the mixed Hyella balani-purple sulphur bacterial community, the rate of acetylene reduction was in all experiments greater in the light than in the dark, the difference being significant (P<0.05) in the majority of cases. The rates were similar in all experiments whether incubation was carried out with sea or brackish water. It seems probable that nitrogen fixation by blue-green algal communities makes an important addition to the lagoon ecosystem, and that not only heterocystous, but also some non-heterocystous, species are involved. Among the latter, the data for Hyella balani and Microcoleus chthonoplastes are especially convincing.