Many symbiotic associations characteristic of tropical and subtropical oceanic waters were observed near shore during a long-term study of the microbiota in the northern part of the Gulf of Aqaba, Red Sea. Among such associations were the heterotrophlc dinophysoid genera Omithocercus, Histioneis and Citharistes with cyanobacterial symbionts. The detection of these heterotroph-autotroph consortia repeatedly coincided with extended nitrogen limitation in the fall season. Populations of free-living cyanobacteria, with known N fixation capability, such as the unicellular Synechococcus/ Synechocystis spp. and colonial forms, e.g. Trichodesmiurn spp., also peaked at the same time. We propose that heterotrophic dinoflagellate hosts may provide the cyanobacterial symbionts with the anaerobic microenvironment necessary for efficient N fixation. Thus, these self-supporting consortia increase in numbers during the long period of stratification and nitrogen limitation in the oligotrophic subtropical waters of the Gulf of Aqaba.
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