HETEROTROPHIC DINOFLAGELLATES WITH SYMBIOTIC CYANOBACTERIA AND NITROGEN LIMITATION IN THE GULF-OF-AQABA

Many symbiotic associations characteristic of tropical and subtropical oceanic waters were observed near shore during a long-term study of t he microbiota in the northern part of the Gulf of Aqaba, Red Sea. Amon g such associations were the heterotrophic dinophysoid genera Ornithoc ercus, Histioneis and Citharistes with cyanobacterial symbionts. The d etection 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 t he unicellular Synechococcus/Synechocystis spp. and colonial forms, e. g. Trichodesmium spp., also peaked at the same time. We propose that h eterotrophic dinoflagellate hosts may provide the cyanobacterial symbi onts with the anaerobic microenvironment necessary for efficient N fix ation. Thus, these self-supporting consortia increase in numbers durin g the long period of stratification and nitrogen limitation in the oli gotrophic subtropical waters of the Gulf of Aqaba.