Spatial distribution of symbiont-bearing dinoflagellates in the Indian Ocean in relation to oceanographic regimes

The spatial distribution of symbiont-bearing dinoflagellates was investigated during a cruise from Cape Town, South Africa to Broome, Australia. Endo- and ectosymbionts were only found in the order Dinophysiales. The genera Ornithocercus, Histioneis, Parahistioneis and Citharistes had cyanobacteria as ectosymbionts, while the genera Amphisolenia and Triposolenia contained both intact cyanobacterial and eukaryotic endosymbionts. The symbiont-bearing dinoflagellates were mainly found in the upper 100 m of the water column. Their distribution was restricted to water temperatures exceeding 16.5 degrees C, and the highest species diversity and cell concentrations were found at temperatures around 20 to 30 degrees C. The symbiont-bearing dinoflagellates were always associated with water masses with low nutrient (N-limited) and chl a concentrations. Special attention was given to the ectosymbiont-bearing dinoflagellates. Under light microscopy, some of the food vacuoles of Ornithocercus spp. resembled ectosymbionts in size, shape and colour. Transmission electron microscopy of O. magnificus and O. quadratus revealed the presence of a peduncle and many rhabdosomes; both may serve in prey capture. Also, numerous food vacuoles were present, but their content was generally too degraded for a proper identification of prey type. However, occasionally remnants of eukaryotes were observed, indicating that Ornithocercus spp. may feed on ciliates. Thus, our data suggest that the ectosymbiont-bearing dinoflagellates use a multi-resource strategy (photosynthesis and phagotrophy) to cope with a low-nutrient environment.