Cell specific density of symbiotic dinoflagellates in tropical anthozoans

Symbiotic dinoflagellates are abundant in the endoderm cells of tropical ma rine anthozoans, but the cell-specific density (CSD) of symbionts has not y et been investigated. In this study we used mechanical and enzymatic method s of maceration, and staining with substrate-specific fluorochromes, to obs erve a large number of individual host cells from 33 species of tropical an thozoans collected in Florida, Hawaii and Jamaica or cultured in Monaco. In the majority of species, most of the host cells contained a single algal c ell (singlet). Host cells with two or more (up to six) algae were much less abundant. The average CSD for the 33 species was 1.54 +/- 0.30 (range 1.11 to 2.19). Singlets arranged in a monolayer can account for the areal densi ty of algae observed in many anthozoans. The dinoflagellates occupy most of the interior of macerated host cells, leaving the host cytoplasm and cell membrane as a thin outer layer, often unresolvable by light microscopy. Thi s spatial arrangement may favor diffusion and transport of CO2, bicarbonate ions, and nutrients from the environment to the algae. The effect of nutri ent enrichment on CSD was determined by exposing eleven species to chronica lly elevated levels of ammonium-N. After four weeks all species exhibited a dramatic increase in algal mitotic index and CSD. The potential consequenc es of environmentally induced increases in CSD in tropical anthozoans are d iscussed in terms of the decreased cell-specific photosynthesis (CO2 limita tion) and decreased rates of calcification observed in other studies.