Preferential expulsion of dividing algal cells as a mechanism for regulating algal-cnidarian symbiosis.

A wide range of both intrinsic and environmental factors can influence the population dynamics of algae in symbiosis with marine cnidarians. The present study shows that loss of algae by expulsion from cnidarian hosts is one of the primary regulators of symbiont population density. Because there is a significant linear correlation between the rate of algal expulsion and the rate of algal division, factors that increase division rates (e.g., elevated temperature) also increase expulsion rates. Additionally, 3H-thymidine is taken up to a greater extent by algae destined to be expelled than by algae retained in the host cnidarians. Taken together, data for rates of expulsion, rates of division at different temperatures, and uptake of 3H-thymidine suggest that dividing algal cells are preferentially expelled from their hosts. The preferential expulsion of dividing cells may be a mechanism for regulation of algal population density, where the rate of expulsion of algae may be an inverse function of the ability of host cells to accommodate new algal daughter cells. This kind of regulation is present in some cnidarian species (e.g., Aiptasia pulchella, Pocillopora damicornis), but not in all (e.g., Montipora verrucosa, Porites compressa, and Fungia scutaria).

Temperature Stress Causes Host Cell Detachment in Symbiotic Cnidarians: Implications for Coral Bleaching.

During the past decade, acute and chronic bleaching of tropical reef corals has occurred with increasing frequency and scale. Bleaching, i.e., the loss of pigment and the decrease in population density of symbiotic dinoflagellates (zooxanthellae), is often correlated with an increase or decrease in sea surface temperature. Because little is known of the cellular events concomitant with thermal bleaching, we have investigated the mechanism of release of zooxanthellae by the tropical sea anemone Aiptasia pulchella and the reef coral Pocillopora damicornis in response to cold and heat stress. Both species released intact host endoderm cells containing zooxanthellae. The majority of the released host cells were viable, but they soon disintegrated in the seawater leaving behind isolated zooxanthellae. The detachment and release of intact host cells suggests that thermal stress causes host cell adhesion dysfunction in these cnidarians. Knowledge of the cellular entity released by the host during bleaching provides insight into both the underlying release mechanism and the way in which natural environmental stresses evoke a bleaching response.