Meeting Abstract
Several possible mechanisms of bleaching (loss of symbiotic dinoflagellates) have been reported in corals and sea anemones. These include in situ degradation of algal cells, exocytosis of algal cells, detachment of host cells containing algae, and death of host cells containing algae (by either apoptosis or necrosis). However, these mechanisms have not been studied in parallel, quantitative studies, and the observations were made in different species under different stress conditions. In addition, the molecular mechanisms underlying these four possible bleaching mechanisms remain unclear. We have developed assays to monitor all four possible bleaching mechanisms in parallel, using animals from a clonal population of the small sea anemone Aiptasia that have been exposed to a variety of precisely controlled temperature-stress and light-stress conditions. The overall bleaching responses are assessed by counting the numbers of algae remaining in the hosts using a flow cytometer that allows precise and rapid counting of a large number of samples. To distinguish exocytosis from host-cell detachment, cells in the seawater surrounding the stressed anemones are collected and examined by fluorescence and electron microscopy. To look at host-cell death and in situ degradation, we are using western blotting, immunohistochemistry, protease-activity assays, and qPCR to study the possible roles of different cellular pathways including apoptosis and autophagy. Under the several stress conditions tested (heat, cold, high light, and heat plus high light), it seems that host-cell detachment, host-cell death, and in situ degradation of algae contribute little to the overall bleaching of Aiptasia, and expulsion of algae appears to be the major cause of bleaching.
