Studies of symbiosis breakdown in Aiptasia, a sea-anemone model for coral biology [electronic resource]
Keywords: Biology.

Coral reefs are one of the world's most productive ecosystems and harbor an immense diversity of organisms. The energetic basis of this ecosystem is the symbiosis between corals and unicellular dinoflagellate algae of the genus Symbiodinium. Despite the importance of corals, little is known about the molecular and cellular bases of the establishment, maintenance, and breakdown of this symbiosis. Currently, the health and abundance of coral reefs are declining rapidly due to anthropogenic activities. Stresses such as rising seawater temperatures cause the symbiosis to break down, leading to a loss of the algal symbiont from the coral host, a phenomenon known as coral bleaching. Even though it is well known that corals bleach under stress, the cellular mechanisms of this process are not well understood. The study of bleaching mechanisms has been confounded in part by the difficulty of maintaining corals under laboratory conditions and their intractability to experimental manipulation. Published studies have been performed using different species and different experimental conditions, making it difficult to compare the results. To date, four possible cellular bleaching mechanisms have been reported. However, it is unclear whether there is a quantitatively predominant mechanism of bleaching and whether the bleaching mechanisms change depending on the stress type and/or duration. To answer these questions, I used a clonal strain of the small sea anemone Aiptasia, which houses Symbiodinium types similar to those in corals but is far more amenable to experimental manipulation. I exposed animals to precisely controlled stress conditions and monitored all four potential bleaching mechanisms in parallel. I found that expulsion of algae seems to be by far the most important bleaching mechanism under all stress conditions tested, with the possible exception of acute cold shock, which also induced in situ degradation of algae and detachment of host cells containing algae. It has been hypothesized that ocean acidification might exacerbate the bleaching of corals under thermal stress. However, only a few studies have examined the effect of low-pH seawater on cnidarian bleaching, and even fewer have looked at the possible synergism of these two stressors. As with studies of the cellular mechanisms of bleaching, the reports on effects of low pH are highly variable and often conflicting, perhaps because of the different study conditions and organisms used. To examine these questions, I exposed Aiptasia either to low-pH seawater as a single stressor, or to low pH in combination with mild thermal stress. The results suggest that at least in Aiptasia, low pH alone causes either no or only moderate bleaching, and that thermal bleaching is only slightly exacerbated by low pH. Using anemone strains with different combinations of host and algal genotype, I also obtained evidence that both genotypes influence the sensitivity of a given strain to low-pH seawater. The knowledge gained in my studies should lay the foundation for future studies of the molecular pathways of symbiosis breakdown under stress and help to enable a rational approach to assessing, and potentially partially remediating, the threats of anthropogenic stresses to coral reefs.