Seminar Details

Molecular mechanisms underlying corals’ response to thermal stress and reduced pH conditions: The apoptotic pathway




Hagit Kvitt - University of Haifa


Global warming and ocean acidification have recently emerged as key threats to the long-term survival of coral reefs. However, some coral species are known to survive and recover from thermal stress and reduced pH conditions, although the mechanisms enabling such survival are unclear. Indeed, several studies have suggested (i) that periodic global warming and coral bleaching events have occurred in the past, and (ii) that there have been periods with unfavorable conditions for calcification, characterized by absence of fossilized coral. These data indicate that corals have survived bleaching and mass extinction events, the latter possibly by alternating between soft bodies and calcified fossilizing forms, and are able to survive drastic environmental changes. The goal of this work was to elucidate the role of apoptosis in corals’ response to environmental stress (i.e. thermal stress and reduced pH conditions). First the involvement of apoptosis in death or survival of corals following thermal stress and bleaching was experimentally established for the first time and a model explaining the variability in response to thermal stress in corals was presented. Second, thermal stress-induced molecular and cellular apoptotic responses were monitored in the stony coral Stylophora pistillata and associated with the coral’s ability to survive thermal stress and bleaching. Our results suggest a “two-stage response” model, including (i) the onset of apoptosis in response to environmental stress, accompanied by rapid activation of antioxidant/anti-apoptotic mediators that block the progression of apoptosis to other cells and (ii) the subsiding of apoptosis concomitant with acclimation of the coral to the chronic stress. Interestingly, this second stage includes not only down-regulation of apoptosis but also up-regulation of anti–apoptotic/pro-cell survival regulators that could play an important role in acclimation of the coral to the sustained stress. Moreover, our study reports for the first time functional analysis of coral protein, establishing the anti-apoptotic role of S. pistillata Bcl-2 family member. Third, the involvement of apoptosis in the ability of scleractinian corals (Oculina patagonica and Pocillopora damicornis) to alternate between calcifying colonial form and non-calcifying soft body solitary polyps under reduced pH conditions was established. We show that apoptosis is initiated in the polyps and that once dissociation between polyp and coenosarc terminates, apoptosis subsides. Following re-exposure of the resulting solitary polyps to normal pH, both coral species regenerated coenosarc tissues and initiated calcification. This indicates that regulation of coloniality is under control of the polyp, the basic modular unit of the colony. A “two-stage” model is suggested, accordingly, apoptosis is utilized by the coral to shift from colonial to solitary form. A mechanistic explanation for several key evolutionarily important phenomena occurring throughout coral evolution is provided. This work contribute to our understanding of corals’ response to environmental stress, establishing that controlled apoptotic response provides an essential mechanism enabling the coral to respond to changing environmental conditions, emphasizing the importance of this process in these sessile, simple metazoans. Elucidating the response of corals to changing environmental conditions may lead to better understanding of their evolution as well as to their future under global climate change. Moreover, increasing understanding of this mechanism in one of the most primordial animal groups (Eumetazoa), may shed light on the origin and evolution of the immune system and tissue-specific apoptosis in higher organisms.

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