Marine science: Coral’s tipping point

The pathways involved in apoptoptic cell death in coral during environmental stress are elucidated in Scientific Reports this week. Coral bleaching, which involves the loss of dinoflagellates from the coral symbiosis, is an ecologically devastating global phenomenon occurring on coral reefs in response to environmental stress ? from increased sea surface temperatures, for example. Apoptotic cell death has been implicated in coral bleaching but the mechanisms governing cellular change and the initiation of stress responses in the coral-dinoflagellate symbiosis have remained unclear. Tracy Ainsworth and colleagues studied the expression of coral homologues of six key regulators of apoptosis in the coral species Acropora aspera during environmental stress. They observed significant changes in both pro- and anti-apoptotic gene expression, suggesting that the control of apoptosis is highly complex in this symbiosis. Moreover, apoptotic cell death cascades have a potentially important role in tipping the cellular life-death balance during environmental stress prior to the onset of coral bleaching. The authors show that the molecular machinery governing cell death in the coral-dinoflagellate symbiosis responds significantly to subtle, daily changes in the environment and at temperatures generally considered to have little impact on the function of coral and its microbial symbionts. Further study of the tissue function and cellular differentiation and recovery processes in coral is needed before this complicated cell death system can be fully understood.