Bleaching in Reef-dwelling Foraminifers: Implications for Reef Decline

Reef-dwelling larger foraminifers share key characteristics with reef-building corals, representatives of both are prolific calcium carbonate producers, physiologically dependent upon algal endosymbionts, and both have suffered bleaching episodes in recent decades. Bleaching was first observed in symbiontbearing foraminifers in the early 1980s in laboratory experiments aimed at determining optimal culture conditions for the most common Caribbean and Indo- Pacific larger species, Amphistegina gibbosa and A. lessonii. Bleaching in field specimens was first noted during a coral post-bleaching survey in the Bahamas in 1988. Since 1991, bleaching has been observed in populations of Amphistegina in all subtropical oceans, with peak bleaching in 1992, and a secondary peak in 1998. Amphistegina populations exhibiting chronic, intermediate-intensity bleaching show anomalous incidences of shell breakage, shell deformities, evidence of predation, and microbial infestation. A key difference between bleaching in corals and foraminifers is that coral bleaching correlates most consistently with elevated seasurface temperatures, while bleaching in Amphistegina is induced primarily by photoinhibitory stress. Since 1992 bleaching incidences in Florida Keys populations of A. gibbosa have increased through the spring and peaked near the summer solstice, preceding late summer temperature maxima. Bleaching in field-collected Amphistegina, and specimens exposed to chronic photoinhibitory stress in laboratory cultures was cytologically indistinguishable. Corals susceptible to bleaching apparently live near their upper thermal thresholds, while Amphistegina thrive near their photoinhibition thresholds and are particularly sensitive to shorter wavelengths of solar radiation. Recognizing the similarities and differences between these taxonomically very different symbiotic systems facilitates understanding of global decline of coral reefs