Temperature Stress Causes Host Cell Detachment in Symbiotic Cnidarians: Implications for Coral Bleaching

Abstract

During the past decade, acute and chronic bleaching of tropical reef corals has occurred with increasing frequency and scale. Bleaching, i.e., the loss of pigment and the decrease in population density of symbiotic dinoflagellates (zooxanthellae), is often correlated with an increase or decrease in sea surface temperature. Because little is known of the cellular events concomitant with thermal bleaching, we have investigated the mechanism of release of zooxanthellae by the tropical sea anemone Aiptasia pulchella and the reef coral Pocillopora damicornis in response to cold and heat stress. Both species released intact host endoderm cells containing zooxanthellae. The majority of the released host cells were viable, but they soon disintegrated in the seawater leaving behind isolated zooxanthellae. The detachment and release of intact host cells suggests that thermal stress causes host cell adhesion dysfunction in these cnidarians. Knowledge of the cellular entity released by the host during bleaching provides insight into both the underlying release mechanism and the way in which natural environmental stresses evoke a bleaching response.

Heat stress induces different forms of cell death in sea anemones and their endosymbiotic algae depending on temperature and duration

“…Another explanation is that host cell PCD may act as a mechanism to remove dysfunctional symbionts. 11,[14][15] The release of zooxanthellae Figure 4 Frequency histograms of cell death morphologies observed in (a) endoderm tissue, (b) ectoderm and (c) zooxanthellae in response to different temperatures over a logarithmic set of time intervals. N ¼ necrosis-like, H ¼ healthy and PCD ¼ apoptosis-like cell death.…”

“…PCD and necrosis of host animal cells were first proposed by Gates et al 11 as possible mechanisms for release of the intracellular symbionts during bleaching. Later it was recognised that death and in situ degradation of the zooxanthellae may also be a significant pathway for loss of symbionts during bleaching events.…”

“…Later it was recognised that death and in situ degradation of the zooxanthellae may also be a significant pathway for loss of symbionts during bleaching events. 12,13 However, the importance of these processes relative to exocytosis of zooxanthellae or detachment of host cells 11 has not yet been resolved. Dunn et al [14][15][16] recently showed evidence for the simultaneous occurrence of PCD and necrosis in both zooxanthellae and host cells in the symbiotic sea anemone Aiptasia sp during bleaching caused by heat stress.…”

Lipid biomarkers in Symbiodinium dinoflagellates: new indicators of thermal stress

“…The symptoms of bleaching include expulsion of the Symbiodinium from host tissue and loss of photosynthetic pigment within the remaining Symbiodinium cells (Gates et al 1992;Douglas 2003). Proposed mechanisms include damage to photosystem II proteins, disruption of carbon fixation pathways, or caspase-mediated apoptosis induced by reactive oxygen species (Warner et al 1999;Jones et al 1998;Tchernov et al 2011).…”

The distribution of Symbiodinium diversity within individual host foraminifera

“…An array of regulation mechanisms have been proposed and studied in corals, operating either by controlling reproduction rates of, selectively destroying, or expelling unwanted symbionts (Gates et al 1992;Falkowski et al 1993;Baghdasarian and Muscatine 2000;Dunn et al 2002, Dunn and. Such post-phagocytic winnowing mechanisms may be responsible for the pattern of the distribution of symbiont types found in the foraminifera in this study.…”