Abstract
Coral bleaching is a significant contributor to the worldwide degradation of coral reefs and is indicative of the termination of symbiosis between the coral host and its symbiotic algae (dinoflagellate; Symbiodinium sp. complex), usually by expulsion or xenophagy (symbiophagy) of its dinoflagellates. Herein, we provide evidence that during the earliest stages of environmentally induced bleaching, heat stress and light stress generate distinctly different pathomorphological changes in the chloroplasts, while a combined heat- and light-stress exposure induces both pathomorphologies; suggesting that these stressors act on the dinoflagellate by different mechanisms. Within the first 48 hours of a heat stress (32°C) under low-light conditions, heat stress induced decomposition of thylakoid structures before observation of extensive oxidative damage; thus it is the disorganization of the thylakoids that creates the conditions allowing photo-oxidative-stress. Conversely, during the first 48 hours of a light stress (2007 µmoles m−2 s−1 PAR) at 25°C, condensation or fusion of multiple thylakoid lamellae occurred coincidently with levels of oxidative damage products, implying that photo-oxidative stress causes the structural membrane damage within the chloroplasts. Exposure to combined heat- and light-stresses induced both pathomorphologies, confirming that these stressors acted on the dinoflagellate via different mechanisms. Within 72 hours of exposure to heat and/or light stresses, homeostatic processes (e.g., heat-shock protein and anti-oxidant enzyme response) were evident in the remaining intact dinoflagellates, regardless of the initiating stressor. Understanding the sequence of events during bleaching when triggered by different environmental stressors is important for predicting both severity and consequences of coral bleaching.
Highlights
Coral bleaching is a physiological phenomenon in which the symbiosis between the coral host and its symbiotic dinoflagellate terminates [1]
Experimental treatments To assess the responses of P. damicornis to heat stress, light stress, and their combinations, coral nubbins were subjected to six treatments for two experimental periods, a short term 24–48 hour period and a longer term of up to 127 hours
Despite advances in understanding the mechanisms of coral bleaching, difficulties remain in assessing causation, in (1) determining the role and time-frame in which temperature and light act during a bleaching event, and (2) their relationship to other environmental stressors that induce bleaching [23], [29], [43]
Summary
Coral bleaching is a physiological phenomenon in which the symbiosis between the coral host and its symbiotic dinoflagellate terminates [1]. Yonge [7] and Yonge and Nicholls [8] challenged this theory by arguing that the symbiotic dinoflagellates were expelled from the endoderm of the cnidarian, and not digested. Their expulsion theory was corroborated to occur in sea anemones by Smith [9], and went unchallenged until the work of Steele and Goreau [10], who reasserted that dinoflagellates were digested. Coral bleaching may result from a number of non-exclusive mechanisms, including host-cell detachment, Vibrio infection, viralinduced lysis of zooxanthellae, and zooxanthella programmed-celldeath [18,19,20,21], though the trigger(s) for the initiating these processes, as well as the processes themselves, remain elusive
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