Differential nitric oxide synthesis and host apoptotic events correlate with bleaching susceptibility in reef corals

Abstract

Coral bleaching poses a threat to coral reefs worldwide. As a consequence of the temperature-induced breakdown in coral-dinoflagellate symbiosis, bleaching can have extensive effects on reef communities. However, our understanding of bleaching at a cellular level is limited, and this is particularly true regarding differential susceptibility among coral species. Recent work suggests that bleaching may represent a host innate immune-like response to symbiont dysfunction that involves synthesis of the signalling compound nitric oxide (NO) and the induction of host apoptotic-like cell death. In this study, we examined the activity of apoptosis-regulating enzymes alongside oxidised NO accumulation (a proxy for NO synthesis) in the reef corals Acropora millepora, Montipora digitata, and Pocillopora damicornis during experimental thermal stress. P. damicornis was the most sensitive species, suffering mortality (tissue sloughing) after 5 days at 33°C but non-lethal bleaching after 9 days at 31.5°C. A. millepora bleached at 33°C but remained structurally intact, while M. digitata showed little evidence of bleaching. P. damicornis and A. millepora both exhibited evidence of temperature-induced NO synthesis and, after 5 days of heating, levels of oxidised NO in both species were fivefold higher than in controls maintained at 28.5°C. These responses preceded bleaching by a number of days and may have occurred before symbiont dysfunction (measured as chlorophyll a degradation and oxidised NO accumulation). In A. millepora, apparent NO synthesis correlated with the induction of host apoptotic-like pathways, while in P. damicornis, the upregulation of apoptotic pathways occurred later. No evidence of elevated NO production or apoptosis was observed in M. digitata at 33°C and baseline activity of apoptosis-regulating enzymes was negligible in this species. These findings provide important physiological data in the context of the responses of corals to global change and suggest that early events in the host may be important in the collapse of the coral-dinoflagellate symbiosis.

Differential coral bleaching—Contrasting the activity and response of enzymatic antioxidants in symbiotic partners under thermal stress

“…Furthermore, no photosynthetic dysfunction or significant changes in the symbiont SOD-APX pathway that would indicate enzymatic removal of ROS was observed within the first five days of thermal stress in A. millepora, whereas host CAT activity at 33°C was already elevated by Day 5. The study of Hawkins et al (2014), which was run in tandem with the current study, adds to this observation by illustrating a transient upregulation of host caspase-like enzyme activity -indicative of apoptoticlike cell death -in A. millepora at 33°C, that coincided with higher catalase-mediated hydrogen peroxide turnover in the host (this study) prior to photosynthetic breakdown in the symbionts. A causal relationship between the induction of nitric oxide synthesis and apoptosis under thermal stress in the host has been suggested (Hawkins et al, 2013;Hawkins et al, 2014), and the patterns seen in A. millepora highlight the role that ROS generally might play in the activation of apoptotic-like pathways in susceptible corals (Tchernov et al, 2011).…”

“…The study of Hawkins et al (2014), which was run in tandem with the current study, adds to this observation by illustrating a transient upregulation of host caspase-like enzyme activity -indicative of apoptoticlike cell death -in A. millepora at 33°C, that coincided with higher catalase-mediated hydrogen peroxide turnover in the host (this study) prior to photosynthetic breakdown in the symbionts. A causal relationship between the induction of nitric oxide synthesis and apoptosis under thermal stress in the host has been suggested (Hawkins et al, 2013;Hawkins et al, 2014), and the patterns seen in A. millepora highlight the role that ROS generally might play in the activation of apoptotic-like pathways in susceptible corals (Tchernov et al, 2011). Nevertheless, the point at which ROS production triggers apoptosis in the coral host appears to be species-specific, as a similar response with respect to host CAT in the bleaching-resistant M. digitata (this study) did not coincide with an upregulation of caspases or NO production (Hawkins et al, 2014).…”

“…The observed trends of antioxidant and apoptosis-regulating enzymes and NO accumulation (Hawkins et al, 2014) in the coral holobiont point towards the importance of early physiological events in the host that may precede the photosynthetic dysfunction of the symbionts. Although highly speculative at this point, hydrogen peroxide might have an active role in the host's antipathogenic response towards its symbionts under bleaching conditions, particularly when the host has sufficient catalase activity to attenuate any self-harm.…”

“…Ambient photosynthetically active radiation (PAR) conditions during the experimental period were recorded with an ODYSSEY PAR cosine recorder (DATAFLOW Systems Pty Ltd., Christchurch, New Zealand). Half of the explants were used for analysis of host cellular and apoptotic signalling (Hawkins et al, 2014), whereas the other half were used to assess the activity of enzymatic antioxidants (this study). After sampling on Day 0, temperature in one of the tanks was increased by 1°C day −1 to a final average temperature of 33°C (±0.8°C), while the minimal temperature of the control tank was set to 27.5°C, based on long-term seawater temperature data for the Heron Island reef flat for February (AIMS, 2013).…”

“…A causal relationship between the induction of nitric oxide synthesis and apoptosis under thermal stress in the host has been suggested (Hawkins et al, 2013;Hawkins et al, 2014), and the patterns seen in A. millepora highlight the role that ROS generally might play in the activation of apoptotic-like pathways in susceptible corals (Tchernov et al, 2011). Nevertheless, the point at which ROS production triggers apoptosis in the coral host appears to be species-specific, as a similar response with respect to host CAT in the bleaching-resistant M. digitata (this study) did not coincide with an upregulation of caspases or NO production (Hawkins et al, 2014).…”