Corals under stress: a study of the coral innate immune system

Despite the absence of an adaptive immune system, scleractinian corals possess an array of innate immune mechanisms for defence against environmental and biological stressors. The rising prevalence of coral diseases worldwide, however, indicates that the corals' ability to respond to disturbances is being increasingly hampered, potentially because cumulative impacts of multiple stressors are overwhelming the immune system and threatening their health. A thorough understanding of how the coral immune system is affected by and responds to stressors is needed to understand the role that innate immunity can play in coral resilience in the face of increasing anthropogenic impacts and a changing climate. My primary objectives in this thesis were to investigate: 1) temporal patterns in immune parameters in healthy corals and how seasonal variation in environmental factors affect constitutive levels of these parameters, 2) impacts of anthropogenic stress associated with a tourist platform on the coral immune system and potential implications for reef degradation, 3) the coral immune response elicited by injury and how this response is affected by seawater temperature, and 4) impacts of elevated seawater temperature on the response of members of the coral holobiont to bacterial challenges, including responses of the coral host, its endosymbiont Symbiodinium, and coral-associated bacterial communities. A study of temporal patterns in green fluorescent protein (GFP)-like protein expression and in activity levels of the pro-phenoloxidase (proPO)-activating system over a year in three common Indo-Pacific corals revealed that corals maintain differing constitutive levels of these immune parameters, and that the seasonality of peaks in temporal patterns varies among species (Chapter 2). Overall, high constitutive levels of PO activity in Porites cylindrica versus generally low constitutive levels in Acropora millepora are consistent with ecological characterisations of these species as stress-tolerant and stress-sensitive, respectively. Variation in temporal patterns of both parameters among species and in the manner that patterns correlate with environmental factors, including temperature, salinity and solar radiation, indicates that investment in immune mechanisms is a life history trait that varies among corals, and that species differ in how they prioritise energy allocation to immune parameters in resource investment trade-offs. In Chapter 3, I analysed the effect of an anthropogenic disturbance on baseline levels of immune parameters in the disease-susceptible coral A. millepora. In summer, anthropogenic stressors associated with tourist platforms caused disease in 30% of corals monitored near platforms, while corals at a control site unaffected by recreational activities, physical damage and platform shading remained healthy throughout the 7 month study. GeXP analyses revealed that both healthy and diseased corals adjacent to tourist platforms increased their expression of immune genes involved in Toll-like receptor (TLR) signalling cascades, such as MAPK p38 and MEKK-1, compared to corals at the nearby control site. In addition, diseased corals exhibited a 2 to 3-fold increase in PO activity and fluorescence levels compared to control corals, as well as up to 2-fold increased expression of immune genes, including cFos and Factor B. Multiple stressors associated with platforms and warm temperatures appear to have overwhelmed the coral immune system in summer, resulting in higher coral disease prevalence at these locations. Once seasonal temperatures declined, disease subsided, suggesting that the immune response was able to cope with anthropogenic stressors in the absence of temperature stress. Some corals near tourist platforms sustained significant levels of physical damage in summer, but did not develop disease, raising questions about the interactive effects of elevated seawater temperature and injury on the coral immune response. In a manipulative aquarium-based experiment at Heron Island, branches of A. aspera that were exposed to low level heat stress (32°C) and subsequently injured were found to exhibit a rapid immune response within 24 hours post-injury that was largely unaffected by heat stress (Chapter 4). For example, while immune genes Tx60 and apextrin were downregulated in uninjured corals at 32°C, their expression in injured heat stressed corals was similar to that in injured corals at ambient temperatures. The similarity in the responses of heat-stressed and control corals (maintained at ambient temperatures) to injury suggests that the immune response of A. aspera is robust to minor increases in temperature, and temperatures 1-2°C above typical summer maxima have a limited impact on its ability to recover from lesions. The increasing likelihood of physical damage to reef corals from predator outbreaks, storms and anthropogenic disturbances, highlights the need for further detailed studies of how corals cope with injury. A longer-term field-based study of A. aspera's immune response to injury demonstrates that this species recovers from significant, artificially-induced lesions within ten days (Chapter 5). The immune response was dynamic, consisting of at least three distinct phases involving differing timing in the upregulation of components of the innate immune system, including the proPO-activating system, GFP-like protein expression, Toll-like receptor signalling and the complement system. The initial response involved a 1.5-fold increase in expression of a TLR within 24 hours, which orchestrated the innate immune response that was most pronounced 48 and 96 hours post-injury. Overall, the immune response was sufficient to protect corals against bacterial infection, as bacteria did not infiltrate coral tissues and the coral-associated bacterial community did not change, enabling rapid recovery in the absence of additional stressors. Patterns of higher disease prevalence in summer for many common coral diseases suggest that elevated seawater temperatures affect a coral's immune response to pathogens. In Chapter 6, I investigated the effects of elevated seawater temperatures (29.5°C and 32°C, compared to 27°C (ambient)) on the response of the coral Montipora aequituberculata to bacterial challenges over a 22 day period, concurrently with responses of Symbiodinium and coral-associated bacterial communities. Full transcriptome analysis using RNA Seq revealed that corals exhibit an immune response towards the pathogenic bacterium Vibrio coralliilyticus, regardless of seawater temperature, but not against a non-pathogenic bacterium (Oceanospirillales sp.). This ability to distinguish between pathogenic and non-pathogenic bacteria and respond only to pathogens, suggests that corals may actively regulate their associated bacterial communities. Unexpectedly, the number of disease cases did not differ between V. coralliilyticus-challenged corals and other treatments, regardless of seawater temperature, despite the occurrence of a white syndrome outbreak in the coral population from which V. coralliilyticus was isolated. Results suggest that, either corals have acquired higher disease resistance over time, or that virulence of the V. coralliilyticus strain used had been reduced. RNA Seq analysis of the coral response showed that under heat stress, a large suite of immune response mechanisms was upregulated, including genes involved in phagocytosis, TLR and pro-inflammatory cytokine signalling, and the complement system. These responses were potentially directed towards a temperature-induced shift in bacterial communities observed in heat stressed corals. Gene enrichment analysis revealed that Symbiodinium also exhibited an immune response toward this biotic stressor; concurrently, a significant photophysiological response was detected under elevated seawater temperatures, although no bleaching was observed. Taken together, this study shows that M. aequituberculata is relatively stress-resistant, possesses a complex suite of immune responses and is capable of distinguishing pathogenic bacteria. Overall, the work presented in this thesis shows that corals are well equipped to cope with disturbances such as injury and bacterial challenges, even when exposed to mild temperature stress or single environmental disturbances. However, the immunocompetence and performance of the coral immune system is species-specific and cumulative environmental and anthropogenic pressures may overpower the coral's immune system, leading to disease. Identifying impacts of environmental and anthropogenic stressors on the coral immune system provides important information for coral reef managers faced with the challenge of prioritising resources to reduce anthropogenic pressures on increasingly threatened coral reef ecosystems