Introduction
Reef building stony corals live in a mutually beneficial partnership with photosynthetic algae that provide them with essential nutrients and oxygen. Stressful conditions such as high temperature or oceanic acidification lead to the disruption of symbiosis known as coral bleaching, and eventual coral death. Climate-change related thermal mass bleaching events have become increasingly common, affecting about 75% of coral reefs in Hawaii and 93% of surveyed corals in Great Barrier Reef, with over 50% mortality in some regions (Couch et al., 2017; Hoegh-Guldberg et al., 2015). Current climate-change mitigation measures (e.g. the Paris agreement), aiming to limit global warming to 2℃, seem not being aggressive enough to effectively moderate impacts on coral reefs, leaving these ecosystems under an imminent threat of collapsing and disappearing by 2050 (Ainsworth et al., 2016; Frieler et al., 2013).
Coral reefs are of particular importance for life on Earth. They produce atmospheric oxygen – while absorbing carbon dioxide – through their photosynthesizing symbiotic algae, they also offer necessary shelter for marine life, serve as nurseries for fishery species, protect coastal regions from wave exposure and provide food for more than 3 billion people (Vercelloni et al., 2018). The World Wildlife Fund for Nature estimates that the range of products and services that coastal and marine environments provide can be valued conservatively at $2.5 trillion each (Hoegh-Guldberg et al., 2015). So, not surprisingly, searching for ways to better protect coral reefs became the kernel of coral research all over the world.
Animals and plants are known to acclimatize to stress conditions and several recent studies show that heat acclimatization might help corals better withstand thermal stress and reduce the severity of coral bleaching and mortality (Bay & Palumbi, 2015; Bellantuono, Granados-Cifuentes, Miller, Hoegh-Guldberg, & Rodriguez-Lanetty, 2012; Palumbi, Barshis, Traylor-Knowles, & Bay, 2014). Unfortunately, this natural protection mechanism could be lost under future climate-change scenarios (Ainsworth et al., 2016). One proposed solution for helping corals withstand future climate conditions is human-assisted evolution (Committee on Interventions to Increase the Resilience of Coral Reefs, Ocean Studies Board, Board on Life Sciences, Division on Earth and Life Studies, & National Academies of Sciences, Engineering, and Medicine, 2019; Oppen et al., 2017). Whether it is selective breeding, managed relocation (assisted gene flow and migration), or genetic manipulations, the main setback is the fundamental lack of knowledge underlying mechanisms of coral-algal symbiosis maintenance and disruption. If we don’t understand the mechanism of bleaching and natural acclimatization/adaptation of corals to heat stress, we can’t responsibly define the attributes of resilience. Reversely, with a proper understanding of coral resilient traits and their trade-offs, current restoration projects could be scaled up to dramatically increase impact on coral reefs worldwide.
Despite global ecological relevance, surprisingly little is known about the cellular and molecular mechanisms of bleaching. In general, the observations point to two prevalent strategies involved in bleaching – apoptosis and autophagy/symbiophagy (Downs et al., 2009; Dunn, Schnitzler, & Weis, 2007; Kvitt, Rosenfeld, & Tchernov, 2016; Pernice et al., 2011; Tchernov et al., 2011). These two evolutionary conserved programmed cell death (PCD) pathways may occur independently but often simultaneously interplay or compensate for each other (Denton & Kumar, 2019; Dunn et al., 2007). For example, key pro-survival genesBcl-2 (B-cell lymphoma 2) and BI-1 (Bax inhibitor 1) inhibit both apoptosis and autophagy, and pro-death genes BAX (Bcl2 Associated X) and BAK (Bcl2 Antagonist/Killer) activate them (Castillo et al., 2011; Karch et al., 2017; Xu et al., 2013).
Molecular analyses from previous bleaching experiments showed correlation of – among others - HSP70 (heat shock protein 70), and PCD genes Bcl-2, BI-1, BAK, and BAX expressions with bleaching, and proved the involvement of caspases – apoptotic proteases - in the process (Kvitt et al., 2016; Pernice et al., 2011; Tchernov et al., 2011). Transcriptomic studies in preconditioned/acclimatized corals focused mostly on the longer-term changes in gene expression profiles and found differences in the expressions of e.g. heat shock family proteins, genes involved in oxidative stress and various pleiotropic cell signaling and transcription factors such as TNFR (tumor necrosis factor receptor) or NFkB inhibitor (NFkBI)(Bay & Palumbi, 2015; Bellantuono et al., 2012; Palumbi et al., 2014; Thomas et al., 2018). The early phase acute heat stress response and the role of PCD in preconditioning and acquired thermal resilience in corals is therefore not well understood. Moreover, the functional analyses of particular genes in the process are missing.
In our study, we used the heat susceptible stony coral Pocillopora acuta and we show that three-day exposure to sublethal temperature makes it more resilient to subsequent acute thermal stress via modulations in cell signaling. In preconditioned corals, the expression of pro-survival gene pBcl-2 increases relatively to pro-death genes pBak and pBax during early phase of the thermal stress. After pBcl-2 activity inhibition, preconditioned corals lose the acquired phenotype and bleach at the same rate as non-preconditioned corals, which determines the crucial role of pBcl-2 and programmed cell death in coral bleaching and acclimatization. Detailed analysis points to the involvement of autophagy/symbiophagy rather than apoptosis in the process. Corals exposed to natural oceanic summer temperatures show stress-induced gene expression profile similar to the experimentally preconditioned corals which suggests that during periods with high probability of extreme temperature events, corals can naturally increase their resilience through alterations in cell signaling.