Low levels of ultra-violet radiation mitigate the deleterious effects of nitrate and thermal stress on coral photosynthesis

Triggers, cascades, and endpoints: connecting the dots of coral bleaching mechanisms

The intracellular coral-dinoflagellate symbiosis is the engine that underpins the success of coral reefs, one of the most diverse ecosystems on the planet. However, the breakdown of the symbiosis and the loss of the microalgal symbiont (i.e. coral bleaching) due to environmental changes are resulting in the rapid degradation of coral reefs globally. There is an urgent need to understand the cellular physiology of coral bleaching at the mechanistic level to help develop solutions to mitigate the coral reef crisis. Here, at an unprecedented scope, we present novel models that integrate putative mechanisms of coral bleaching within a common framework according to the triggers (initiators of bleaching, e.g. heat, cold, light stress, hypoxia, hyposalinity), cascades (cellular pathways, e.g. photoinhibition, unfolded protein response, nitric oxide), and endpoints (mechanisms of symbiont loss, e.g. apoptosis, necrosis, exocytosis/vomocytosis). The models are supported by direct evidence from cnidarian systems, and indirectly through comparative evolutionary analyses from non-cnidarian systems. With this approach, new putative mechanisms have been established within and between cascades initiated by different bleaching triggers. In particular, the models provide new insights into the poorly understood connections between bleaching cascades and endpoints and highlight the role of a new mechanism of symbiont loss, i.e. 'symbiolysosomal digestion', which is different from symbiophagy. This review also increases the approachability of bleaching physiology for specialists and non-specialists by mapping the vast landscape of bleaching mechanisms in an atlas of comprehensible and detailed mechanistic models. We then discuss major knowledge gaps and how future research may improve the understanding of the connections between the diverse cascade of cellular pathways and the mechanisms of symbiont loss (endpoints).

The interactive impacts of a constant reef stressor, ultraviolet radiation, with environmental stressors on coral physiology

Reef-building corals create one of the most biodiverse and economically important ecosystems on the planet. Unfortunately, global coral reef ecosystems experience threats from numerous natural stressors, which are amplified by human activities. One such threat is ultraviolet radiation (UVR) from the sun; a genotoxic stressor that is a double-edged sword for corals as they rely on sunlight for energy. More intense UVR has been shown to have greater direct impacts on animal physiology, and these may be exacerbated by co-occurring stressors. The aim of this systematic literature review was to examine if the same applies to corals; that is, if the co-exposure of a constant stressor (UVR) with other stressors has a greater impact on coral physiology than if these stressors occurred separately. We reviewed the biochemical and cellular processes impacted by UVR and the defenses corals have against UVR. The main stressors investigated with UVR were temperature, nitrate, nutrient, oil, water motion, and photosynthetically active radiation (PAR). UVR generally worsened the physiological impacts of other stressors (e.g., by decreasing zooxanthellae and chlorophyll densities). There were species-specific differences in their tolerance to UVR (differences in zooxanthellae populations, sunscreen production and depth) and environmental stress (e.g., resilience to some oils), and that ambient levels of UVR were often beneficial (i.e., nullifying impacts of nitrates). We highlight areas of future investigation including examining and assessing other interacting stressors and their impacts (e.g., ocean acidification, ocean deoxygenation, toxins and pollutants), investigating impacts of multiple stressors with UVR on the coral microbiome, and elucidating the effects of multi-stressors with UVR across early-life history stages (especially larvae). UVR is a pervasive stressor to corals and can interact with other environmental conditions to compromise the resilience of corals. This environmental driver needs to be more comprehensively examined alongside climate change stressors (e.g., temperature increases, ocean acidification and hypoxia) to better understand future climate scenarios on reefs.

Competitive effects of the macroalga Caulerpa taxifolia on key physiological processes in the scleractinian coral Turbinaria peltata under thermal stress

An increased abundance of macroalgae has been observed in coral reefs damaged by climate change and local environmental stressors. Macroalgae have a sublethal effect on corals that includes the inhibition of their growth, development, and reproduction. Thus, this study explored the effects of the macroalga, Caulerpa taxifolia, on the massive coral, Turbinaria peltata, under thermal stress. We compared the responses of the corals' water-meditated interaction with algae (the co-occurrence group) and those in direct contact with algae at two temperatures. The results show that after co-culturing with C. taxifolia for 28 days, the density content of the dinoflagellate endosymbionts was significantly influenced by the presence of C. taxifolia at ambient temperature (27 °C), from 1.3 × 106 cells cm-2 in control group to 0.95 × 106 cells cm-2 in the co-occurrence group and to 0.89 × 106 cells cm-2 in the direct contact group. The chlorophyll a concentration only differed significantly between the control and the direct contact group at 27 °C. The protein content of T. peltata decreased by 37.2% in the co-occurrence group and 49.0% in the direct contact group compared to the control group. Meanwhile, the growth rate of T. peltata decreased by 57.7% in the co-occurrence group and 65.5% in the direct contact group compared to the control group. The activity of the antioxidant enzymes significantly increased, and there was a stronger effect of direct coral contact with C. taxifolia than the co-occurrence group. At 30 °C, the endosymbiont density, chlorophyll a content, and growth rate of T. peltata significantly decreased compared to the control temperature; the same pattern was seen in the increase in antioxidant enzyme activity. Additionally, when the coral was co-cultured with macroalgae at 30 °C, there was no significant decrease in the density or chlorophyll a content of the endosymbiont compared to the control. However, the interaction of macroalgae and elevated temperature was evident in the feeding rate, protein content, superoxide dismutase (SOD), and catalase (CAT) activity compared to the control group. The direct contact of the coral with macroalga had a greater impact than water-meditated interactions. Hence, the competition between coral and macroalga may be more intense under thermal stress.

Individual and combined effects of herbicide prometryn and nitrate enrichment at environmentally relevant concentrations on photosynthesis, oxidative stress, and endosymbiont community diversity of coral Acropora hyacinthus

Nitrogen pollution and pesticides such as photosystem II (PSII) inhibitor herbicides have several detrimental impacts on coral reefs, including breakdown of the symbiosis between host corals and photosynthetic symbionts. Although nitrogen and PSII herbicide pollution separately cause coral bleaching, the combined effects of these stressors at environmentally relevant concentrations on corals have not been assessed. Here, we report the combined effects of nitrate enrichment and PSII herbicide (prometryn) exposure on photosynthesis, oxidative status and endosymbiont community diversity of the reef-building coral Acropora hyacinthus. Coral fragments were exposed in a mesocosm system to nitrate enrichment (9 μmol/L) and two prometryn concentrations (1 and 5 μg/L). The results showed that sustained prometryn exposure in combination with nitrate enrichment stress had significant detrimental impacts on photosynthetic apparatus [the maximum quantum efficiency of photosystem II (Fv/Fm), nonphotochemical quenching (NPQ) and oxidative status in the short term. Nevertheless, the adaptive mechanism of corals allowed the normal physiological state to be recovered following 1 μg/L prometryn and 9 μmol/L nitrate enrichment individual exposure. Moreover, exposure for 9 days was insufficient to trigger a shift in Symbiodiniaceae community. Most importantly, the negative impact of exposure to the combined environmental concentrations of 1 μg/L prometryn and 9 μmol/L nitrate enrichment was found to be significantly greater on the Fv/Fm, quantum yield of non-regulated energy dissipation [Y(NO)], NPQ, and oxidative status of corals compared to the impact of individual stressors. Our results show that interactions between prometryn stress and nitrate enrichment have a synergistic impact on the photosynthetic and oxidative stress responses of corals. This study provides valuable insights into combined effects of nitrate enrichment and PSII herbicides pollution for coral's physiology. Environmental concentrations of PSII herbicides may be more harmful to photosystems and antioxidant systems of corals under nitrate enrichment stress. Thus, future research and management of seawater quality stressors should consider combined impacts on corals rather than just the impacts of individual stressors alone.

The response of aquatic ecosystems to the interactive effects of stratospheric ozone depletion, UV radiation, and climate change

Variations in stratospheric ozone and changes in the aquatic environment by climate change and human activity are modifying the exposure of aquatic ecosystems to UV radiation. These shifts in exposure have consequences for the distributions of species, biogeochemical cycles, and services provided by aquatic ecosystems. This Quadrennial Assessment presents the latest knowledge on the multi-faceted interactions between the effects of UV irradiation and climate change, and other anthropogenic activities, and how these conditions are changing aquatic ecosystems. Climate change results in variations in the depth of mixing, the thickness of ice cover, the duration of ice-free conditions and inputs of dissolved organic matter, all of which can either increase or decrease exposure to UV radiation. Anthropogenic activities release oil, UV filters in sunscreens, and microplastics into the aquatic environment that are then modified by UV radiation, frequently amplifying adverse effects on aquatic organisms and their environments. The impacts of these changes in combination with factors such as warming and ocean acidification are considered for aquatic micro-organisms, macroalgae, plants, and animals (floating, swimming, and attached). Minimising the disruptive consequences of these effects on critical services provided by the world's rivers, lakes and oceans (freshwater supply, recreation, transport, and food security) will not only require continued adherence to the Montreal Protocol but also a wider inclusion of solar UV radiation and its effects in studies and/or models of aquatic ecosystems under conditions of the future global climate.