Abstract
Coral reefs have been detrimentally impacted causing health issues due to elevated ocean temperatures as a result of increased greenhouse gases. Extreme temperatures have also exacerbated coral diseases in tropical reef environments. Numerous studies have outlined the impacts of thermal stress and disease on coral organisms, as well as understanding the influence of site-based characteristics on coral physiology. However, few have discussed the interaction of all three. Laboratory out-planting restoration projects have been of importance throughout impacted areas such as the Caribbean and southern Florida in order to increase coral cover in these areas. This study analyzes photosynthetic efficiency of Porites astreoides from the lower Florida Keys after a two-year reciprocal transplant study at inshore (Birthday reef) and offshore (Acer24 reef) sites to understand acclimation capacity of this species. Laboratory experiments subjected these colonies to one of three treatments: control conditions, increases in temperature, and increases in temperature plus exposure to an immune stimulant (lipopolysaccharide (LPS)) to determine their influence on photosynthetic efficiency and how stress events impact these measurements. In addition, this study is a continuation of previous studies from this group. Here, we aim to understand if these results are static or if an acclimation capacity could be found. Overall, we observed site-specific influences from the Acer24 reef site, which had significant decreases in photosynthetic efficiencies in 32 °C treatments compared to Birthday reef colonies. We suggest that high irradiance and lack of an annual recovery period from the Acer24 site exposes these colonies to significant photoinhibition. In addition, we observed significant increases in photosynthetic efficiencies from LPS exposure. We suggest host-derived antioxidants can mitigate the negative impacts of increased thermal stress. Further research is required to understand the full complexity of host immunity and symbiont photosynthetic interactions.
Highlights
Increases in atmospheric greenhouse gasses from anthropogenic activities have contributed to climate change by trapping more solar radiation from the sun and warming the planet [1,2]
(e.g., simulation of disease), potentially influenced from host-derived reactive oxygen species (ROS) suppressants. While it is out of the scope of this paper, this information could be useful for coral repopulation efforts in southern Florida [45] as our results indicate the importance of thermal history to the resiliency of photosynthetic efficiency changes in response to thermal stress
Climate change is an existential threat to corals worldwide, and we propose exacerbated stress from simulated thermal stress based on our results
Summary
Increases in atmospheric greenhouse gasses from anthropogenic activities have contributed to climate change by trapping more solar radiation from the sun and warming the planet [1,2]. Coral reefs exist and thrive within a narrow range of light, temperature, salinity, and turbidity [4]. This is due to the symbiotic relationship that coral organisms have with algae in the family Symbiodiniaceae. Light is a significant impact on coral bleaching and reductions in photosynthetic efficiencies and symbiont pigmentation [8]. Oxidative stress is a significant effect on coral bleaching, where stress events (i.e., temperature and light) impact repair mechanisms and cause cascading events such as reductions in photosynthetic efficiencies, and eventual expulsion of symbionts by means of apoptosis, necrosis, and exocytosis [8]. Bleaching has caused many coral reefs around the world to lose significant amounts of coral cover such as in southern Florida (USA) and the Great Barrier Reef (Australia) [10,11]
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