Abstract
Coral reefs are currently experiencing substantial ecological impoverishment as a result of anthropogenic stressors, and the majority of reefs are facing immediate risk. Increasing ocean surface temperatures induce frequent coral mass bleaching events—the breakdown of the nutritional photo-symbiosis with intracellular algae (genus: Symbiodinium). Here, we report that Stylophora pistillata from a highly diverse reef in the Gulf of Aqaba showed no signs of bleaching despite spending 1.5 months at 1–2°C above their long-term summer maximum (amounting to 11 degree heating weeks) and a seawater pH of 7.8. Instead, their symbiotic dinoflagellates exhibited improved photochemistry, higher pigmentation and a doubling in net oxygen production, leading to a 51% increase in primary productivity. Nanoscale secondary ion mass spectrometry imaging revealed subtle cellular-level shifts in carbon and nitrogen metabolism under elevated temperatures, but overall host and symbiont biomass proxies were not significantly affected. Now living well below their thermal threshold in the Gulf of Aqaba, these corals have been evolutionarily selected for heat tolerance during their migration through the warm Southern Red Sea after the last ice age. This may allow them to withstand future warming for a longer period of time, provided that successful environmental conservation measures are enacted across national boundaries in the region.
Highlights
Coral reefs are among the most diverse and productive ecosystems on the planet
Net coral photosynthesis in the light and coral respiration in the dark was derived from rates of oxygen production/consumption taken during daytime in sealed, stir bar-agitated, temperature-controlled metabolic chambers (250–270 ml)
Symbiont densities were not significantly affected by any of the treatments and symbiont total chlorophyll concentration significantly increased by 44% in the combined stress scenario, predominantly driven by temperature (+45%), compared to pH (−9%)
Summary
Coral reefs are among the most diverse and productive ecosystems on the planet. Their carbonate structures are produced by scleractinian hermatypic corals that live in association with a range of microand macroscopic organisms [1]. Most tropical corals live close to their upper thermal threshold and their symbiotic relationship typically breaks down when exposed to 1–2°C above their local summer maximum for a month or more [14], absolute bleaching thresholds can be as low as 27°C in Rapa Nui or as high as 35°C in the Persian Gulf [15] Building on these findings, research employing large-scale mesocosms to manipulate seawater conditions has investigated whether and how thermal pre-conditioning alters stress performance as well as the traits of subsequent coral generations [16,17,18,19]. We monitored a broad suite of physiological functions including photosynthesis, symbiont carbon/nitrogen fixation and translocation, protein, carbohydrate and antioxidant metabolism of both symbiotic partners, as well as calcification and holobiont appearance (symbiont density and pigmentation)
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