Physiological and Biogeochemical Responses of Super-Corals to Thermal Stress from the Northern Gulf of Aqaba, Red Sea

Abstract

Mass coral bleaching is increasing in frequency and severity, leading to the loss of coral abundance and diversity. However, some corals are less susceptible to bleaching than others and can provide a model for identifying the physiological and biogeochemical traits that underlie coral resilience to thermal stress. Corals from Eilat in the Gulf of Aqaba in the northern Red Sea do not bleach unless seawater temperatures are sustained at +6 • C or higher above their average summer maximum. This extreme thermal tolerance qualifies these as super-corals, as most corals bleach when exposed to temperatures that are only +1-2 • C above their thermal maximum. Here, we conducted a controlled bleaching experiment (+6 • C) for 37 days (equivalent to 32 • heating weeks) on three species of corals from Eilat: Stylophora pistillata, Pocillopora damicornis, and Favia favus. To assess the response of the holobiont to thermal stress, the following variables were measured on each coral: endosymbiotic algal cell density, Chlorophyll a, endosymbiotic mitotic cell division, total lipids, protein, carbohydrate, and the stable carbon (δ 13 C) and oxygen (δ 18 O) isotopic composition of the skeleton and the δ 13 C of the animal host tissue and endosymbiotic algae. While all three species appeared visibly bleached, their physiological and biogeochemical responses were species-specific. S. pistillata catabolized lipids but still maintained total energy reserves and biomass. Increases in both skeletal δ 13 C and δ 18 O indicates that calcification declined in this species. P. damicornis was the least affected by bleaching. It maintained its total energy reserves and biomass, and isotopic evidence suggests that it maintained calcification and was not dependent on heterotrophy for meeting metabolic demand when bleached. Finally, F. favus catabolized protein and carbohydrates, and suffered losses in total energy reserves and biomass. Nevertheless, isotopic evidence suggest that photosynthesis and calcification were maintained, and that this species has a high baseline heterotrophic capacity. Thus, just like their non-super-coral conspecific counterparts, maintaining energy reserves Grottoli et al.Response of Super-Corals to Thermal Stress and biomass, and heterotrophic capacity appear to be traits that underlie the thermal tolerance of these super-corals from Eilat. Given the high thermal tolerance of these super-corals, these populations could provide viable seed stock for repopulating coral losses on other reefs.

Spatial variation in the biochemical and isotopic composition of corals during bleaching and recovery

“…Conversely, light attenuation and low rates of photosynthesis (Muscatine et al ; Laws et al ; Swart et al ; Maier et al ) can decrease both δ 13 C and δ 15 N values in corals (but see also Rost et al ). Lower δ 13 C values can also result from greater feeding on particles (i.e., plankton and organic particles; Levas et al ; Grottoli et al ) and the preferential utilization of heterotrophic nutrition in lipid biosynthesis (Alamaru et al ; Baumann et al ). Short‐term increases in heterotrophic nutrition can be difficult to verify, however, due to uncertainty in rates of tissue turnover and changes in tissue composition, especially following physiological stress (Rodrigues and Grottoli ; Logan et al ).…”

“…Conversely, light attenuation and low rates of photosynthesis (Muscatine et al 1989;Laws et al 1995;Swart et al 2005b;Maier et al 2010) can decrease both δ 13 C and δ 15 N values in corals (but see also Rost et al 2002). Lower δ 13 C values can also result from greater feeding on particles (i.e., plankton and organic particles; Levas et al 2013;Grottoli et al 2017) and the preferential utilization of heterotrophic nutrition in lipid biosynthesis (Alamaru et al 2009;Baumann et al 2014).…”

The Future of Coral Reefs Subject to Rapid Climate Change: Lessons from Natural Extreme Environments

“…The PAG is arguably the world's hottest sea supporting coral reefs; however, other hot seas exist (e.g., the Red Sea) that support corals potentially more tolerant of thermal stress events (Fine et al, 2013;Grottoli et al, 2017;Krueger et al, 2017). The hot, southernmost end of the Red Sea acts as a selective thermal barrier favoring heat-resistant genotypes; once these genotypes spread to the northern, cooler Red Sea, they live well below their bleaching threshold (Fine et al, 2013).…”

Nitrogen eutrophication particularly promotes turf algae in coral reefs of the central Red Sea

“…The increased δ 13 C in the zooxanthellae (Fig. S2) could also be an indicator for a negative effect on hard coral health which was also found in relation to bleached Favia favus (now Dipsastraea favus) corals in the Northern Red Sea (Grottoli, Tchernov & Winters, 2017) but not Montastraea faveolata (now Orbicella faveolata) in Florida (Wall et al, 2019). Given our 8-week observation period and a comparatively cold water temperature, our study did not provide a setting to trace severe bleaching effects and for the Southern Red Sea it was speculated that higher nutrient availability might even benefit P. verrucosa to resist higher water temperature (Sawall et al, 2014a).…”