Ecophysiology of Red Sea Corals in Response to Carbon and Nitrogen Availabilities

Carbon (C) and nitrogen (N) are essential for coral health, growth and energy production. Yet, maintaining a balanced availability of these elements is critical, as both deficiencies and excesses can have negative consequences for coral survival. This thesis explores how key Red Sea coral taxa, including reef-forming hard corals and a dominant soft coral species, respond to C and N availability, providing insights into their ecophysiology and resilience to environmental challenges such as nutrient pollution. Research Question 1 explored the ecophysiological responses of Red Sea corals to natural C fluxes, as detailed in Chapters 2 and 4. Chapter 2 showed that Xenia umbellata’s physiology was negatively impacted by the absence of heterotrophic food, reducing pulsation rates, symbiont density, and mitotic index, though the coral compensated by increasing symbiont chlorophyll-a content. Water flow had no significant effect, likely due to its pulsation-driven flow regulation. Chapter 4 revealed that azooxanthellate corals like Tubastraea coccinea exhibited significantly higher denitrification rates than zooxanthellate species, as denitrifiers utilised environmental C (e.g., DOC), instead of relying solely on photosynthates. High DOC availability (in addition to other environmental and physiological factors) was identified as one of the key drivers of denitrification in Acropora spp., Millepora dichotoma and Tubastrea coccinea, highlighting the role of C in N cycling processes in corals. These findings emphasise the critical role of both autotrophic and heterotrophic strategies in corals' responses to natural variations in C availability and its influence on biogeochemical processes like denitrification. Research Question 2 investigated the ecophysiological responses of Xenia umbellata to excess C availability under eutrophic conditions, as explored in Chapter 3. The study found that excess organic matter (OM) at 20 mg C L-1, provided as dissolved organic matter (DOM) had no negative effect on coral ecophysiology. However, particulate organic matter (POM) in the form of phytoplankton and zooplankton, caused significant damage, including impaired feeding tentacles, reduced pulsation rates, and increased mortality. The severity of these effects was primarily linked to POM dosage, rather than particle size, highlighting X. umbellata’s vulnerability to coastal eutrophication, where excess POM can harm its ecophysiology. Research Question 3 explored the ecophysiological responses of Red Sea corals to natural fluctuations in N availability, addressed in Chapter 4. High ammonium levels drive denitrification in Acropora spp., as ammonium supports nitrification and nitrate production, a key substrate for denitrifying bacteria. Unexpectedly, T. coccinea showed elevated denitrification under low nitrate availability, likely due to co-occurring N2 fixation and denitrification, characteristic of oligotrophic Red Sea conditions. These results emphasize that coral responses to N fluxes are highly species-specific and influenced by local nutrient dynamics, underscoring the need to consider both biological and environmental variability when assessing coral reef resilience. This thesis highlights Xenia umbellata’s adaptability to low C availability and variable flow but reveals its vulnerability to excess C inputs, exposing soft corals to anthropogenic threats. Species with higher heterotrophic capacities may better withstand inorganic N pollution, potentially driving shifts toward heterotrophic-dominated reefs with significant biodiversity and ecosystem implications. This thesis offers critical insights into the physiological responses of Red Sea corals to ambient and excess nutrient levels, helping to predict reef resilience and shifts in community composition. The findings provide a basis for targeted management strategies to mitigate nutrient-related stress, especially in light of expanding coastal development projects in the Central Red Sea region.