Interactive effects of black sand and heat stress on coral reefs at Red Sea, Egypt

Coral reefs, vital marine ecosystems, face increasing threats from climate change and localized stressors like black sand deposits, yet their combined impacts are underexplored. This pioneering study investigates the synergistic effects of rising temperatures and black sand on reef-building corals in the Red Sea, Egypt, focusing on Stylophora pistillata and Acropora hyacinthus. Coral samples were collected from Hurghada and exposed to a three-day laboratory experiment involving thermal stress (31 °C), black sand (300 mg/L), nano black sand (300 mg/L), and combinations thereof. Key biochemical markers, including malondialdehyde (MDA), catalase (CAT), total antioxidant capacity (TAC), superoxide dismutase (SOD), total protein, lysozyme activity (LYZ), and metallothionein, were analyzed to assess oxidative stress and antioxidant responses. Additionally, thermal stress trends were evaluated using Degree Heating Weeks (DHW) derived from satellite sea surface temperature data (2022-2024), and coral composition was characterized geochemically. Results revealed a significant rise in DHW, particularly near northern reef areas, indicating heightened thermal stress. Both coral species exhibited comparable responses, with combined treatments (thermal stress with black sand or nano black sand) intensifying oxidative damage and reducing antioxidant defenses. Geochemical analyses showed no significant changes in coral composition post-treatment. These findings underscore the exacerbating role of black sand in amplifying thermal stress, threatening coral health and resilience. Urgent conservation measures, including monitoring black sand impacts and mitigating coastal stressors, are critical to prevent widespread bleaching and mortality in Red Sea coral reefs, especially under ongoing climate change pressures.