The combined effects of nitrate with high temperature and high light intensity on coral bleaching and antioxidant enzyme activities

• We studied the multiple effects of high nitrate, high temperature and high light on coral bleaching.
• Nitrate alone was not enough to induce stress with ambient temperature and low light.
• Enrichment of nitrate accelerated coral bleaching under high light or high temperature.
• The negative correlation between zooxanthellae density and antioxidant enzyme activities was found.

The frequency and severity of coral bleaching events have increased in recent decades. Regionally, human-attributed nutrient pollution, particularly nitrate, has increased in coastal areas due to inflow from rivers and groundwater. The combined effects of increased nitrate concentrations with high temperature or high light on bleaching events and reactive oxygen species levels (ROS) were tested. Coral fragments of Montipora digitata were incubated for 3 and 6 days at different light intensities (200 and 600 μmol m−2  s−1), temperatures (27 and 32 °C), and nitrate concentrations (<1 and 10 μM). Quantum yields of the photosynthetic electron transport system were significantly decreased under high-temperature conditions. Because there were strong correlations between zooxanthellae density and antioxidant enzyme activities (superoxide dismutase and catalase), we hypothesized that a decrease in zooxanthellae occurred due to oxidative stresses. Our results show that enrichment of nitrate can accelerate coral bleaching under high light or high temperature conditions compared with single stress conditions. If the concentration of nitrate increases damage to coral reefs in coastal areas could significantly increase under high temperature or high light stress.