Abstract
The nitrogen-to-phosphorus (N:P) ratio of dissolved inorganic nutrients may influence coral susceptibility to heat stress, but comparative studies on how different N:P ratios affect soft and hard corals do not exist. This study thus investigated the effects of two different N:P ratios on two common Indo-Pacific reef corals: the pulsating soft coral Xenia umbellata and the hard coral Montipora digitata. Corals were exposed for five weeks to N:P 19:1 (37 µM nitrate, two µM phosphate) and N:P 5:1 (37 µM nitrate, eight µM phosphate), relative to a low nutrient control (N:P 3:1, 0.003 µM nitrate, 0.001 µM phosphate). After two weeks, temperatures were gradually increased from 26 to 32 °C. Nutrient enrichment alone did not affect any response parameter for M. digitata, whereas for X. umbellata, 9% mortality was observed, along with a reduction in pulsation rates (-100% under 5:1 ratio, -58% under 19:1 ratio). Heat stress alone significantly reduced Symbiodiniaceae density and chlorophyll a content of M. digitata, while X. umbellata was unaffected. Combined factors significantly increased mortality for M. digitata (100% and 92%) and X. umbellata (87% and 33%) under the 5:1 and 19:1 ratio, respectively. Symbiodiniaceae density and chlorophyll a content in M. digitata showed the same response as under heat stress alone, while these parameters were unaffected for X. umbellata. Pulsation of X. umbellata was reduced by 100% in both combined treatments. Nutrient enrichment alone likely compromised X. umbellata's metabolism through the energy-intensive reduction of nitrate to ammonium, halting pulsation. Yet, under combined warming and nutrient addition, X. umbellata showed better fitness than M. digitata, suggesting it may better cope with the predicted environmental changes. Still, both corals were negatively affected, particularly by the 5:1 N:P ratio. The stronger impact of this low ratio likely resulted from exacerbated energy depletion by nitrogen assimilation, while the imbalanced 19:1 ratio may have limited nitrogen assimilation, thereby mitigating stress. These findings suggest that high phosphorus, in combination with high nitrogen, may accelerate coral stress. Hence, tailored nutrient management strategies to aid coral survival in a warming ocean should reduce nutrient concentrations and keep N:P ratios close to the Redfield ratio of 16:1.