How did nutrient ratios change the effects of heat on two corals?

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The ratio of dissolved inorganic nitrogen to phosphorus (N:P) may influence corals’ susceptibility to heat stress, but comparative studies of different ratios in soft and hard corals had not been conducted. Researchers tested two N:P ratios in two common Indo-Pacific reef corals: the pulsating soft coral Xenia umbellata and the hard coral Montipora digitata.

For five weeks, corals were exposed to N:P 19:1 (37 µM nitrate and 2 µM phosphate) or 5:1 (37 µM nitrate and 8 µM phosphate), alongside a low-nutrient control of 3:1 (0.003 µM nitrate and 0.001 µM phosphate). After two weeks, temperatures were gradually raised from 26 to 32 °C.

Nutrient enrichment alone did not affect any measured response in M. digitata. In X. umbellata, it was associated with 9% mortality and reduced pulsation rates by 100% at 5:1 and 58% at 19:1. Heat stress alone significantly reduced Symbiodiniaceae density and chlorophyll a content in M. digitata, but did not affect X. umbellata.

Combined warming and nutrient enrichment significantly increased mortality: at 5:1 and 19:1, respectively, mortality was 100% and 92% in M. digitata and 87% and 33% in X. umbellata. In M. digitata, Symbiodiniaceae density and chlorophyll a content responded as they did to heat stress alone; in X. umbellata, these measures were unaffected. X. umbellata pulsation fell by 100% under both combined treatments.

The researchers suggest nutrient enrichment alone likely impaired X. umbellata’s metabolism through the energy-intensive reduction of nitrate to ammonium, stopping pulsation. Under combined warming and nutrient addition, however, X. umbellata showed better fitness than M. digitata, suggesting it may cope better with predicted environmental changes. Both corals were negatively affected, especially at 5:1.

The stronger effect of 5:1 likely resulted from greater energy depletion through nitrogen assimilation, while the imbalanced 19:1 ratio may have limited nitrogen assimilation and mitigated stress. The findings suggest high phosphorus combined with high nitrogen may accelerate coral stress. The researchers recommend tailored nutrient management that reduces nutrient concentrations and keeps N:P ratios close to the Redfield ratio of 16:1 to help corals survive in a warming ocean.