Heterotrophic feeding modulates the effects of microplastic on corals, but not when combined with heat stress

Plastic pollution is an increasing stressor adding pressure on coral reefs. Microplastic (MP) affects photosynthetic performance and growth of corals and may lead to bleaching. When corals are bleached and autotrophy is impaired, corals are more dependent on heterotrophic feeding to complement energy acquisition. While MP pollution is suspected to interfere with heterotrophic feeding, the effect of food availability on the tolerance of corals to MP pollution is still unknown. Here, we investigated how food availability affects the physiology of corals under MP pollution, by measuring coral growth, photophysiology, and tissue composition. Finally, we also investigated how the physiological response to MP and feeding are connected to bleaching susceptibility of the corals. To this end, we conducted a six-week aquarium experiment followed by a short-term heat stress phase with the coral species Pocillopora verrucosa and Stylophora pistillata exposed to three treatments: MP-free control and twice-weekly feeding (Control), MP-treatment and twice-weekly feeding (MP), and MP-treatment and daily feeding (MP + HF). Coral growth was similar across all treatments. However, MP treatment significantly decreased the tissue energy content of P. verrucosa, although it increased photosynthesis and respiration. High food availability partially mitigated the loss of tissue energy content observed in the MP treatment while maintaining photosynthesis and respiration rates comparable to control conditions. S. pistillata was not affected by MP exposure alone, but when combined with high feeding, photosynthesis decreased below that of the Control. When exposed to short-term heat stress, all corals bleached severely, however, both species bleached less in the MP treatment. These findings highlight that food availability and MP exposure elicit complex responses that influence the effect of other stressors such as heat stress. As MP continues to accumulate in rapidly warming oceans, further research is needed to understand the interactions between food availability and multivariate stressors on coral stress tolerance.