Symbiont-mediated tradeoffs between growth and heat tolerance are modulated by light and temperature in the coral Montipora capitata

Abstract

Corals’ endosymbiotic algae (Symbiodiniaceae) supply much of the energetic demands of the host, and some symbionts, such as Durusdinium spp., may confer higher thermal tolerance than others. However, there may be physiological tradeoffs during non-bleaching periods or across environmental gradients. The common Hawaiian reef-building coral, Montipora capitata, hosts the symbionts Cladocopium C31 and/or Durusdinium glynnii. Here, we measured growth of these holobionts over two months to test whether the higher heat tolerance of D. glynnii involved a tradeoff of lower growth, which could impact survivorship and competition for space on the reef. Additionally, we tested whether any such tradeoffs were modulated by light, which is a major driver of the ecological distribution of these symbionts in Kāneʻohe Bay. During the first month, when water temperatures were warmer (median 25.5 °C), there was no effect of symbiont on growth. However, over the cooler second month (median 23.4 °C), C31-dominated corals grew up to 77% faster than D. glynnii-dominated corals, and this growth advantage increased in higher light treatments (up to a daily light integral of 3.8 mol m−2 d−1, corresponding to 3.8–6.4 m depth). This advantage may explain why C31-dominated colonies are more prevalent at most depths; however, their increasing advantage at higher light levels is not reflected in the shallowest parts of Kāneʻohe Bay, where D. glynnii-dominated colonies become more prevalent. This may indicate that in these warmer surface waters, D. glynnii’s higher heat tolerance outweighs the growth advantage that would otherwise favor C31 during the cooler parts of the year, providing a net ecological benefit. As oceans continue to warm, understanding how symbionts and the environment mediate coral performance outcomes will help predict reef futures and inform conservation interventions where coral stocks are selected for specific traits or symbionts.