Abstract
Evidence has shown that individually feeding or reduced light can mitigate the negative effects of elevated temperature on coral physiology. We aimed to evaluate if simultaneous low light and feeding would mitigate, minimize, or exacerbate negative effects of elevated temperature on coral physiology and carbon budgets. Pocillopora damicornis, Stylophora pistillata, and Turbinaria reniformis were grown for 28 days under a fully factorial experiment including two seawater temperatures (ambient temperature of 25 °C, elevated temperature of 30 °C), two light levels (high light of 300 μmol photons m−2 s−1, low light of 150 μmol photons m−2 s−1), and either fed (Artemia nauplii) or unfed. Coral physiology was significantly affected by temperature in all species, but the way in which low light and feeding altered their physiological responses was species-specific. All three species photo-acclimated to low light by increasing chlorophyll a. Pocillopora damicornis required feeding to meet metabolic demand irrespective of temperature but was unable to maintain calcification under low light when fed. In T. reniformis, low light mitigated the negative effect of elevated temperature on total lipids, while feeding mitigated the negative effects of elevated temperature on metabolic demand. In S. pistillata, low light compounded the negative effects of elevated temperature on metabolic demand, while feeding minimized this negative effect but was not sufficient to provide 100% metabolic demand. Overall, low light and feeding did not act synergistically, nor additively, to mitigate the negative effects of elevated temperature on P. damicornis, S. pistillata, or T. reniformis. However, feeding alone was critical to the maintenance of metabolic demand at elevated temperature, suggesting that sufficient supply of heterotrophic food sources is likely essential for corals during thermal stress (bleaching) events.
Highlights
Under predicted future ocean conditions, mean global ocean temperature is estimated to increase 1–4 ◦ C by 2100 [1]
Chlorophyll a content was lower under elevated temperature compared to ambient temperature and under high light compared to low light (Figure 3a; Table S3)
Calcification was lower in the low light + fed corals compared to all other treatments, irrespective of temperature (Figure 3c; Table S3), while no significant differences were observed in biomass (Figure 3d; Table S3)
Summary
Under predicted future ocean conditions, mean global ocean temperature is estimated to increase 1–4 ◦ C by 2100 [1]. Under thermal stress, the loss of the endosymbiotic algae and/or photosynthetic pigments causes photosynthesis to decline (e.g., [12,13,14]), and corals are no longer able to meet metabolic demand through photosynthesis alone [7] To compensate for this decrease in photosynthetic capacity, coral can undertake one or more of the following strategies to aid in the recovery and maintenance of metabolic demand: (1) increase heterotrophy (e.g., [7,15,16,17]), (2) catabolize energy reserves (e.g., [7,18,19]), (3) decrease respiration (e.g., [14,20,21]), and (4) decrease calcification rates (e.g., [22,23,24]). Heterotrophy can mitigate bleaching damage to algal endosymbionts by stimulating photosynthesis and the re-establishment of host–algal symbioses [20,25,26], and by providing fixed carbon for tissue building and lipid synthesis [17,27,28]
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