Photoautotrophic and heterotrophic carbon in bleached and non-bleached coral lipid acquisition and storage

Lipids are key biomolecules within the coral holobiont, serving as energy reserves and promoting bleaching tolerance. To better understand the physiological implications of bleaching and recovery on lipid acquisition and storage, an isotope pulse-chase labeling experiment was conducted on treatment (bleached) and control (non-bleached) Hawaiian corals Porites compressa and Montipora capitata after 1 and 11 months on the reef. After 1 month on the reef, 75% of the carbon in newly acquired lipids in treatment corals of both species was heterotrophic in origin, which is two times higher than that in control corals. However, stored lipids in both treatment and controls were hugely dominated by heterotrophic C in P. compressa (i.e., 75-100%) and by photoautotrophic C in M. capitata (i.e., 90-100%). After 11 months on the reef, elevated levels of heterotrophic C input in lipid acquisition and storage were only seen in treatment P. compressa. Many previously measured physiological parameters are recovered in P. compressa after 8 months, suggesting that the need for increased heterotrophic C input for lipids appears to last beyond recovery of all other parameters, indicating that P. compressa is not yet fully recovered even after 11 months. In M. capitata, lipid acquisition and storage were 28-52% heterotrophic in origin and 48-72% photoautotrophic and, along with many previously measured physiological parameters, were fully recovered after 11 months on the reef. However, the evidence suggests that carbohydrate and possibly protein acquisition was not fully recovered even after 11 months. These species-specific differences in lipid acquisition and storage mechanisms, and response patterns to temperature stress indicate that corals may take more than 11 months to recover from bleaching and that heterotrophic carbon is critical to coral lipids, especially when bleached.