Acquisition and allocation of carbon via autotrophic and heterotrophic pathways: Implications for sexual reproduction in corals after bleaching

The success of future reefs will depend on a coral’s ability to continue sexual reproduction. Yet, little is known about the trophic pathways that corals use to successfully reproduce in stressful environments. We followed the acquisition of carbon by autotrophic and heterotrophic pathways with a pulse-chase experiment in corals Montipora capitata and Porites compressa, 8 months after a natural bleaching event. Adult colonies that bleached and did not bleach were exposed to ¹³C-labelled seawater or zooplankton. Allocation to host tissue, symbionts and developing eggs was measured at 24 and 168 hours; M. capitata released egg-sperm bundles at 72-120 hours and were also measured for carbon isotopic values. In both species, carbon acquisition to eggs and egg-sperm bundles occurred via autotrophy with minimal acquisition of labelled ¹³C occurring via heterotrophy. Carbon acquisition by adults was influenced by bleaching, while allocation to gametes was not. At 24 hours, tissues of previously bleached colonies of M. capitata incorporated 25-50% more labelled ¹³C via autotrophy than colonies that did not bleach. Yet there was no difference in the allocation of labelled carbon to egg-sperm bundles for bleached and non-bleached colonies. By 168 hours, bleached colonies of M. capitata were depleted in labelled ¹³C, while non-bleached colonies retained a labelled pool. Labelled ¹³C was also maintained in developing eggs at 168 hours in all colonies, indicating that carbon allocation for egg growth is an important priority for this species despite bleaching. Similar patterns were observed in P. compressa, except that the labelled ¹³C pool was retained at 168 hours in previously bleached colonies. Species-specific differences in carbon allocation may be due to their different gametogenic cycles and/or energetic requirements. This study provides an important baseline to assess the impacts of bleaching on reproduction.