Contrasting physiological adaptation strategies to natural environmental change in two Red Sea coral holobionts

Coral holobionts acquire energy and nutrients from heterotrophic feeding, Symbiodiniaceae symbiosis, and additional metabolic functions (e.g. nitrogen (N) fixation) from associated bacterial communities. Since symbioses often require stable environmental conditions, corals in environments with seasonal variability have likely evolved adaptation strategies by either maintaining (i.e. regulating) or shifting (i.e. conforming) key functional traits, but empirical data is needed. We investigated carbon (C) and N elemental and stable isotope ratios alongside bacterial community composition in the hydrocoral Millepora dichotoma and the scleractinian coral Stylophora pistillata every two months over one year. These data were integrated with environmental parameters to investigate potential adaptation strategies of the coral holobionts over a seasonal cycle. S. pistillata showed temporal changes in δ13C, δ15N and C:N ratios in both host and Symbiodiniaceae tissues (indicating stable host-Symbiodiniaceae C/N cycling), in combination with stable bacterial communities. M. dichotoma, did not exhibit temporal changes in elemental and stable isotope ratios, but higher δ15N and C:N variability, and 61% higher C:N ratios in Symbiodiniaceae compared to host tissue. Temporal shifts in bacterial communities resulted in significantly enriched predicted metabolic functions for C, N, and sulfur cycling in winter. Stable C/N cycling and bacterial community composition suggest a regulator-like life history strategy of S. pistillata, whereas variable C/N cycling and flexible bacterial communities indicate a conformer-like life history strategy for M. dichotoma. Both contrasting adaptation strategies enable these organisms to succeed amid current environmental change, yet to what extent this can be maintained under future climate scenarios remains to be investigated.