Co-occurrence patterns of photosymbiont genera in reef-building corals: a global review and meta-analysis

Coral reefs are experiencing unprecedented global declines driven by intensifying marine heatwaves, deoxygenation, and other consequences of climate change. One trait that may enhance coral resilience is their ability to harbour diverse communities of microalgal symbionts (Symbiodiniaceae). Cladocopium and Durusdinium are two common Symbiodiniaceae genera harbouring functionally different traits; however, the extent to which they co-occur within coral hosts has not yet been explored. Here, we conducted a systematic literature review and meta-analysis to assess global patterns of Cladocopium-Durusdinium co-occurrence and co-phylogeny, synthesising data from ~36,000 coral colonies across 378 studies, 98 coral genera, 77 countries and territories, 11 genetic markers, and 13 genetic analysis techniques. Co-occurrence prevalence across these studies was 9.6%, with estimates varying across methodological, biological, spatial, and study variables. Incorporation of more recent high-sensitivity genotyping and intra-colony sampling increased this estimate by up to 7.1-fold, suggesting that Cladocopium-Durusdinium co-occurrence may be more common than previously recognised. We also identified a co-phylogenetic signal between putative Cladocopium and Durusdinium taxa, suggesting that in some cases, these symbionts have co-evolved. Together, these findings contribute to the understanding of eco-evolutionary relationships between symbionts within coral hosts, providing important insight into the adaptive capacity of corals in a changing climate.