Highly Diverse Symbiodiniaceae Types Hosted by Corals in a Global Hotspot of Marine Biodiversity.

Symbiotic dinoflagellates in the genus Symbiodiniaceae play vital roles in promoting resilience and increasing stress tolerance in their coral hosts. While much of the world's coral succumb to the stresses associated with increasingly severe and frequent thermal bleaching events, live coral cover in Papua New Guinea (PNG) remains some of the highest reported globally despite the historically warm waters surrounding the country. Yet, in spite of the high coral cover in PNG and the acknowledged roles Symbiodiniaceae play within their hosts, these communities have not been characterized in this global biodiversity hotspot. Using high-throughput sequencing of the ITS2 rDNA gene, we profiled the endosymbionts of four coral species, Diploastrea heliopora, Pachyseris speciosa, Pocillopora acuta, and Porites lutea, across six sites in PNG. Our findings reveal patterns of Cladocopium and Durusdinium dominance similar to other reefs in the Coral Triangle, albeit with much greater intra- and intergenomic variation. Host- and site-specific variations in Symbiodiniaceae type profiles were observed across collection sites, appearing to be driven by environmental conditions. Notably, the extensive intra- and intergenomic variation, coupled with many previously unreported sequences, highlight PNG as a potential hotspot of symbiont diversity. This work represents the first characterization of the coral-symbiont community structure in the PNG marine biodiversity hotspot, serving as a baseline for future studies.