Homogenization of Endosymbiont Communities Hosted by Equatorial Corals during the 2016 Mass Bleaching Event

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.

Decadal stability in coral cover could mask hidden changes on reefs in the East Asian Seas

Coral reefs in the Central Indo-Pacific region comprise some of the most diverse and yet threatened marine habitats. While reef monitoring has grown throughout the region in recent years, studies of coral reef benthic cover remain limited in spatial and temporal scales. Here, we analysed 24,365 reef surveys performed over 37 years at 1972 sites throughout East Asia by the Global Coral Reef Monitoring Network using Bayesian approaches. Our results show that overall coral cover at surveyed reefs has not declined as suggested in previous studies and compared to reef regions like the Caribbean. Concurrently, macroalgal cover has not increased, with no indications of phase shifts from coral to macroalgal dominance on reefs. Yet, models incorporating socio-economic and environmental variables reveal negative associations of coral cover with coastal urbanisation and sea surface temperature. The diversity of reef assemblages may have mitigated cover declines thus far, but climate change could threaten reef resilience. We recommend prioritisation of regionally coordinated, locally collaborative long-term studies for better contextualisation of monitoring data and analyses, which are essential for achieving reef conservation goals.

Molecular diversity and assemblages of coral symbionts (Symbiodiniaceae) in diverse scleractinian coral species

The scleractinian coral-associated symbiotic algae Symbiodiniaceae plays an important role in bleaching tolerance and coral resilience. In this study, coral-associated Symbiodiniaceae communities of 14 reef sites of Perhentian and Redang Islands Marine Parks (Malaysia, South China Sea) were characterized using the high-throughput next-generation amplicon sequencing on the ITS2 rDNA marker to inventory the Symbiodiniaceae diversity from a healthy tropical reef system and to generate a baseline for future studies. A total of 64 coral-Symbiodiniaceae associations were characterized in 18 genera (10 families) of scleractinian corals using the SymPortal analytical framework. The results revealed the predominance of Symbiodiniaceae genera Cladocopium (average 82%) and Durusdinium (18%), while Symbiodinium, Breviolum, Fugacium, and Gerakladium were found as minor groups (<0.01%). Of the 39 Cladocopium and Durusdinium major ITS2 sequences, 14 were considered dominant/sub-dominant, with C3u as the predominant type (63.3%), followed by D1 (15%), C27 (10.1%), and C15 (6.9%). A total of 19 and 13 Cladocopium and Durusdinium ITS2-type profiles were detected across the coral species, respectively. Symbiodiniaceae diversity and richness recorded in this study were higher when compared to other reefs in the proximity. With the increasing coral-Symbiodiniaceae associations archived, the database would provide a baseline to assess the changes of Symbiodiniaceae communities in the coral hosts and to explore the potential adaptive roles of this coral-algal association.