Reconstructing the dynamics of past coral endosymbiotic algae communities using coral ancient DNA (coraDNA)

Symbiodiniaceae endosymbiotic algae are essential for the development and survival of most corals. Some corals can shift and/or shuffle their associated Symbiodiniaceae to cope with temporal environmental stress. However, the large-scale temporal dynamics of Symbiodiniaceae at the coral colony level is largely under documented mainly due to technical limitations.We here combined the use of ancient DNA and metabarcoding (ITS2) to monitor the dynamics of Symbiodiniaceae community associated with a colony of Porites lobata from New Caledonia over the last century. Seventeen samples were extracted at successive coral skeletal bands dating from 2018 back to the 1870's. Four additional samples were extracted just before and after two past extreme climatic events in 1997-1998 (ENSO) and 2010 (hot winter).Overall, we detected 13 OTUs, nine of which were affiliated to the Symbiodiniaceae Cladocopium clade, one to P. lobata, one to Azadinium spinosum (Dinophycae); the two other OTUs remained unidentified. One OTU was largely predominant and was ubiquitous overall samples. The number of OTUs was marginally correlated to the total number of sequences obtained per sample but not to the age of the coraDNA sample. The Symbiodiniaceae community diversity was generally stable over time but decreased after the 2010 winter which potentially indicates a temporal expulsion of most Symbiodiniaceae at this time.This study paves the way for more investigations on the large-scale temporal evolutionary dynamics of coral holobionts at the colony scale.