Molecular diversity of Symbiodiniaceae in temperate scleractinian corals from the Ligurian Sea (NW Mediterranean Sea)

Photosynthetic, endosymbiotic dinoflagellates of the family Symbiodiniaceae play a pivotal role in the growth and health of stony corals. However, this mutualistic symbiosis remains relatively understudied in temperate scleractinians from the Mediterranean Sea, particularly when compared to the extensive research on coral-algae symbiosis in tropical and subtropical reef-building corals. Here, we assessed the genetic diversity of Symbiodiniaceae associated with four scler- actinian coral species from the Bergeggi Island Marine Protected Area (Ligurian Sea, NW Mediterranean Sea) using high-throughput sequencing of the internal transcribed spacer 2 rDNA (ITS2) and Sanger sequencing of the chloroplast 23S rDNA (cp23S). Oculina patagonica hosted Breviolum psygmophilum (ITS2 type profiles B2-B2j-B2aa and B2-B2f- B2m) while also having a low number of post-MED ITS2 sequences belonging to Philozoon sp. (P1bo/P1fb). Conversely, Madracis pharensis showed mixed proportions of both Philozoon and Breviolum ITS2 sequences. Balanophyllia europaea was associated with P. balanophyllum (P1fb/P1fc-P1fd-P1od-P1oe and P1fb), characterised by multiple ITS2 intragenomic variants within each individual. Cladocora caespitosa harboured Philozoon sp. (P1fb), with the exception of a sample hosting also B. psygmophilum. These results aligned with previous studies from other Mediterranean localities and con- tributed to a broader understanding of the ecology of temperate coral-Symbiodiniaceae associations. Given the increasing frequency of coral mortality events in the Mediterranean Sea, further research into this symbiotic relationship is essential for assessing the resilience and adaptive capacity of temperate scleractinian corals.