An overview of diatom and bacterial diversity associated with Mediterranean loggerhead sea turtles

Sea turtles are known for a long time as a habitat for numerous and diverse microbes that happily live on the external and internal surfaces of their different body parts. Shells are especially known to be often covered with various macroepibionts and colourful biofilms formed by diatoms and bacteria. These microbes can be specialized and found almost exclusively associated with sea turtles but also opportunistic and found elsewhere in the turtle environment. In this presentation I am going to show you an overview of results obtained by research conducted within the TurtleBIOME project running in the period of 2018-2023 that involved a lot of very enthusiastic and dedicated participants and we are extremely thankful for their involvement. The main focus of our research was to characterize the microbial communities found in both the external (skin and shell) and the internal (gut, cloaca, oral cavities) habitats provided by loggerheads. We chose this species of sea turtle (Caretta caretta) as it is the most common turtle species in the Mediterranean Sea. We collected microbial samples from >120 animals from several locations in the Mediterranean area (Pula, Vis, and Lošinj in Croatia, Bari in Italy, and Amvrakikos Bay and Rethymnos in Greece) and analysed them using amplicon profiling with 16S, 18S, rbcL and ITS as molecular markers. The collected carapace and skin biofilm samples were also analysed using classical microscopical approach and that resulted in the description of several new diatom species. We have also isolated, identified, and maintained ca. 200 non-axenic diatom strains, which were then used in several experiments investigating the ecological preferences of diatoms themselves and their associated bacterial partners. We characterized the bacterial community of turtle-associated diatoms using metabarcoding and cultivation approach. This presentation will summarize these and other results we obtained while exploring the prokaryotic and microeukaryotic communities of loggerheads. Our multidisciplinary investigations provide the first inventory of loggerhead sea turtle endo- and epimicrobiota, revealing high diversity, different levels of host fidelity, and local biogeography of sea turtle associated microbes.