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Diatoms and bacteria are known for being the first colonizers of submerged surfaces including the skin of marine reptiles. Sea turtle carapace and skin harbor diverse prokaryotic and eukaryotic microbes, including several epizoic diatoms. However, the importance of diatom-bacteria associations is hardly investigated in biofilms associated with animal hosts. This study provides an inventory of diatoms, bacteria and diatom-associated bacteria originating from loggerhead sea turtles using both metabarcoding and culturing approaches. Amplicon sequencing of the carapace and skin samples chloroplast gene rbcL and 16S rRNA gene detected, in total, 634 diatom amplicon sequence variants (ASVs) and 3661 bacterial ASVs, indicating high diversity. Cultures of putative epizoic and non-epizoic diatoms contained 458 bacterial ASVs and their bacterial assemblages reflected those of their host. Diatom strains allowed for enrichment and isolation of bacterial families rarely observed on turtles, such as Marinobacteraceae, Alteromonadaceae and Alcanivoracaceae. When accounting for phylogenetic relationships between bacterial ASVs, we observed that related diatom genera might retain similar microbial taxa in culture, regardless of the turtle's skin or carapace source. These data provide deeper insights into the sea turtle-associated microbial communities, and reveal the potential of epizoic biofilms as a source of novel microbes and possibly important diatom-bacteria associations.

The chain-forming diatom Chaetoceros vixvisibilis is one of the most abundant and frequent diatoms in the northeastern Adriatic Sea. This species had not been previously studied by electron microscopy. Its general morphology is similar to those species allocated in the subgenus Hyalochaete: cells and chains (straight, of variable length, usually 4-8 cells per chain, but can be longer) of delicate appearance, valves thinly silicified, with slightly eccentric annulus and costae radiating from it, a single rimoportula present only at terminal valves, long, thin and delicate setae perforated by tiny poroids with no spines, and one plate-like chloroplast per cell. Setae with no spines are not common among members of Hyalochaete (and the whole genus Chaetoceros), except C. socialis. The most characteristic feature is, however, the resting spores commonly found, which also show morphological variability, from solitary to paired, both valves convex to domed and surface smooth or with small granules, and one to four strong spines often branching dichotomically. Abundances of C. vixvisibilis are positively correlated to the Po River inflow. Maximum abundances (> 10 6 cells L -1 ) were found in the period April-July, in the temperature range between 12 and 16°C, salinity between 33 and 38, when total phosphorus concentration is higher than 0.4 μmol L -1 , but occasionally also in nitrogen limited conditions.