engleski

Growth dynamics of epizoic Achnanthes elongata and non-epizoic Psammodictyon panduriforme in co- cultures

Biofilm-forming diatoms occur on external surfaces of marine vertebrates such as killer whales and loggerhead sea turtles. There are indications that certain diatom taxa are host- specific but to date, conclusive evidence is lacking. Loggerhead sea turtles (Caretta caretta) represent a peculiar habitat, as sea turtles exhibit complex behaviors such as ontogenetic shifts, courtship behavior, and long-distance migrations. Furthermore, the sea turtle carapace and skin have shown high potential for new diatom species discovery. The mechanisms underlying substrate specificity in putative turtle-specific (epizoic) versus non-specific (non-epizoic) diatom taxa remain unclear as some epizoic taxa have not yet been found on any other substrates in the turtles’ environment, but seem to be easily cultured once isolated (e.g., Achnanthes, Poulinea, Proschkinia). In this study, we build on our previous work that focused on establishing monocultures and a polyphasic approach (morphology, molecular markers) to identifying diatoms from carapace and skin biofilms of incidentally caught loggerhead sea turtles in the Adriatic Sea. Morphological and DNA metabarcoding investigations of sea turtle- associated diatom assemblages regularly find putative epizoic Achnanthes elongata on different sea turtle species in high abundances regardless of A. elongata slower growth rates (in culture) and usually higher incidences of non-epizoic opportunists. We hypothesize that, in laboratory co-cultures, A. elongata and non-epizoic Psammodictyon panduriforme change their respective growth rates in favor of A. elongata. We suppose similar interactions could take place on the turtle carapace or skin, thus explaining an aspect of A. elongata prevalence on the sea turtle surfaces despite faster-growing opportunistic taxa. Although laboratory co- cultures cannot fully reflect the complex interactions between diatoms and other microorganisms on turtles’ surfaces, they provide a basis for further exploration and interpretation of data obtained on epizoic diatoms. To test our hypothesis and investigate potential interactions among turtle- associated diatoms we assessed changes in growth and biomass of A. elongata and P. panduriforme by co- culturing. The co-cultivations were performed in cell culture plates and quantified by a multi- mode reader (bright field and CY5 fluorescence images). We present the initial results pointing to minor changes in growth rates of A. elongata when co-cultured with P. panduriforme, while P. panduriforme tended to grow slower in co-cultures than in monocultures.

diatoms, co-culture, epizoic

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