Impact of plastic debris on the gut microbiota of Caretta carretta from Northwestern Adriatic sea

Plastics are nowadays a relevant threat for the ecological balance in marine ecosystems. Small plastic debris can enter marine food webs through various marine organisms, with possible consequences on their physiology and health. The loggerhead sea turtle (Caretta caretta), widespread across the whole Mediterranean Sea, is a flagship species, useful as indicator of the general pollution level of marine ecosystems. Intestinal microbiome, being at the interface between water, ingested food and the host physiology, could mediate, at least partly, the interaction between plastic pollution and animal health. Aiming at exploring possible influences of plastic debris ingestion on the intestinal microbiome, we collected fecal samples taken from 45 turtles rescued between 2017 and 2019 in the Northwestern Adriatic Sea (Italy). Due to the great availability of food and warm shallow waters, this area is an ideal foraging and over-wintering site for Mediterranean loggerheads of different age classes. We assessed occurrence and content of plastic debris in the samples and subsequently characterized for their bacterial composition of by using 16S rRNA next generation sequencing. Plastic debris were found in all samples (except for one), with a significant negative correlation between particles amount and the sea turtle size, suggesting a possible relationship between life stage features and plastic ingestion. Plastic debris content did not significantly affect the general asset of the fecal microbiome. However, the amount of plastic debris showed to affect the relative abundance of several specific operational taxonomic units, some of which assigned to bacterial species previously isolated from polluted environmental sites. Besides confirming the role of the sea turtle as relevant flagship species for the degree of plastic pollution of the marine environment, our study paves the way to the exploration of the impact that plastic debris ingestion can have on the microbial counterpart of large marine organisms, with potential feedback consequences on the animal and ecosystem health.