Genetic diversity of single-celled microorganism Blastocystis sp. and its associated gut microbiome in free-ranging marine mammals from North-Western Mediterranean Sea

Blastocystis sp. is frequently identified in humans and several animal hosts exhibiting a wide genetic diversity. Within One Health perspective, data on Blastocystis sp. distribution and its circulating subtypes (STs) from the terrestrial environment are available, while those from the marine environment remain still scare. A genetic and 16S rRNA gene sequencing analysis were conducted over the period 2022–2024 by screening fecal samples from four different species of free-ranging marine mammals (sperm, fin, long-finned pilot and Cuvier's beaked whales) circulating within North-Western Mediterranean Sea. 10 out of 43 fecal samples (23.2 %) were found positive to Blastocystis sp. using molecular tools. A predominance of zoonotic subtype ST3 among different species of marine mammals as well as the presence of ST1 allele 4 subtype and even untypable subtype within the fin whale specimen was reported. Moreover, Firmicutes, Bacteroidetes and Proteobacteria within the different Blastocystis-carrier marine mammal species as well the identification of Archaebacteria from Methanomethylophilaceae family within the fin whale isolate were detected by Illumina V3-V4 generated data. The present survey presents new insights regarding Blastocystis sp. prevalence and its circulating zoonotic ST1-ST3 subtypes from the marine environment, as well as its associated gut microbiome, providing hence baseline data for a better understanding of the associated risk and to prevent human and marine ecosystem exposure to these anthropogenic microorganisms.