First report on yeasts Meyerozyma guilliermondii and Trichosporon asahii associated with Crassostrea sp. oysters in an intertidal region and in an oyster farm on the Amazonian coast of Maranhão State, Brazil

Yeasts are widely distributed in marine ecosystems and play essential roles in maintaining their productivity. However, the identification of yeasts in marine invertebrates, especially in organisms used for human consumption, such as oysters, remains limited. This study aimed to identify yeast species in Crassostrea sp. oysters, collected from wild-harvesting and aquaculture sites along the Amazonian coast of Maranhão, Brazil. A total of 40 oyster samples were collected over two sampling months and subjected to microbiological analysis using Potato Dextrose Agar following serial decimal dilutions. Molecular identification was performed through ITS rRNA sequencing, and species similarity was assessed in dendrograms. Microbiological results revealed the presence of both mycelial fungi and yeasts, with higher densities of mycelial fungi found in the extraction and cultivation areas, whose averages were approximately 1.7 × 103 and 2.1 × 103 CFU/g, respectively, with high variability among samples. Yeasts were more abundant in cultivated oysters (4.3 × 103 CFU/g) compared to wild oysters (6.2 × 102 CFU/g). This study focused exclusively on yeast strains and molecularly identified Meyerozyma guilliermondii and Trichosporon asahii among the isolates. This is the first report of these species in oysters. These yeasts may have an allochthonous origin associated with anthropogenic factors and natural dispersion, as both are frequently found in warm-blooded animals. Therefore, this study provides relevant data for future investigations into the ecological roles of yeasts in marine invertebrates and their impacts on coastal ecosystems.