Malassezia is a highly contaminating fungus in environmental studies

Malassezia is the most prevalent member of the human skin mycobiota, known for its lipid dependence and inability to synthesize long-chain fatty acids, necessitating reliance on external sources. It is known as an opportunistic pathogen and causes several common skin diseases in humans. Malassezia exhibits remarkable niche breadth, inhabiting soils, dust, as well as various marine environments from polar regions to deep-sea vents. The consistent presence of Malassezia in marine habitats, including those associated with marine animals suggests that the fungus is able to grow and play a fundamental role in marine ecosystems. In this study, we explored metagenomic sequences from the SRA database to investigate the presence, distribution, and origins of Malassezia species within marine ecosystems, with the goal of assembling Malassezia genomes. Using various search methods we found that about 1-10 percent of the shotgun or amplicon data contained Malassezia sequences. Malassezia was also detected in all non-marine habitats and across various sequencing strategies. However, the relative abundance of Malassezia in each data set was low, and we also considered whether Malassezia could have been introduced as a contaminant. We found a significant correlation between human and Malassezia sequences in marine environments, suggesting potential contamination from human sources. From the most Malassezia-rich datasets, we attempted to generate metagenome-assembled genomes. Genome assembly and purification of Malassezia contigs showed low completeness levels, with a maximum of 2.2%. Phylogenetic analysis identified the contigs as closely related to human-associated M. globosa and M. restricta, along with M. sympodialis and M. pachydermatis, which were the most prevalent species across all studied environments. Our data point to a high likelihood that many observations of Malassezia in marine and other ecosystems are due to the movement of Malassezia to the sequenced sample from humans. However, the data do not exclude the possibility that these fungi can exist in a free-living or symbiotic state in marine ecosystems after their transfer, nor can we rule out the possibility that novel Malassezia species in marine habitats escaped our detection strategy.