Malassezia in environmental studies is derived from human inputs

The fungus Malassezia is the most prevalent member of the human skin mycobiota, known for its lipid dependence and inability to synthesize long-chain fatty acids. 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 Sequence Read Archive 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 up to 10% of the shotgun and amplicon data sets analyzed contained Malassezia sequences. However, the relative abundance of Malassezia in each data set was low, and there was a strong correlation between human and Malassezia sequences in marine environments, consistent with contamination. From the most Malassezia-rich data sets, we attempted to generate metagenome-assembled genomes. However, Malassezia contigs together showed low genome completeness levels, with a maximum of 2.2% in a sample. Phylogenetic analysis identified the contigs as closely related to human-associated Malassezia globosa and Malassezia restricta, along with Malassezia sympodialis and Malassezia pachydermatis, which were the most prevalent species across all studied environments. Our data suggest that many observations of Malassezia in the environment are derived from human sources and that widespread contamination confounds the exploration of Malassezia diversity and ecological role in the marine environment.

Malassezia is the singular fungus most associated with humans. It colonizes mammalian skin and requires host-derived fatty acids to grow. Widespread sequencing of environmental DNA surprisingly showed that Malassezia is also ubiquitous outside of mammalian hosts. Malassezia is frequently found in marine habitats where it is associated with corals, deep sea vents, diatoms, and more. Given its widespread presence, we reasoned that public metagenomic data could be used to assemble a genome sequence of an uncultured marine Malassezia. However, we found that Malassezia was ubiquitous but never abundant in marine samples and that the few metagenomes we could assemble were consistent with recent human introduction. We also found that the presence of human DNA in sequencing data sets is strongly correlated with the presence of Malassezia DNA, and while not ruling out the growth and survival of Malassezia in marine habitats, they suggest widespread contamination of public data with Malassezia.