Evaluating the diversity of the enigmatic fungal phylum Cryptomycota across habitats using 18S rRNA metabarcoding

Fungi in the phylum Cryptomycota have been recovered in numerous environmental DNA (eDNA) surveys but are only known from five described genera of intracellular parasites. These fungi are common in aquatic and soil habitats, but little is known about their relative diversity and specificity among particular habitats. We surveyed Cryptomycota from 80 eDNA samples including freshwater, soil, and marine habitats using Cryptomycota-preferential primers coupled with long-amplicon PacBio sequencing (1.2 kb of the 18S rRNA gene region). We found that freshwater samples were the most diverse, comprising 175 operational taxonomic units (OTUs) of Cryptomycota and also showed a high abundance of the related algae-parasitic group Aphelidiomycota, while marine samples were the least diverse with 25 OTUs. The composition of Cryptomycota communities was influenced by habitat, with freshwater and soil showing statistically distinct communities. Phylogenetic analyses showed that the present survey recovered most previously sampled major clades of Cryptomycota, but most (61%) OTUs were novel to this study, indicative of an extensive diversity of the group that remains largely uncharacterized.