Pezizomycotina dominates the fungal communities of South China Sea sponges Theonella swinhoei and Xestospongia testudinaria.

Compared with the knowledge of sponge-associated bacterial diversity and ecological roles, the fungal diversity and ecological roles of sponges remain largely unknown. In this study, the fungal diversity and protein synthesis potential in two South China Sea sponges Theonella swinhoei and Xestospongia testudinaria were investigated by rRNA vs. rRNA gene analysis. EF4/fung5 was chosen after a series of PCR tests to target fungal 18S rRNA and 18S rRNA gene. Altogether, 283 high-quality sequences were obtained, which resulted in 26 Operational taxonomic units (OTUs) that were assigned to Ascomycota, Basidiomycota, and Blastocladiomycota. At subphylum level, 77.3% of sponge-derived sequences were affiliated with Pezizomycotina. The fungal compositions of T. swinhoei and X. testudinaria were different from that of ambient seawater. The predominant OTU shared between two sponges was rare in seawater, whereas the most abundant OTUs in seawater were not found in sponges. Additionally, the major OTUs of sponge cDNA datasets were shared in two sponges. The fungal diversity illustrated by sponge cDNA datasets correlated well with that derived from sponge DNA datasets, indicating that the major members of sponge-associated fungi had protein synthesis potential. This study highlighted the diversity of Pezizomycotina in marine sponge-fungi symbioses and the necessity of investigating ecological roles of sponge-associated fungi.