Seasonal dynamics of pelagic mycoplanktonic communities: interplay of taxon abundance, specialization grade, and biotic interactions.

Marine fungi are an important component of pelagic planktonic communities. However, our understanding towards the fungal involvement in marine degradation processes and implications on food web structures is still severely lacking. Fungal degradation activity and biotic interactions depend on taxon-specific characteristics like abundance, life strategy, and specialization. Here, we investigate the association of marine fungal taxon dynamic, environmental factors including fungal biotic interactions, and the role they hold in the fungal community. Marine pelagic surface water was sampled weekly over the course of one year in the vicinity of the island of Helgoland in the German Bight (North Sea). Marine fungal communities were 18S rRNA tag-sequenced and correlated to environmental data including phytoplankton, zooplankton, and abiotic factors. Finally, co-occurrence network analyses were carried out. Four different pattern types of fungal OTUs were identified reflecting the relationship with environmental factors and the trophic mode of the taxon. Interestingly, abiotic factors played a minor role for fungal community dynamics while significant correlations to diverse zooplankton groups pointed towards highly complex and manifold relationships such as parasitic, predaceous, or grazing. Furthermore, the sequence abundance of many fungal OTU's strongly increased in seasons with low total phytoplankton cell counts, which could suggest a niche separation for fungi and bacteria. The grade of specialization and abundance of fungal OTUs was crucial to the role they hold within the fungal community. Thus, mainly rare OTUs contributed to the temporal persistence of the fungal community by a switching -on and off- of single members. In contrast, the generalists may ensure continuous functionality while cross-linking the community to other organisms and higher trophic levels. The results presented here suggest diverse ways in which marine fungi can be embedded into the marine microbial loop and food web. This knowledge now serves as a basis for the design of new experimental studies focusing on the functionality and interaction of single OTUs.