Five-Year Time Series Reveals Short-Term Blooms of Planktonic Fungi in a Coastal Mediterranean Site

Fungi have gained recognition as key organisms within the pelagic marine food webs over the past few decades, with studies showing they constitute a significant proportion of eukaryotes in different marine ecosystems. However, how this proportion varies with time, what triggers fungal blooms, and which fungal clades are involved in those blooms are largely open questions. Here, we used a 5-year, high-frequency 18S V4 metabarcoding time series from a well-documented coastal site of the North West Mediterranean Sea to address these questions. This time series has one of the highest temporal resolutions (up to two samples a week) ever used to investigate marine fungal dynamics. We showed that the dynamics of the fungal relative abundance at this site are mainly chaotic, with short-term blooms dominated by 41 Amplicon Sequence Variants (ASVs), mainly assigned to Ascomycota. Most of these ASVs are not restricted to the Mediterranean Sea or the marine environment. We found weak links between biotic or abiotic parameters and the relative abundance of Fungi. Our study highlights the relevance of high-frequency time series to study marine fungal dynamics, as it lowers the risk of aliasing and spurious conclusions.