Annual recurrence of climactic microbial communities in shallow waters of the North Mediterranean

In temperate coastal environments, wide fluctuations of biotic and abiotic factorsgovern microbiome dynamics. To unveil the relationship between recurrentecological patterns and planktonic microbial communities, we analyzed a monthly-sampled 3-year time series of 16S rRNA amplicon sequencing data, alongside coreenvironmental variables, collected at two adjacent stations in the northern AdriaticSea. Through the multivariate exploration of our dataset, we identified threerecurrent climactic communities, whose behaviour was mainly driven by changes indaily heliophany and temperature. Mixotrophs (e.g., Ca. Nitrosopumilus, SUP05clade) thrived under oligotrophic, low-light conditions, whereas copiotrophs (e.g.,NS4, NS5, NS9) flourished at higher temperatures and organic matter availability.The third group of communities, centered around April, was characterized by anoisier set of amplicon sequence variants (ASVs), including both copiotrophic (i.e.,NS4 and NS5) and photo-auto/heterotrophic organisms (e.g., Synechococcus sp.,Roseobacter clade), suggesting that the day length increase supportsphotoheterotrophic organisms and triggers phytoplankton blooms and theconsequent organic matter load. At the interannual scale, we identified three mainenvironmental stable states, each one exploited by a set of distinct ASVs pointing tothe emergence of climactic communities according to their trophic modes. ASVsbelonging to genera shared among recurring communities showed seasonal andenvironmental differentiation, pinpointing the need to take full advantage of thehighest possible taxonomic resolution since even closely phylogenetically relatedASVs may represent distinct ecotypes with different ecological roles.