Impact of the invasive diatom species <i>Cymbella janischii</i> on riverine microbial biofilm communities and a potential role of bacterially produced zeatin

The diatom Cymbella janischii is an invasive species in Japan, causing nuisance blooms by forming thick mats in rivers. To date, there are no documented studies on the microbiome associations in C. janischii mats or the processes that drive bloom formation. This study used metabarcoding of diatoms, bacteria, and fungi to identify key species and assess the effects of C. janischii blooms on the benthic microbial communities. C. janischii blooms reduced diatom and bacterial species diversity, while fungal diversity remained stable. In addition, the diatom Nitzschia paleacea and the bacterium Flavobacterium sp. were observed to co-occur and vary in abundance, indicating a possible ecological link that may affect mat structure or function. Metagenomic predictions of bacterial functions showed that compared to benthic stones without visible C. janischii mats, mat-associated bacteria had enriched pathways related to the metabolism of carbohydrates, nucleotides, and amino acids, along with zeatin biosynthesis. Zeatin is a cytokinin phytohormone that stimulates plant growth and development. In vitro exposure of C. janischii to varying zeatin concentrations confirmed its growth-promoting effects, inducing cell proliferation and stalk formation. This study shows that zeatin stimulates the growth of C. janischii. The findings of this study provide new insights into microbiome diversity, identifying key taxa associated with C. janischii mats to help better understand bloom formation.