The complex relationships between diatoms, bacterial communities, and dissolved organic matter: Effects of silicon concentration

Complex relationships exist between diatoms, bacterial communities and dissolved organic matter in aquatic ecosystems. Clarifying their relationships is essential to reveal biogeochemical cycles. In this study, the diatom Cyclostephanos invisitatus was cultured in standard silicon (SS), no silicon (NS), and excessive silicon (ES). Integrated 16S rDNA high-throughput sequencing and excitation-emission matrix parallel factor analysis (EEM-PARAFAC) were used to investigate diatom growth, bacterial community and dissolved organic matter (DOM) characteristics. It was found that the abundance of diatoms decreased in no silicon and excessive silicon conditions. Further, the composition and function of the associated bacterial communities were affected. The Proteobacteria and Cyanobacteria were more highly represented in the attached bacteria, whereas the Proteobacteria and Bacteroidetes were more highly represented in the free-living bacteria. Alpha diversity showed different trends with incubation time in all treatments. Moreover, genes (K01995 to K01998) involved in environmental information processing were more abundant in diatoms during the stationary phase in attached bacterial community. Stochastic processes play a key role in the formation of bacterial communities, both for attached and free-living bacteria. In addition, the differences in fluorescence components (protein and humic-like) and spectral indices (fluorescence index, biological index, freshness index, and humification index) revealed intricate relationship between microalgae and bacteria. Overall, this work confirmed the importance of silicon concentration in regulating diatom-associated bacterial communities and DOM characteristics.