Phycosphere microbiome contributes to ecological dominance of diatoms: A comparative study of <i>Cyclotella atomus</i> and <i>Ulnaria ulna</i>

Abstract Diatoms play a crucial role in aquatic ecosystems, yet the mechanisms underlying their long-term dominance remain poorly understood. This study investigated the relationship between diatom ecological persistence and their phycosphere bacterial communities by comparing the long-term dominant species Cyclotella atomus with the short-term dominant species Ulnaria ulna. 16S rRNA gene sequencing combined with predictive functional profiling revealed that the bacterial community associated with C. atomus was more diverse, stable, and interconnected than that associated with U. ulna. Taxonomic analysis identified key bacterial taxa such as Gemmatimonas, Sphingobium, and Pseudorhodoferax enriched in C. atomus. Co-occurrence network analysis demonstrated higher microbial interaction complexity in C. atomus, enhancing functional redundancy and ecosystem stability. Functional predictions indicated significant enrichment in carbohydrate metabolism (glycosaminoglycan degradation, pentose/glucose interconversion) and stress response pathways (betaine biosynthesis, xenobiotic metabolism by cytochrome P450) in the C. atomus microbiome, supporting a mutualistic relationship in which diatom-derived EPS sustains specialized bacteria that reciprocate with vitamin B12, phytohormones, and chemical defenses. Based on these results, a mutually reinforced symbiotic cycle model was proposed to illustrate how the diatom and its phycosphere microbiome established a resilient holobiont capable of prolonged ecological dominance. The bacterial community associated with each diatom species exhibited host specificity and contributed to the maintenance of host dominance. These findings highlight the critical role of microbial partnerships in diatom success, offering new insights for predicting phytoplankton community dynamics and managing aquatic ecosystems.