Algae and bacteria engage in ubiquitous interactions ranging from symbiosis to parasitism, forming one of the most fundamental ecological interactions. These interactions transcend mere physical proximity, relying on sophisticated bidirectional chemical communication, material exchange, and signal transduction. Specific manifestations include coordinated nutrient exchange, mutual provision of growth factors, quorum sensing-mediated signaling, and occasional predation or lysis of partners for resource acquisition.

Among algae, diatoms represent globally dominant primary producers and contribute ∼20% of global primary production. Their distinctive bio-silica frustules not only mediate the marine silicon cycle but also increase particulate organic carbon export via tight coupling between the silica pump and the biological carbon pump, thereby influencing global climate regulation. Conventional research has attributed diatom population dynamics primarily to abiotic conditions and the inherent physiological traits of diatoms themselves, while overlooking the phycosphere as a key modulator of diatom performance. The mechanisms by which specific diatom species maintain long-term or transient dominance in phytoplankton communities remain poorly understood.

A research team led by Professor BI Yonghong from the Institute of Hydrobiology (IHB) of the Chinese Academy of Sciences has successfully isolated and purified two diatom species from the Middle Route of China’s South‑to‑North Water Diversion Project: Cyclotella atomus, a long‑term dominant species, and Ulnaria ulna, a short‑term dominant species. The team characterised the community structure, interaction networks and metabolic potential of their phycosphere-associated microorganisms. The findings are published in ISME Communications, the official journal of the International Society for Microbial Ecology.

Each diatom harbours a distinct, host‑specific phycosphere bacterial community. The phycosphere microbiome of C. atomus was significantly enriched with Gemmatimonas, Sphingobium and Pseudorhodoferax, accompanied by higher bacterial species richness and evenness. By comparison, the phycosphere microbiome of U. ulna was enriched with Acidovorax, Methylobacterium, Novosphingobium and other bacterial taxa, with relatively lower richness and evenness.

Co‑occurrence network analyses revealed that the phycosphere microbial community of C. atomus formed a densely connected, intricate network characterised by close inter‑species microbial relationships and elevated functional redundancy. These features strengthened the resistance and adaptability of the diatom‑bacteria holobiont to fluctuating environmental conditions. In contrast, the phycosphere bacterial network of U. ulna was structurally fragmented with limited functional redundancy, rendering its holobiont much less resilient to environmental perturbations.

Functional predictions revealed marked upregulation of key metabolic pathways in the C. atomus phycosphere microbiome, including exopolysaccharide degradation, pentose‑glucose interconversion, betaine biosynthesis, cytochrome P450‑mediated xenobiotic metabolism and tryptophan metabolism. C. atomus secreted extracellular polymeric substances to feed symbiotic bacteria. In return, symbionts produced vitamin B₁₂ and phytohormones, and provided detoxification and allelochemical defenses, enabling the host to withstand stress and outcompete other phytoplankton and forming a positive mutually‑beneficial feedback loop. In contrast, the U. ulna phycosphere microbiome was dominated by basal growth‑ and nitrogen‑related metabolic pathways. Its diatom‑bacteria partnership was loose and stochastic; bacteria gained survival benefits primarily from the phycosphere niche.

Building upon these observations, the researchers proposed a conceptual “mutually reinforced symbiotic cycle” model to explain sustained diatom dominance: dominant diatoms secreted extracellular polymeric substances that recruited and assembled specialized phycosphere bacterial consortia. Symbiotic bacteria reciprocated by supplying nutrients, stress‑protective metabolites and allelochemical defenses to their host. Collectively, the diatom‑bacteria holobiont enabled diatoms to maintain long‑term dominance under variable environmental regimes.

This study uncovers the mechanistic basis by which diatom‑bacteria symbiosis sustains the dominance of dispersive diatoms in the Middle Route of China's South‑to‑North Water Diversion Project. It delivers theoretical insights for interpreting algal succession across the reservoir‑canal continuum and informs refined ecological management of this large‑scale water diversion system.

(Editor: MA Yun)