Scientists have recently found that a long‑term dominant diatom in China's South-to-North Water Diversion Project maintains its edge through a mutually reinforced cycle with bacteria, whereas a short‑term dominant species lacks such a close partnership and is far less resilient.

The study, led by researchers from the Institute of Hydrobiology (IHB) of the Chinese Academy of Sciences, was published in ISME Communications.

Algae and bacteria engage in ubiquitous interactions—ranging from symbiosis to parasitism—that form one of the most fundamental ecological relationships. These interactions go beyond 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 stand out as globally dominant primary producers, contributing about 20% of global primary production. Their distinctive bio‑silica frustules not only mediate the marine silicon cycle but also increase particulate organic carbon export through the 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.

Recently, a research team led by Professor BI Yonghong from the IHB has 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 then characterized the community structure, interaction networks, and metabolic potential of their phycosphere‑associated microorganisms.

Two diatom species from the Middle Route of China's South‑to‑North Water Diversion Project: C. atomus, a long‑term dominant species, and U. ulna, a short‑term dominant species. (Image by IHB)

Each diatom harbors 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 characterized 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, making its holobiont much less resilient to environmental perturbations.

Functional predictions showed 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 secretes extracellular polymeric substances to feed symbiotic bacteria. In return, symbionts produce vitamin B₁₂ and phytohormones, and provide 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 is loose and stochastic; bacteria gain survival benefits primarily from occupying the phycosphere niche.

Building on these observations, the researchers proposed a conceptual "mutually reinforced symbiotic cycle" model to explain sustained diatom dominance: dominant diatoms secrete extracellular polymeric substances that recruit and assemble specialized phycosphere bacterial consortia. Symbiotic bacteria reciprocate by supplying nutrients, stress‑protective metabolites, and allelochemical defenses to their host. Together, the diatom‑bacteria holobiont enables 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 these diatoms in the Middle Route of China's South‑to‑North Water Diversion Project. It offers theoretical insights for interpreting algal succession across the reservoir‑canal continuum and supports refined ecological management of this large‑scale water diversion system.