Soil protozoa are critical components of the soil food web and represent the most diverse group of eukaryotes in terrestrial ecosystems. They actively shape soil microbial community structures, regulate soil fertility, and aid plant roots in resisting pathogenic bacteria. Despite growing evidence from high-throughput sequencing studies indicating their diversity rivals that of aquatic systems, research into soil protozoa remains notably lagging.

Colpoda, a genus of kidney-shaped ciliates widely distributed in soils, is characterized by ease of collection and exceptional resilience via cyst formation under unfavorable conditions. While most studies have centered on resting cysts, reproductive cysts—vital for cell division and proliferation—have received scant attention, with their underlying molecular mechanisms remaining elusive.

To fill this knowledge gap, a research team led by Prof. MIAO Wei from the Institute of Hydrobiology (IHB) of the Chinese Academy of Sciences has identified an alpha-amylase that promotes the proliferation of Colpoda, a dominant soil protozoan. The study was recently published in Science China Life Sciences.

In the research, the team systematically delineated the complete life cycle of Colpoda inflata and, using high-resolution microscopy, observed unique sugar-coated granules containing bacterial DNA that accumulate during reproductive cyst formation. These granules act as a "nutrient reservoir," providing essential materials for subsequent cell division and offspring development.

Through genome sequencing and protein analysis, the researchers found that spent culture medium containing secreted proteins could induce reproductive cyst formation. Mass spectrometry further pinpointed an alpha-amylase as the key inducing protein.

Laboratory experiments demonstrated that purified alpha-amylase alone drives Colpoda reproduction, achieving cell densities 4.6 times higher than standard culture methods and reducing the generation time by nearly half—from nine hours to five hours.

Notably, the enzyme also induced reproduction across different Colpoda species, with its efficacy correlated to evolutionary distance. This finding uncovers a novel mechanism for microbial communication in soil ecosystems. Complementary analyses of rice root nematode samples revealed that approximately 90% of soil samples host multiple coexisting Colpoda species.

This study elucidates the reproductive mechanism of Colpoda, establishes an efficient method for greatly enhancing their culture density, and identifies alpha-amylase as a key regulator of cyst formation and population growth.

A tubulin staining image shows the morphology of Colpoda inflata. (Image by IHB)