Defense against Paramecium predation via long filament morphology favors the survival of Raphidiopsis raciborskii populations.

Raphidiopsis blooms are notorious for cyanotoxin formation and strong invasiveness, threatening the stability of aquatic ecosystems and human health. The protozoa Paramecium can potentially serve as an organism for controlling Raphidiopsis blooms owing to its grazing effect. However, the grazing ability of Paramecium is largely determined by the size of the prey, and the population of Raphidiopsis consists of filaments of varying lengths and sizes. The selective grazing behavior of Paramecium toward short-length or small-sized filaments in the Raphidiopsis population, as opposed to long filaments, remains unclear. Therefore, in this study, we co-cultured the predator Paramecium sp. with different initial abundances and the prey Raphidiopsis raciborskii to explore this knowledge gap. Our results suggested that: (1) the population of R. raciborskii declined under the selective grazing effect of Paramecium sp. on short filaments, whereas R. raciborskii with long filaments survived; (2) the growth of Paramecium sp. feeding on the same abundance of R. raciborskii was reduced at higher initial abundances, whereas its carrying capacity exhibited an opposite trend; (3) under ingestion by Paramecium sp., the morphology of R. raciborskii developed in the direction of becoming larger, and higher initial abundances of Paramecium sp. intensified this process; (4) increasing initial abundance of Paramecium sp. aggravated the decline of R. raciborskii photosynthetic activity. Therefore, the grazing effect of Paramecium sp. on R. raciborskii mainly affects filaments of short length or small size. Collectively, these results clarify the inter-species interaction between the protozoa Paramecium and filamentous cyanobacteria Raphidiopsis, including population dynamics and morphological and physiological changes in the predator and prey. Such insights into the interactions between Paramecium and R. raciborskii may have implications for the biological control of blooms caused by filamentous cyanobacteria.