Coculture of the picocyanobacterium Merismopedia punctata and the larger cyanobacterium Microcystis aeruginosa under different light treatments

Picocyanobacteria are small-sized photosynthetic prokaryotes (< 2–3 μm) commonly found in aquatic ecosystems. Little attention has been paid to them in freshwater ecosystems because of their small size and rarity in routine sampling. However, they can form blooms and represent potential toxic risks for ecosystems and human health. Picocyanobacteria often compete with larger cyanobacteria during seasonal successions, but the role of light in these interactions remains poorly understood. To better understand interactions between small and large cyanobacteria, we conducted a 35-day experiment with Merismopedia punctata PMC 242.05, a picocyanobacterium, and Microcystis aeruginosa PCC 7806, grown in mono- and cocultures under two light irradiances (14 and 34 μE.m2.s−1). In monocultures, M. punctata developed similarly under both light conditions, whereas M. aeruginosa grew faster under 34 μE.m2.s−1. In coculture under low-light conditions, both species coexisted, in contrast to high-light conditions, where M. aeruginosa thrived and led to a decline in M. punctata population. Notably, M. punctata formed colonies that disappeared starting on Day 7 in cocultures. Regarding toxicity, the highest cellular quotas of microcystin were observed in coculture under high light. These findings highlight the impact of light irradiance and allelopathic interactions on cyanobacterial succession and associated toxic risks.