Differential responses of Chlorella and Micractinium to algicidal bacteria in varying organic carbon environments

Microbial control of algae has emerged as a promising strategy due to its environmental sustainability and high efficiency. Four algicidal bacterial strains (Serratia N-1, Proteus N-2, Pseudomonas F5-2, and Pseudomonas F11) were isolated from lake sediment and identified via 16S rRNA gene sequencing. Their effects on Chlorella and Micractinium under different nutritional conditions were explored. Under autotrophic conditions, all strains showed negligible inhibition on Chlorella but approximately 20% inhibition against Micractinium, with no distinct difference between bacterial cultures and supernatants. Under glucose-supplemented mixotrophic conditions, algicidal performance against Chlorella was remarkably enhanced: Serratia N-1 achieved over 99% inhibition within 3 days, while Proteus N-2, Pseudomonas F5-2 and Pseudomonas F11 reached 79%, 47% and 60% on day 7, respectively. The comparable efficacy of supernatants and cultures indicated that extracellular metabolites dominated algicidal activity. For Micractinium, only the culture of Serratia N-1 exhibited strong inhibition; its supernatant and the other strains showed no inhibitory effects. In sucrose systems, the algicidal capacity of all strains was largely weakened. Notably, Serratia N-1 switched from a strong algicidal agent to a growth promoter for Chlorella, increasing algal biomass by fivefold. In conclusion, the presence and type of organic carbon sources in co-culture systems significantly modulate bacterial algicidal phenotypes. This study provides a scientific basis for understanding algae-bacteria interactions and the nutritional regulation of algicidal activity under varying carbon environments.