Selective cyanobactericidal activity of Nocardioides convexus MS16 against Microcystis aeruginosa, mediated by direct attack

Abstract

A cyanobactericidal bacterium, Nocardioides convexus MS16, was isolated from the co-culture solution of Microcystis aeruginosa and Myriophyllum spicatum (Eurasian watermilfoil), an aquatic plant known for its inhibitory effects on the growth of M. aeruginosa. The present study investigated the cyanobactericidal activity of N. convexus MS16 against M. aeruginosa KW, evaluating its potential as a biocontrol agent for managing Microcystis blooms. To determine the algicidal mode of action, treatments using the bacterial culture, washed cells (direct attack), and cell-free filtrate (indirect allelopathy) were compared. The MS16 strain exhibited a weak allelochemical impact but a strong direct physical attack, demonstrating algicidal activity of 84 % against M. aeruginosa KW within a day and 98 % within two days, using washed cells. The scanning electron microscope images showed that the cells of M. aeruginosa KW were damaged by cell surface-attached N. convexus MS16. Furthermore, N. convexus MS16 exhibited a selective inhibition of M. aeruginosa, compared to other cyanobacteria and green algae. The gene expression levels of ftsH, rbcL, mcyA, SOD, GSH, and recA in M. aeruginosa KW significantly decreased after treatment with N. convexus MS16 culture, indicating that M. aeruginosa had lost the ability to grow and survive. These findings highlight the potential of N. convexus MS16 to enhance the development of eco-friendly methods to mitigate harmful cyanobacterial blooms.