Algicidal effects of Fictibacillus sp. 5A8M on Margalefidinium polykrikoides through attachment and secretion of extracellular metabolites

Abstract

Margalefidinium polykrikoides blooms inflict significant economic losses on the aquaculture industry. Utilizing algicidal bacteria to mitigate harmful algal blooms (HABs) has emerged as an environment-friendly approach. Despite numerous reports on algicidal bacteria, few studies have thoroughly elucidated their mechanisms against M. polykrikoides. In this study, we present the first documentation of the algicidal effect of a Fictibacillus strain on M. polykrikoides. Strain 5A8M exhibited algicidal effects through physical attachment and the secretion of extracellular algicidal metabolites. The algicidal activity of strain 5A8M was dose- and time-dependent. When inoculated at a ratio of 10 % (v/v), strain 5A8M induced lysis in 97.9 % of M. polykrikoides after 24 h of co-culture. The active chlorophyll-a and transcriptional levels of key genes in M. polykrikoides decreased under the algicidal influence of strain 5A8M. Furthermore, exposure to the cell-free filtrate of strain 5A8M for 12 h triggered lipid peroxidation in M. polykrikoides, resulting in elevated malondialdehyde levels. Analysis via ¹H nuclear magnetic resonance (NMR), ¹³C NMR, and electrospray ionization-mass spectra identified l-phenylalanine and anthranilic acid as the extracellular algicidal metabolites produced by strain 5A8M. The algicidal activity of l-phenylalanine and anthranilic acid (10 μg/ml) against M. polykrikoides reached 74.6 and 72.5 %, respectively, within 24 h of exposure. Collectively, our findings underscore the considerable potential of strain 5A8M in controlling M. polykrikoides HABs.