Pathogenic effects of Halomonas on cyanobacteria and biocontrol potential of a deep-sea Bacillus strain

Limnospira fusiformis , a nutritionally valuable cyanobacterium with significant biomanufacturing potential, faces critical challenges from pathogenic invasions. This study identified Halomonas variabilis 2-9 as the primary pathogen of L. fusiformis cultivation collapse, demonstrating broad-spectrum algicidal activity against multiple cyanobacterial species. Additionally, the algicidal substance produced by H. variabilis 2-9 was purified and identified as toxic dibutyl phthalate. At a concentration of 3 µg/mL, the purified algicidal substance caused 90.6% reduction in Fv/Fm within 24 h and 86% degradation of chlorophyll- a within 120 h. To be specific, Fv/Fm as the maximum photochemical quantum yield of photosystem II is a core indicator reflecting the photosynthetic activity and health status of spirulina. A sudden drop in Fv/Fm of spirulina indicates impaired function of Photosystem II. Therefore, the purified algicidal substance severely damaged the photosynthetic system of L. fusiformis . To mitigate the detrimental effects of H. variabilis 2-9, Bacillus velezensis L4 was isolated from deep-sea sediments. This strain produced an antimicrobial compound (C 31 H 42 O 2 ) that specifically inhibited H. variabilis 2-9 without damaging L. fusiformis . In co-culture experiments, B. velezensis L4 reversed Halomonas -induced algal decay, demonstrating its potential as a biocontrol agent. These findings provide both mechanistic insights into Halomonas -mediated cyanobacterial pathogenesis and a practical solution for sustainable aquaculture management. IMPORTANCE This study identifies H. variabilis 2-9 as a novel cyanobacterial pathogen that produces hazardous compound dibutyl phthalate (DBP), causing severe damage to L. fusiformis and exhibiting broad-spectrum algicidal activity against other cyanobacteria. The discovery of DBP-mediated pathogenesis provides crucial insights into microbial threats to aquaculture systems. Significantly, we demonstrate that B. velezensis L4, isolated from deep-sea environments, serves as an effective biocontrol agent through the production of a selective antimicrobial compound that specifically targets H. variabilis 2-9 without harming L. fusiformis . These findings offer both fundamental understanding of cyanobacterial disease mechanisms and a practical, sustainable solution for algal disease management.