Light-oxygen-voltage photoreceptor-regulated virulence in the pathogenic bacterium Pseudoalteromonas piscicida X-8 of a farmed seaweed bleaching disease

Abstract

The outbreaks of bleaching disease of the farmed Saccharina japonica are closely associated with the increased light intensity at the late nursery stage. Based on the light-oxygen-voltage (LOV) photoreceptor in the causative bacterium Pseudoalteromonas piscicida X-8 (PpX-8), we hypothesize that PpX-8 has a light-regulated virulence mechanism through LOV photoreceptors as those in phytopathogenic bacteria. In this study, combining infection assays, bioinformatics approaches, and gene knockout techniques, we investigated the light-regulated virulence mechanism of PpX-8. Our results showed that white light negatively affected the growth but enhanced both motility and virulence of PpX-8 compared to darkness. The light-oxygen-voltage photoreceptor of PpX-8 (PpX-8-LOV) was found to negatively regulate growth and biofilm formation under white light while enhancing motility and virulence at the initial stage of infection. Furthermore, the photoreceptor PpX-8-LOV up-regulated the expression of genes involved in quorum sensing, iron acquisition, and the type II secretion system, thereby enhancing the virulence of PpX-8. This study provides valuable insights for S. japonica farms to optimize light management strategies, thereby effectively preventing or reducing the occurrence of bleaching disease.