Bacteriophage therapy on the conchocelis of Pyropia haitanensis (Rhodophyta) infected by Vibrio mediterranei 117-T6

Abstract

Yellow spot disease (YSD) is a destructive disease that usually occurs at the conchocelis nursery stage of commercially cultivated Pyropia species. Vibrio mediterranei 117-T6 has been identified as the pathogenic bacteria causing YSD. To develop a biological control program for YSD, we investigated the effects of bacteriophage vB_VmeM-Yong XC31 (phage XC31) on the density of the pathogenic bacteria, the physiological and biochemical parameters of the free-living conchocelis (FLC) and the shell-borne conchocelis (SBC) of Pyropia haitanensis infected by V. mediterranei 117-T6. The highest survival rate (83%) of infected FLC was obtained when the density of V. mediterranei 117-T6 was 108 CFU/mL and the multiplicity of infection (MOI) of phage XC31 was 1. Under these conditions, phage XC31 could kill the pathogens and significantly decrease the Vibrio density in the infection system (P < 0.05). The activity of superoxide dismutase, and the contents of phycocyanin and allophycocyanin in the infected FLC co-cultured with phage XC31 were significantly higher than those that were not co-cultured with phage XC31 (P < 0.05). These results indicated that phage XC31 could effectively control YSD caused by V. mediterranei 117-T6 in Pyropia species by killing the pathogen and increasing the survival rate of infected Pyropia conchocelis. Meanwhile, the algal photosynthesis and antioxidant capacity were improved in the bacteriophage protected groups. Hence, bacteriophage therapy can be used as an alternative and effective treatment for the diseases caused by bacterial pathogens in seaweed cultivation.