Isolation and identification of Vibrio mediterranei 117-T6 as a pathogen associated with yellow spot disease of Pyropia (Bangiales, Rhodophyta)

Abstract Pyropia is an important cultivated seaweed in Northeast Asia. Yellow spot disease (YSD) commonly occurs in conchocelis sporeling cultures of Pyropia and causes huge economic losses. In this study, the isolated strain 117-T6 was identified as a pathogenic bacterium of YSD in Pyropia by Koch's postulates. Based on the morphological, physiological, and biochemical characteristics and multi-locus analysis of house-keeping gene sequences, this gram-negative bacterium was named as Vibrio mediterranei 117-T6. V. mediterranei 117-T6, including its intracellular and the extracellular extracts, caused both free-living conchocelis (FLC) and shell-borne conchocelis (SBC) to exhibit YSD symptoms within 12 days, indicating that the bacteria might attack Pyropia by secreting certain compounds that can be transported into algal cells, breaking down their structure and leading to death. During the infection, four stages were observed. 1) Prophase: a large number of pathogenic bacteria gathered around the algae. The conchocelis was in healthy condition at that time. 2) Early stage: organelle edges of Pyropia were blurred, mitochondria and chloroplasts became fluffy and unraveled, autophagosomes appeared. 3) Middle stage: cellular structures and contents of the algae kept being lost and degraded, cell walls became thin. 4) Late stage: the conchocelis bleached completely, and as the cell wall disintegrated gradually, the bacteria took over the remains of Pyropia. Finally, the whole algal cell disappeared, which may be the reason for shell bleaching. Our results show that YSD caused by V. mediterranei 117-T6 is a severe disease of algal disintegration caused by bacterial secretion, with a short onset and severe symptoms.