Isolation and pathogenicity identification of bacterial pathogens in bleached disease and their physiological effects on the red macroalga Gracilaria lemaneiformis

Marine macroalga, Gracilaria lemaneiformis, is affected by bleached disease in recent years, but the bacterial pathogens for bleached disease in G. lemaneiformis are either known or unknown. In this study, we identified eight bacterial pathogens, of which Agarivorans albus, Aquimarina latercula (T) and Brachybacterium sp. had the strongest pathogenicity on healthy G. lemaneiformis. These pathogens had strong agarase activity, suggesting a possible infection mechanism through the degradation of algal-agar. Furthermore, analysis of the macroalgal defense against pathogens in terms of hydrogen peroxide and malondialdehyde content indicated that oxidative burst started at an early stage during the bacterial infection. On the other hand, the increasing activity of peroxidase and the occurrence of different peroxidases were to protect algae from the oxidative damage, by compensating for the decrease in catalase and total antioxidant capacity in bleached tissue. The photosynthetic pigment - phycobiliprotein was dramatically decreased after the bacterial infection, with the beta subunit but not the alpha subunit of phycoerythrin being degraded first as the disease progressed. The findings here provide very important fundamental information for future research on understanding the algae-bacteria interaction mechanism, the mechanism of algal defense against bacterial infection, which is important for the efficient and healthy cultivation of G. lemaneiformis.