Effects of Bacillus sp. on Pyropia haitanensis at high temperature

Pyropia haitanensis is an important economic seaweed cultivated in the East China Sea. Its optimum growth temperature is about 20 °C, and the extremely high temperature is about 29 °C.P. haitanensis is vulnerable to disease due to high temperature or hostile epiphytic bacterial flora in the phycosphere. In this study, the thalli of P. haitanensis were co-cultured with Bacillus sp. WPySW2 which was proved to be probiotic to Pyropia at 20 °C. However, the experiment was conducted at 28 °C, an extremely high temperature to Pyropia species. The physiological indexes such as relative growth rate (RGR), activities of antioxidant enzymes (SOD and POD), contents of soluble proteins, free proline (Pro), malondialdehyde (MDA), and phycobiliproteins (phycorythrin, phycocyanin, and allophycocyanin) were detected. The results showed that: at 28°C, the RGR of co-cultured P. haitanensis was higher than the control at the beginning, while the difference became less significant when the co-culture process continued. Both the control and co-cultured samples rotted 4 days later, and the conditions of co-culture groups were worse. After co-culturing for 24 h, the activities of SOD and POD, and the content of MDA in co-culture samples were higher than the control. The contents of soluble proteins and free Pro were much lower than those in the control. The contents of phycobiliproteins in co-cultured groups were higher than the control. It revealed that at the high temperature such as 28 °C, Bacillussp. WPySW2 could promote the growth of P. haitanensis at the beginning of co-culture, however, when the co-culture process continued, the bacteria inhibited the growth and the physiological conditions of the algae. The results indicated the phycospheric bacteria could change the ecological functions when the environment changed.