Abstract: The health of coral holobiont is closely linked to their internal microbial interactions. In particular the interactions between Symbiodiniaceae and bacteria are crucial for responding to environmental stress. However, how these interactions are regulated under heat stress remains unclear. In this study, we used that were both isolated from the tissue of the coral Pocillopora damicornis: Symbiodinium sp. CCMP 2433 and a vitamin-producing bacterium SCSIO 12696 (“Corallivirga polyvitaminum”). This study systematically compared the dynamics of Symbiodinium physiology, bacterial (SCSIO 12696) attachment behavior, and their interactions under control (25°C) and heat stress (32°C) conditions. Heat stress significantly inhibited the growth (cell density), maximum photochemical efficiency (Fv/Fm), photosynthetic pigment content (chlorophyll a) of Symbiodinium sp. CCMP 2433. At the same time, heat stress significantly enhanced the attachment of the bacterium SCSIO 12696 to the Symbiodinium cell surface and altered the ratio in the co-culture system and resulting in a decreased Symbiodinium/SCSIO 12696 cell abundance ratio. Microscopic observations further confirmed that heat stress enhanced the attachment of the bacterial to Symbiodinium cells, and SCSIO 12696 was observed entering the Symbiodinium cells by the fifth day of co-culture. These results indicated that temperature plays a regulatory role in the Symbiodinium-SCSIO 12696 interaction. Under heat stress (32°C), the growth and photosynthetic physiology of Symbiodinium were inhibited, while the attachment and proliferation of bacterium SCSIO 12696 were promoted. This study provides experimental evidence for understanding the interaction between Symbiodiniaceae and bacteria, and contributes to the understanding of the response and adaptability of the coral holobiont under heat stress.

Key words: heat stress, Symbiodiniaceae-bacteria interaction, co-culture, coral holobiont, photosynthetic physiology