Community level physiological study of algicidal bacteria in the phycospheres of Skeletonema costatum and Scrippsiella trochoidea

•Bacteria in phycosphere of S. costatum and S. trochoidea can lyse their host algae.•The bacterial community in the phycospheres had low metabolic activity.•The bacterial community showed low physiological diversity and simple structures.•The bacterial community structure changed significantly during lysis of algal cells.

Bacteria in the phycosphere have a unique ecological relationship with host algae due to their utilization of algal extracellular products as nutrients. Some bacteria control the growth of algal cells and even lyse them. The diversity of bacteria and their community dynamics in the phycosphere of microalgae are still relatively little understood, especially of those associated with red tide-causing algae. In this study, scanning electron microscope (SEM) images of algal cell morphology revealed that the phycosphere bacteria of the red tide-causing algae, Skeletonema costatum and Scrippsiella trochoidea, could lyse them within 72 h. The community level physiology of the algicidal bacteria was studied using Biolog ECO microplates, a common method for the ecological study of microbial communities. The average well color development (AWCD) values of bacteria in the phycospheres of both species were low, indicating that the bacteria had low metabolic activity overall. The diversity indices were both lower than the bacterial diversity from natural environments. However, the bacteria associated with S. trochoidea demonstrated a higher AWCD value and diversity than those in the phycosphere of S. costatum. The utilization of carbon sources significantly changed at different lytic times, reflecting that the bacterial community structure changed during the algae-lysing process. These results revealed that the bacterial communities in phycospheres had a simple structure and low diversity. When the balance between algae and bacteria broke down, the total bacterial density increased while the algicidal bacteria accumulated and became the dominant species, changing the bacterial community structure in this micro-ecosystem.