Figure 7. Conceptual model of the lifestyle switch of Sulfitobacter D7 in response to Emiliania huxleyi-derived metabolites.

During its interactions with E. huxleyi, Sulfitobacter D7 exhibits a lifestyle switch from coexistence to pathogenicity. In the coexistence phase, E. huxleyi secretes to the phycosphere various metabolites such as benzoate, dimethylsulfoniopropionate (DMSP), and other growth substrates, which bacteria can uptake and consume for growth. Based on the observation that benzoate hindered the pathogenicity-inducing effect of DMSP, we hypothesize that such energy-rich metabolic currencies hinder DMSP signaling in Sulfitobacter D7. When the algal physiological state is compromised, for example, stationary phase, the amount of available growth substrates decreases, due to bacterial consumption and less secretion by the alga. In this context, high concentration of algal DMSP acts as a signal that alters the transcriptional profiles of the bacterium and leads to high expression of pathogenicity-related genes, such as flagellar and transport genes, and yet unknown virulence factors that kill E. huxleyi cells. This leads to a surge of alga-derived growth substrates that are taken up efficiently by Sulfitobacter D7. The flagellum can mediate the dispersal of Sulfitobacter D7 and to forage for an alternative host.