Original and recruited microbiomes associated with the bloom-forming Haptophyte alga, Phaeocystis globosa

Phaeocystis is a cosmopolitan, bloom-forming phytoplankton genus that contributes significantly to global carbon and sulfur cycles. Under bloom conditions, Phaeocystis produces large carbon-rich, mucilaginous colonies, thus creating a unique interface for bacterial interactions. While bacteria are known to interact with phytoplankton - e.g., by promoting growth through phytohormone or vitamin production - such interactions have not been described with Phaeocystis. Therefore, we investigated the composition and function of Phaeocystis globosa microbiomes. Specifically, we tested whether microbiome communities are consistent across individual colonies from four P. globosa strains, whether similar communities are recruited after antibiotic treatment, and how bacteria affect P. globosa growth under different limiting conditions. Results illuminated a core microbiome associated with P. globosa colonies and showed that similar bacterial communities are recruited after microbiome disruption. Growth experiments with antibiotic-treated and untreated cultures demonstrated that intact microbiomes allow continued growth when B-vitamins are withheld, suggesting bacteria provide Phaeocystis with essential B-vitamins. In contrast, intact microbiomes accelerated culture collapse when nitrogen was withheld, indicating competition for this resource. These results reveal specific interactions between Phaeocystis colonies and bacteria that may influence bloom dynamics, e.g., by allowing Phaeocystis to thrive when B-vitamins are limiting or by advantaging other phytoplankton when nitrate is limiting.