Proliferation of a bloom-forming phytoplankton via uptake of polyphosphate-accumulating bacteria under phosphate-limiting conditions

Harmful algal blooms negatively impact the ecosystem and fisheries in affected areas. Eutrophication is a major factor contributing to bloom occurrence, and phosphorus is particularly important in limiting the growth of bloom-forming algae. Although algae efficiently utilize orthophosphate (Pi) as a phosphorous source over other molecular forms, Pi is often limited in the marine environment. While uptake and utilization of soluble inorganic and organic phosphorous by bloom-forming algae has been extensively studied, the details of geochemical and biological phosphorous cycling remain to be elucidated. Here, we report for the first time that the bloom-forming alga Heterosigma akashiwo can phagocytose bacteria and grow under phosphate-depleted conditions. The addition of Vibrio comitans to Pi-depleted H. akashiwo enabled the alga propagate to high cell densities, whereas other bacterial strains had only a minor effect. Importantly, V. comitans accumulates polyphosphate—a linear polymer of Pi—at high levels. The extent of algal proliferation induced by the addition of Vibrio species and polyphosphate-accumulating Escherichia coli correlated strongly with their polyphosphate content, indicating that bacterial polyphosphate served as an alternative PO43− source for H. akashiwo. The direct uptake of polyphosphate-accumulating bacteria through algal phagocytosis may represent a novel biological phosphorous-cycling pathway in marine ecosystems. The role of polyphosphate-accumulating marine bacteria as a hidden phosphorous source required for bloom formation warrants further investigation.