Regulation of the relationship between phytoplankton Scenedesmus acutus and heterotrophic bacteria by the balance of light and nutrients

Culture experiments were conducted with the alga Scenedesmus acutus and heterotrophic bacteria to examine if the nature of their relationship changes according to the balance of light and nutrient supplies. Mixtures of algae and bacteria were grown in vanous combinations of 6 light intensities and 4 phosphorus (P) concentrations at high N:P ratio (80:l). We used an artificial medium composed of inorganic nutrients so that bacteria relied on organic matter released by algae as carbon (C) source. Every 2 d, 25 % of the culture suspension was replaced by fresh medium. At the end of incubation when both bacterial and algal densities were stabilized, bacteria were separated from algae. Bioassays with glucose and/or inorganic P enrichment were then performed to assess the extent to which bacterial growth rate was limited by organic C or inorganic P. The algal density in the semibatch culture was low under the light intensity 1 5 5 FE rn-'s-' regardless of P concentrations, while it was higher at higher light and P supply rate above that light intensity. The bacterial density was higher in the cultures where algal density was higher. The bioassay revealed that bacteria were C limited at the light intensity <55 pE m-2 S-', indicating a commensal relationship between algae and bacteria.