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 het erotrophic 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 various combinations of 6 light intensities and 4 phosphorus (P) concentrations at high N:P ratio (80:1). We used an artifi cial medium composed of inorganic nutrients so that bacteria relied on orga nic matter released by algae as carbon (C) source. Every 2 d, 25 % of the c ulture suspension was replaced by fresh medium. At the end of incubation wh en both bacterial and algal densities were stabilized, bacteria were separa ted from algae. Bioassays with glucose and/or inorganic P enrichment were t hen performed to assess the extent to which bacterial growth rate was limit ed by organic C or inorganic P. The algal density in the semibatch culture was low under the light intensity <55 mu E m(-2) s(-1) regardless of P conc entrations, while it was higher at higher light and P supply rate above tha t light intensity. The bacterial density was higher in the cultures where a lgal density was higher. The bioassay revealed that bacteria were C limited at the light intensity <55 mu E m(-2) s(-1), indicating a commensal relati onship between algae and bacteria. Above that light intensity, bacteria suf fered from deficiency of organic carbon rather than P at lowest P supply ra te, because of low algal biomass due to a shortage in P supply. At moderate P supply rates and Light intensities greater than or equal to 55 mu E m(-2 ) s(-1), however, bacterial growth was limited by P rather than organic C, because supply of organic C from algae exceeded P supply relative to bacter ial demand. Further increase in P supply released both algae and bacteria f rom P limitation. Thus, competitive interaction for P was most intense at a moderate P supply rate. These results demonstrate that there is a shift be tween commensalism for C and competition for P depending on light intensity and nutrient supply rate.