Effect of Anthracene on the Interaction BetweenPlatymonas helgolandicavar. tsingtaoensis andHeterosigma akashiwo in Laboratory Cultures

Two species of marine phytoplankton,Platymonas helgolandicavar. tsingtaoensis andHeterosigma akashiwo, were cul-tivated in bi-algal cultures to investigate the effect of anthracene (ANT) on the interaction between them. Without ANT,H. akashiwo out-competedP. helgolandica at low initial biomass ratios (P. helgolandica (P): H. akashiwo (H)=1:4 and 1:1), but not at the highest (P:H=4:1). This observation was consistent with the description in Lotka-Volterra two species competition model. It was found that P. helgolandica was excluded at low initial biomass ratios, while the unstable equilibrium between two species was predicted at the highest. For both species, carrying capacity and maximal specific growth rate decreased in bi-algal cultures compared to those in monocultures.H. akashiwo exhibited a higher sensitivity to ANT thanP. helgolandica. This resulted markedly in a reduced cell den-sity ofH. akashiwo but an increased cell density ofP. helgolandica. Carrying capacity ofP. helgolandica was consistently higher in bi-algal cultures with ANT than those without ANT, suggesting that ANT, through the elimination ofH. akashiwo, generated the dominance ofP. helgolandica independently of initial biomass ratios. This study showed a density-dependent effect of harmful alga (H. akashiwo) on dietary alga (P. helgolandica), and indicated that ocean pollutant ANT could induce the succession of marine phytoplankton.