Effect of anthracene on the interaction between Platymonas helgolandica var. tsingtaoensis and Heterosigma akashiwo in laboratory cultures

Two species of marine phytoplankton, Platymonas helgolandica var. tsingtaoensis and Heterosigma akashiwo , were cultivated in bi-algal cultures to investigate the effect of anthracene (ANT) on the interaction between them. Without ANT, H. akashiwo out-competed P. 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 than P. helgolandica . This resulted markedly in a reduced cell density of H. akashiwo but an increased cell density of P. helgolandica . Carrying capacity of P. helgolandica was consistently higher in bi-algal cultures with ANT than those without ANT, suggesting that ANT, through the elimination of H. akashiwo , generated the dominance of P. 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.