MODELING INTERACTIONS BETWEEN PHYTOPLANKTON AND BACTERIA UNDER NUTRIENT-REGENERATING CONDITIONS

The interactions of phytoplankton and bacteria in a nitrogen-limited s teady-state system with an organic nitrogen compound or ammonium as th e sole nitrogen source were modelled. The effects of various algal exc retion rates and two different mathematical representations of excreti on were examined. The model predicted that higher excretion elevated t he bacterial steady-state biomass, and lowered the algal biomass. Bact erial respiration, which directly determined nitrogen regeneration, ha d an important effect on the system. The bacterial growth yield in the model was mainly a function of the growth rate, and not of the nitrog en:carbon ratio of the substrate. In one version of the model, where t he excretion of organic carbon increased with decreasing growth rate, the model started to oscillate when the multiplication product of maxi mum specific excretion of excreted organic carbon (EOC) and the bacter ial yield on EOC exceeded the dilution rate, irrespective of the form of nitrogen (ammonium or dissolved organic nitrogen) in the medium. Th e model results were compared with chemostat experiments with the alga Emiliania huxleyi and a bacterial isolate in pure and mixed culture a t two different dilution rates. The carbon and nitrogen biomass of the bacteria was similar to 1.5 times higher in mixed culture than in pur e culture. In the experiments with low dilution rate, the recovery of nitrogen in the form of biomass, ammonium or amino acids was low, sugg esting the excretion by the algae of a refractory nitrogen-containing product which the bacteria could not use.