Modelling interactions between phytoplankton and bacteria under nutrient-regenerating conditions

Abstract

The interactions of phytoplankton and bacteria in a nitrogen-limited steady-state system with an organic nitrogen compound or ammonium as the sole nitrogen source were modelled. The effects of various algal excretion rates and two different mathematical representations of excretion were examined. The model predicted that higher excretion elevated the bacterial steady-state biomass, and lowered the algal biomass. Bacterial respiration, which directly determined nitrogen regeneration, had 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 nitrogen:carbon ratio of the substrate. In one version of the model, where the excretion of organic carbon increased with decreasing growth rate, the model started to oscillate when the multiplication product of maximum specific excretion of excreted organic carbon (EOC) and the bacterial yield on EOC exceeded the dilution rate, irrespective of the form of nitrogen (ammonium or dissolved organic nitrogen) in the medium. The model results were compared with chemostat experiments with the alga Emiliania huxleyi and a bacterial isolate in pure and mixed culture at two different dilution rates. The carbon and nitrogen biomass of the bacteria was similar to 1.5 times higher in mixed culture than in pure culture. In the experiments with low dilution rate, the recovery of nitrogen in the form of biomass, ammonium or amino acids was low, suggesting the excretion by the algae of a refractory nitrogen-containing product which the bacteria could not use.