The influence of the initial composition of an algal-bacterial microcosm constituted of Chlorella sorokiniana and Ralstonia basilensis was tested for the fed-batch degradation of salicylate at 5 mM. Salicylate degradation was always limited by the O-2 generation rate, which was initially proportional to the algal density, but rapidly became limited by the availability of light once the algae started to grow. The decrease of the salicylate removal rate observed at high algal densities was likely caused by mutual shading within the algal population and the increase of O-2 consumption due to algal dark respiration. With repeated salicylate amendments, all systems converged towards the same characteristics, reaching an optimum rate of salicylate degradation at 1 mmol l(-1) day.