Effect of dilution rate on competitive interactions between the cyanobacterium Microcystis novacekii and the green alga Scenedesmus quadricauda in mixed chemostat cultures

Abstract

We examined the competition between the cyanobacterium Microcystis novacekii (Kom.) Comp. and the green alga Scenedesmus quadricauda (Turpin) Brébisson using unialgal and mixed chemostat cultures with various supply rates of culture medium where NH4+-N limited algal growth. In unialgal cultures, both species grew at all of the dilution rates examined (0.1, 0.3 and 0.8 day-1): steady-state cell densities were 1 x 104 to 8 x 104 cells mL-1 for M. novacekii and 0.5 x 105 to 2.1 x 105 cells mL-1 for S. quadricauda. Microcystis novacekii was dominant in mixed cultures at a dilution rate of 0.1 day-1, where the steady-state cell density was 1 x 104 to 7 x 104 cells mL-1 for M. novacekii and 1 x 102 to 5 x 102 cells mL-1 for S. quadricauda. Scenedesmus quadricauda was dominant in mixed cultures at the higher dilution rates (0.3 and 0.8 day-1), where the final cell density was 0.5 x 102 to 6.4 x 102 cells mL-1 for M. novacekii and 0.2 x 105 to 7 x 105 cells mL -1 for S. quadricauda. This result indicates that the dilution rate affects the competitive interaction. We conclude that it is necessary to consider water exchange in the study of mechanisms of cyanobacterial blooms.