Ammonium supply mode and the competitive interaction between the cyanobacterium Microcystis novacekii and the green alga Scenedesmus quadricauda

We performed competition culture experiments using the bloom-forming cyanobacterium Microcystis novacekii (Kom.) Comp. and the green alga Scenedesmus quadricauda, (Turpin) Brebisson to study the effects of nutrient supply modes on the competition of phytoplankton. The cells were grown in continuous cultures under nitrogen limitation. Ammonium, as the sole nitrogen source, was supplied either continuously or in pulses as one pulse per two days, and the effects of the mode of nutrient supply were examined. While both the species grew well in monoculture, the growth of one species was suppressed by the presence of the other species in the mixed cultures, indicating the occurrence of competition for the supplied ammonium. The competitive outcomes depended on the mode of ammonium supply: under continuous supply, though both the species tended to grow together, the growth of S. quadricauda was suppressed by the presence of M. novacekii, compared to that in the monoculture. Under the pulsed supply, M. novacekii tended to be suppressed by S. quadricauda. This suggests that M. novacekii is a species that grows better in low ammonium supply, while S. quadricauda is a species that grows better in high ammonium supply. The temporally high concentration of ammonium immediately after the pulsed nutrient supply provided an opportunity for S. quadricauda to outgrow or almost exclude M. novacekii.