Grazing Effects on Toxic and Non-Toxic Microcystis aeruginosa by the Mixotrophic Flagellate Ochromonas sp.

We investigated grazing and specific growth rates of the mixotrophic flagellate (MxFl) Ochromonas sp. when fed toxic Microcystis aeruginosa (NIES-88, NIES-298, and NIES-44 strains) and a non-toxic strain (NIES-101). The maximum growth rates of Ochromonas sp. were 1.28 d(-1) for M. aeruginosa NIES-44, 1.24 d(-1) for NIES-88, 2.05 d(-1) for NIES-298, and 1.53 d(-1) for NIES-101. Also, the maximum ingestion rates on each Microcystis strain by Ochromonas sp. were 4.32 prey MxFl(-1) d(-1) for M. aeruginosa NIES-44, 5.71 prey MxFl(-1) d(-1) for NIES-88, and 2.12 prey MxFl(-1) d(-1) for NIES-101. Interestingly, although the growth rate of Ochromonas sp. with the NIES-298 strain was higher than it was for any of the other strains, its ingestion rate for NIES-298 was low and similar to that with NIES-101. There are two possible explanations for this difference. First, the colony formation by the NIES-298 and NIES-101 strains may have interfered with ingestion. Second, the medium nutrients available to Ochromonas sp. were sufficient to support its population growth despite low grazing on prey, since this flagellate gains its nutrients through a combination of autotrophy and phagotrophy. The flagellate Ochromonas sp. showed active growth regardless of toxic and non-toxic prey types.We thank the National Institute for Environmental Studies (NIES) for providing the Microcystis strains. This study was supported by grants from Korean Research Foundation (KRF-2006-005-J01901)