Ingestion rate and grazing impact by the marine red tide ciliate Mesodinium rubrum on marine eubacteria

Mesodinum rubrum is a ubiquitous photosynthetic ciliate. This species often becomes dominant as a major planktonic primary producer and recurrently cause red tides in many bays and estuaries. To understand the population dynamics of M. rubrum, we investigated the ingestion rates of M. rubrum on bacterioplankton as a function of bacterial concentration in the laboratory. In addition, the ingestion rates of the natural populations of M. rubrum on the natural populations of eubacteria were measured in three char­acteristic Korean coastal waters.The ingestion rates of M. rubrum on eubacteria or Synechococcus sp. rapidly increased with increasing prey concentrations up to 2.0 × 10⁶ cells ml^-1, but slowly at higher prey concentrations. The maximum ingestion rates of M. rubrum on eubacteria and Synechococcus sp. reached 34.4 and 2.1 cells predator^-1 h^-1, respectively. During the field experiments, the ingestion rates and grazing coefficients of cryptophytes on natural populations of heterotrophic bacteria were 2.3–16.8 cells predator^-1 h^-1 and 0.001–0.089 d^-1, respectively. Based on the present study, it is suggested that M. rubrum may be an important grazer of heterotrophic bacteria and sometimes have consider­able grazing impact on natural populations of eubacteria.