Effect of protistan bacterivory on coastal bacterioplankton diversity

Four protist exclusion experiments were conducted to test the hypothesis that marine bacterivorous protists selectively feed on different bacterioplankton genotypes and affect the taxonomic diversity of coastal marine bacterioplankton communities. In these experiments, the changes in bacterial community composition of seawater samples from which protists were removed by filtration were followed and compared to those of untreated control water samples. Bacterioplankton community structure was inferred from the relative abundance of bacterial small subunit rRNA genes (SSU rDNAs) by a recently developed technique (length heterogeneity analysis by PCR (LH-PCR; Suzuki et al. 1998). The results of Me experiments show that the community structure did not dramatically change up to a 24 h incubation period in any of the treatments. However there were significant differences in filtered water samples and controls between 24 and 48 h of incubation. In the absence of bacterivores some SSU rDNAs that were rare in the original water samples dominated the bacterioplankton SSU rDNA pool after 48 h of incubation. Protists appeared to be capable of controlling bacterioplankton taxonomic diversity under these manipulated conditions. The results also agree with the hypothesis that aquatic bacterioplankton communities are composed of small cells that escape predation, and are in a state of low physiological turnover.