The ecology of Paraphysomonas imperforata based on studies employing oligonucleotide probe identification in coastal water samples and enrichment cultures

The geographical distribution and seasonal abundance of the cosmopolitan he terotrophic flagellate Paraphysomonas imperforata in several coastal waters was examined using species-specific oligonucleotide hybridization probes w hich target small subunit ribosomal RNA. P. imperforata was found to occur in several coastal environments, but at very low abundances (typically <50 cells ml(-1)). The seasonal abundance of P. imperforata examined at one sam pling site remained consistently low and constituted no more than 1% of the total nanoplankton at any time during a 17-month sampling period. Ln contr ast to the low abundances observed in natural water samples, P. imperforata frequently dominated heterotrophic enrichment cultures prepared from these same samples, comprising up to 98% of the total nanoplankton. Based on the se findings, we conclude that P. imperforata is an opportunistic species ca pable of growing rapidly to high abundances when prey concentrations are hi gh. Water and enrichment temperature as well as the temperature tolerance r ange of P. imperforata appear to have played a role in the seasonal differe nces observed in P. imperforata dominance. Experiments with enrichment cult ures indicated that the absolute abundances of P. imperforata in the water samples and the activity of consumers of nanoplankton also influenced the d egree to which P. imperforata dominated the heterotrophic nanoplankton asse mblages of enrichment cultures. Seasonal changes in water temperature might also affect these latter factors, and, as a consequence, indirectly influe nce the ability of P. imperforata populations to dominate enrichments. Our results support the notion that enrichment cultivation of heterotrophic fla gellates, and perhaps incubations in general, can select for species such a s P. imperforata that may not be representative of nanoplanktonic protists that numerically dominate natural assemblages.