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 heterotrophic flagellate Paraphysomonas imperforata in several coastal waters was examined using species-specific oligonucleotide hybridization probes which target small subunit ribosomal RNA. P. imperforata was found to occur in several coastal environments, but at very low abundances (typically ,50 cells ml21). The seasonal abundance of P. imperforata examined at one sampling site remained consistently low and constituted no more than 1% of the total nanoplankton at any time during a 17-month sampling period. In contrast 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 these findings, we conclude that P. imperforata is an opportunistic species capable of growing rapidly to high abundances when prey concentrations are high. Water and enrichment temperature as well as the temperature tolerance range of P. imperforata appear to have played a role in the seasonal differences observed in P. imperforata dominance. Experiments with enrichment cultures indicated that the absolute abundances of P. imperforata in the water samples and the activity of consumers of nanoplankton also influenced the degree to which P. imperforata dominated the heterotrophic nanoplankton assemblages of enrichment cultures. Seasonal changes in water temperature might also affect these latter factors, and, as a consequence, indirectly influence the ability of P. imperforata populations to dominate enrichments. Our results support the notion that enrichment cultivation of heterotrophic flagellates, and perhaps incubations in general, can select for species such as P. imperforata that may not be representative of nanoplanktonic protists that numerically dominate natural assemblages. Heterotrophic nanoflagellates (2–20 mm in diameter) play a major role in the flow of energy in planktonic food webs, as the primary grazers of bacteria and other picoplankton and as prey for larger protists and metazoa (Fenchel 1982b,c; Sieburth and Davis 1982; Azam et al. 1983; Sieburth 1984; Sherr and Sherr 1994). These heterotrophs are also potential sources of nutrients and dissolved organic material, which are regenerated as a result of their grazing activities (Goldman and Caron 1985; Caron and Goldman 1990). Although our understanding of the role of these microorganisms in aquatic ecosystems has improved considerably, very little is known about the spatial and temporal abundance and dynamics of individual species in the water column. The abundance of total nanoplanktonic protists in nature can be routinely enumerated by standard staining and counting methods using fluorochrome dyes and epifluorescence microscopy (Sherr et al. 1993), but these methods do not reveal ultrastructural features for identifying individual flagellate taxa. Several investigators have recently begun to document the diversity and distribution of heterotrophic nanoflagellates in different marine waters, but such studies have not focused on obtaining quantitative information for individual species (Vørs 1992, 1993a,b). Studies documenting the diversity and distribution of individual species of nanoflagellates have not