Limnol. Oceanogr., 44(8), 1999, 1917–1927

Theory suggests that variation in the attractive solvation force associated with cell-surface hydrophobicity can significantly affect contact rates among small cells in aqueous environments and consequently may influence rates and selective impacts of marine nanoflagellate grazers feeding on picoplankton assemblages. To investigate this hypothesis, we assayed the natural range in hydrophobic characteristics of subtropical picoplankton from the oligotrophic subtropical Pacific (Station Aloha, 228459N, 1588W) and mesotrophic Kaneohe Bay, Hawaii, using hydrophobic interaction chromatography (HIC) in conjunction with analytical flow cytometry. Variability in a relative index of cell-surface hydrophobicity (HIC index) for heterotrophic bacteria, Prochlorococcus and Synechococcus, exhibited some consistent spatial patterns. The HIC index for Prochlorococcus at Station Aloha varied about threefold, being consistently more hydrophobic in the upper 80 m of the water column and dropping abruptly below this depth. Heterotrophic bacteria were more hydrophobic near the surface and decreased slightly, but steadily, with increasing depth. The hydrophobicity of heterotrophic bacteria steadily increased along a Kaneohe Bay transect extending from oligotrophic to mesotrophic conditions. In experiments involving nanoflagellates grazing on laboratory cultures of Prochlorococcus, cell cultures exhibiting the highest HIC indices were grazed upon at the highest rates. An additional experiment involving mixtures of Prochlorococcus cells exhibiting high and low hydrophobicities showed that the average hydrophobicity of the uningested prey mixture was driven progressively toward lower hydrophobicity as the more hydrophobic cells were selectively removed through time. If these laboratory grazing results hold in nature, the rate at which picoplankton cells are cleared from suspension by nanoflagellates could vary by as much as twofold due solely to natural variation in cell surface hydrophobicity. Heterotrophic nanoflagellates are the principal consumers of picoplankton in many oceanic systems (Fenchel 1982a,b; Azam et al. 1983; Sherr and Sherr 1994). In the oligotrophic open ocean, where photosynthetic picoplankton can dominate total autotrophic carbon fixation (Li et al. 1983; Platt et al. 1983; Goericke and Welschmeyer 1993; Vaulot et al. 1995), heterotrophic nanoflagellates are typically the most important herbivores (Herbland et al. 1985; Campbell and Carpenter 1986; Murray et al. 1994; Landry et al. 1995, 1997; Monger et al. 1997). In coastal ecosystems where picophytoplankton are less important relative to larger forms, small flagellates are the main pathway through which the considerable secondary production of heterotrophic bacteria is transferred to higher trophic levels or remineralized back