Experimental studies on an omnivorous microflagellate: implications for grazing and nutrient regeneration in the marine microbial food chain

Abstract

A phagotrophic marine microflagellate Paraphysomonas imperforata was found to graze on a wide assortment of phytoplankton species as well as bacteria; it also resorted to cannibalism when food was in short supply. Growth rates of the microflagellate were higher than those of the phytoplankton prey, although in some cases lower measured growth rates were found when there was aggregation of cells. This aggregation seemed to be bacterially mediated. The ratio of predator to prey cell length varied from about 2 for the phytoplankton prey to 7 for bacteria. Nitrogen regeneration by the microflagellate, primarily as NH 4 +, never exceeded 50% of the nitrogen originally incorporated by the phytoplankton and bacterial prey. The role of bacteria in regenerating nutrients was found to be minimal relative to microflagellates. These results imply that both omnivory by small protozoa and a greater flexibility in the relationship between predator and prey sizes should be incorporated into the contemporary “microbial food loop” concept. In addition, to achieve nutrient regeneration efficiencies in pelagic surface waters of 80–90%, as is generally believed to occur, requires that the microbial food web be exceedingly complex with a hierarchy of at least several grazing steps. Alternatively, nutrient regeneration efficiencies in surface waters may be lower than envisioned.