Nutrient cycling in a microflagellate food chain: I. Nitrogen dynamics

Abstract

In a series of grazing experiments we found that the phagotrophic microflagellate Paraphysomonas imperforata (Lucas) (7 to 12 pm dia.) fed equally well on the diatom Phaeodactylum tricornutum and bacteria under batch conditions. Growth rates of the microflagellate were considerably higher than those of the phytoplankton prey. Regeneration of NH,' was negligible in control cultures of phytoplankton alone, bacteria alone, or phytoplankton and bacteria. However, when the microflagellate was grazing on either phytoplankton or bacteria there was considerable regeneration of NH,+ This result was a clear indication that only the microflagellate was responsible for the excretion. Rates of NH,+ excretion were highest during exponential growth of the microflagellate and slackened off considerably with onset of the stationary phase when growth of the prey was not nutrientlimited. Regeneration efficiencies, however, were lowest during exponential growth (15 to 30 %), but increased to 50 % if calculated on the basis of the entire experiment which included the stationary phase. Regeneration efficiencies during exponential growth decreased to 8 % when the prey was grown under nitrogen-limitation. Nitrogen turnover rates per body weight are generally much higher in microflagellates than in macrozooplankton, but can b e greatly influenced by many environmental factors including prey nutritional state.