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

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Abstract
In a series of grazing experiments the phagotrophic microflagellate Paraphysomonas imperforata (Lucas) (7-12 .mu.m diameter) 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 NH4+ was negligible in control cultures of phytoplankton alone, bacteria alone, or phytoplankton and bacteria. When the microflagellate was grazing on either phytoplankton or bacteria there was considerable regeneration of NH4+. Only the microflagellate was responsible for the excretion. Rates of NH4+ excretion were highest during exponential growth of the macroflagellate and slackened off considerably with onset of the stationary phase when growth of the prey was not nutrient-limited. Regeneration efficiencies were lowest during exponential growth (15-30%), 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 N-limitation. N turnover rates per body weight are generally much higher in microflagellates than in macrozooplankton, but can be greatly influenced by many environmental factors including prey nutritional state.

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