Nutrient cycling in a microflagellate food chain: IV. Phytoplankton-microflagellate interactions

Abstract

Phagotrophic microflagellate Paraphysomonas imperforata was capable of grazing 2 marine phytoplankton species, the diatom Phaeodactylum tricornutum and the chlorophyte Dunaliella tertiolecta. The phytoplankton species, which are grossly different in size, shape and morphology, were first grown under different degrees of nitrogen and phosphorus limitation. Patterns of nutrient regeneration appeared to be a function of the physiological state of the prey: lags in NH: regeneration untd the end of exponential growth occurred when the prey were N-limited and phosphorus regeneration was negligible dunng the entire growth cycle of the microflagellate when the prey were P-limited. There was, however, evldence for dark uptake of NH; by N-limited prey that were ungrazed, which could have biased the observed lags in nutrient regeneration. Regeneration efficiencies reached as high as 70 O/O for nitrogen and up to 50 % for phosphorus only after prolonged penods in the stationary phase. Because protozoa are able to convert prey nutrients to their own biomass with great efficiency during exponential growth, particularly when the prey are nutrient-limited, the size and complexity of the microbial food web may be related to the nutritional state of the phytoplankton.