Abstract
We experimentally assess nutrient feedback of oligotrichous ciliates and phagotrophic flagellates on growth in naturally occurring Mediterranean autotrophic microbial populations as well as on the microphytoplankton. Dialysis bags were used to separate cultured protozoa from the <7‐µm plankton filtrates yet allow nutrient exchange. Growth rates in microbial autotrophic populations increased with protozoan densities added to bags. Microbial growth half‐saturation constants, expressed in terms of protozoan densities added, varied from 11 to 17 ciliates ml−1 and 2–5 × 103 flagellates ml−1. The “transdialysis contact” of protozoa with microbial planktonic populations led to growth rates 2 to > 10 times higher than those observed without added protozoa. These findings suggest that in situ, autotrophic pico‐ and nanoplanktonic growth is limited by protozoan standing stocks. Lower generation times and half‐saturation constants were obtained for autotrophic nanoplankton (∼3 h, Kµ = 10 ciliates ml−1 and 1.7 × 103 phagotrophic flagellates ml−1, respectively). Microphytoplankton (e.g. diatoms and dinoflagellates) showed no significant functional responses to the addition of microprotozoa, suggesting a possible dependence of the latter on new rather than on recycled nutrients.
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