Nonlinear Time-Course Uptake of Carbon and Ammonium by Marine Phytoplankton

In a series of laboratory and field experiments with natural and cultured marine phytoplankton the shapes of the time-dependent uptake curves of 15NH: and Hy4C0? were determined. Non-linearity in ''C uptake in laboratory cultures did not seem to be a function of steady state growth rate. However, temperature did appear to affect the degree to which 14C is incorporated in a linear manner for Thalassiosira weissflogii between 8 " and 25 "C and Dunaliella tertiolecta below 10 "C, but not for the other species investigated. Deviations from linearity for both H14C0; and 15NH: uptake in both laboratory and field experiments could be correlated with NH: depletion, especially when NH: levels at the start of the incubations dld not exceed a few tenths pg at I-'. The distribution of C and N among subcellular components was also investigated during the field experiments. The results demonstrated that by analyzing compositional changes among subcellular components a much improved estimate of the metabolic state of confined phytoplankton may be obtained. Our results demonstrate that there are severe incornpatabilities between choosing a n incubation period based solely on analwcal requirements from one based on the best representation of the time scale of physiological responses by phytoplankton. Time-course experiments allow us to understand better the short-term responses by phytoplankton, environmental influences on uptake, such as light and temperature, and to identify analytical problems or bottle effects, such as nutrient depletion.