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

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Abstract
In a series of laboratory and field experiments with natural and cultured marine phytoplankton the shapes of the time-dependent uptake curves of 15NH4+ and H14CO3- were determined. Non-linearity in 14C uptake in laboratory cultures did not seem to be a function of steady state growth rate. Temperature did appear to affect the degree to which 14C is incorporated in a linear manner for Thalassiosira weissflogii between 8-25.degree. C and Dunaliella tertiolecta below 10.degree. C but not for the other species investigated. Deviations from linearity for both H14CO3- and 15NH4+ uptake in both laboratory and field experiments could be correlated with NH4+ depletion, especially when NH4+ levels at the start of the incubations did not exceed a few tenths .mu.g at 1-1. 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. There are apparently severe incompatabilities between choosing an incubation period based solely on analytical requirements from one based on the best representation of the time scale of physiological responses by phytoplankton. Time-course experiments elucidate the short-term responses by phytoplankton, environmental influences on uptake, such as light and temperature, and help to identify analytical problems or bottle effects, such as nutrient depletion.