Dark Protein Synthesis: Physiological Response to Nutrient Limitation of a Natural Phytoplankton Population

Abstract

Dark 14CO2 incorporation into protein was determined from 24-hr incubations using size-fractionated natural phytoplankton populations from Kane'ohe Bay, Hawai'i, enriched with either ammonium or ammonium plus phosphorus. Response to ammonium addition was maximum at an ammonium concentration of 3-4 μM. Dark 14C02 assimilation was suppressed by addition of both ammonium and phosphorus, but percentage incorporation into protein was not significantly different from addition of ammonium alone. About 75 ± 1% of the 14C taken up by the cells was incorporated into either protein or low-molecular-weight intermediate compounds. Cells smaller than 10 μm showed little response to nutrient additions. However, cells in the 10- to 35-μm size fraction incorporated significantly more 14C into protein when nutrients were added. C:N ratios calculated from the percentage of 14C incorporated into protein were most variable temporally in the 10- to 35-μm size group and least variable in the picoplankton (0.2-2.0 μm). Nutrient limitation indices (NUs) calculated from the quotient of C:N ratios in control and nutrient-enriched cultures were not significantly different for the picoplankton and 2- to 10-μm size fraction. The NLI for the 10- to 35-μm size fraction was significantly lower and implied a modest degree of nutrient limitation. The results suggest that cells smaller than 10 μm are growing at close to nutrient-saturated rates much of the time in Kane'ohe Bay. However, larger cells appear to experience a significant degree of nutrient limitation at some times, particularly when chlorophyll a concentrations are less than about 1 mg m-3 • Dark protein synthesis appears to be a useful modification of previous methods based on the dark uptake of 14C02 for studying nutrient limitation.