The kinetics of nitrate and ammonia uptake by natural populations of marine phytoplankton

Abstract

The response of natural marine populations of phytoplankton to nitrate and ammonia concentrations has been investigated using nitrogen-15 tracer techniques. Experiments made in the Bering Sea, the waters of southeastern Alaska, and the northeastern tropical Pacific suggest that the uptake of both compounds follows the Michaelis-Menten expression for enzyme kinetics. A hyperbola, therefore, describes the relationship between the concentration of nitrate or ammonia and the uptake of that nutrient. Such a hyperbola can be obtained easily in experiments with ammonia and, under suitable conditions, with nitrate. The value of K t , the nutrient concentration at which the maximum uptake rate is reduced by one-half, appears to be related to the nutrient and productivity regime of the region inhabited by the population. In the tropical oligotrophic area investigated, K t(NO 3 −) ⩽ 0.2 μg-atom/l., while in corresponding eutrophic regions, K t(NO 3 −) ⩾ 1.0 μg-atom/l. The values suggest that the phytoplankton populations of oligotrophic regions are adapted to the low ambient nutrient concentrations and are able to take up nutrients at a higher rate under these conditions than would phytoplankton species characteristics of eutrophic conditions and showing a higher value of K t .