Ammonium uptake by phytoplankton cells on a filter: a new high-resolution technique

Abstract

Ammonium uptake by the marine diatom Thalassiosira pseudonana was measured by trapping chemostat-grown cells on an in-line filter and following changes in the concentration of ammonium pulses pulled across the filter. Over time intervals longer than 1 min, this technique yielded results comparable to those obtained using conventional techniques on cell suspensions. The new technique also allowed changes in uptake kinetics within the first minute of exposure to ammonium to be resolved. Saturated uptake rates were highest (26 d-l) within the first 15 S, and declined to 11 d-I after 1 min. The affinity for ammonium remained constant at 17 d-' PM-'. Implications of these results for models of uptake and assimilation, and possible adaptation of the technique for use in the field, are discussed