Dark CO2 uptake by the diatom Chaetoceros simplex in response to nitrogen pulsing

Abstract

In a continuing investigation of dark CO2 uptake by nitrogen-limited cultures of the marine diatom Chaetoceros simplex (Bbsm), we expanded on several of our earlier conclusions regarding the potential application of this physiological response for measuring the degree and type of nitrogen limitation in phytoplankton populations. First, the duration over which the maximal enhancement of dark 14CO2 uptake was sustained after NH4+enrichment was a function both of the concentration of added NH4+and the standing crop of phytoplankton nitrogen — in effect, the total N demand. Second, pulsing with NH4+for a given degree of N-limitation always produced the same level of enhanced dark CO2 uptake regardless of whether the cultures were preconditioned with oxidized or reduced nitrogen. In contrast, urea pulsing led to reduced dark CO2 uptake, but the effect was most pronounced in cells grown on NO3−. And third, the assay could be used to distinguish readily between no, moderate, and severe N limitation. The degree of severe N limitation was quantitatively correlated with the degree of enhanced dark CO2 uptake, but this relationship was not so clear in the region of moderate N limitation. The main advantage of the assay is that it is a relatively simple and effective alternative to more complicated techniques for gauging the degree and form of N limitation in phytoplankton. Further evaluation will be required, both in the laboratory and field, before the assay can be calibrated for quantitative use.