Table 1.
Measurements of culture cell density (cells L−1×106), dissolved nitrate+nitrite concentrations (NO3− + NO2−, µmol L−1) and N2-fixation rates (fmol cell−1 hr−1) at different time points (hours since culture dilution) in cultures used in the short- and long-term exposure experiments.
Figure 1.
Short-term inhibitory effects of ammonium (NH4+, 0–1.5 µmol L−1) and nitrate (NO3−, 0–40 µmol L−1) on N2 fixation by Crocosphaera watsonii (WH0003) (percent of control with no added nitrogen).
Cultures were grown in steady state under high light (175 µmol quanta m−2 s−1, growth rate (µ) = 0.68 d−1, open symbols) and low light (25 µmol quanta m−2 s−1, µ = 0.23 d−1, closed symbols) before adding nitrogen. Error bars represent standard deviations on means from 3 culture replicates.
Figure 2.
Effects of long-term exposure to 30 µmol L−1 nitrate (NO3−).
(a) cellular growth rates and (b) nitrogen-assimilation rates of Crocosphaera watsonii (WH0003) acclimated to high light (175 µmol quanta m−2 s−1) and low light (25 µmol quanta m−2 s−1). (b) N2-fixation rates (solid bars) are overlain on total N assimilation (N2 + NO3− assimilation, hashed bars). Control cultures did not receive added NO3−. Error bars represent standard deviations on means from 3 culture replicates.
Figure 3.
Changes in nitrate (triangles) and cell concentrations (circles) in cultures of Crocosphaera watsonii (WH0003) in long-term exposure experiments.
Cultures were grown in steady state under (a) high light (175 µmol quanta m−2 s−1) and (b) low light (25 µmol quanta m−2 s−1) with added nitrate (30 µmol L−1; open symbols) or with N2 only (closed symbols). *Calculated NO3− concentrations (see Methods section for details). Error bars represent standard deviations on means from 3 culture replicates.
Figure 4.
Growth-specific assimilation rates of nitrate (NO3−; open bars) and dinitrogen (N2; closed bars) in cultures of C. watsonii (WH0003) with added NO3− (30 µmol L−1).
Growth-specific NO3− and N2-assimilation rates change inversely relative to each other as a function of light-limited growth. Error bars represent standard deviations on means from 3 culture replicates.