Influence of low light and a light:: Dark cycle on NO3- uptake, intracellular NO3-, and nitrogen isotope fractionation by marine phytoplankton

The nitrogen isotope enrichment factor (epsilon) of four species of marine phytoplankton grown in batch cultures was determined during growth in continuous saturating light, continuous low light, and a 12:12-h light:dark cycle, with nitrate as a nitrogen source. The low growth rate that resulted from low irradiance caused an increased accumulation of the intracellular nitrate pool and/or a reduction in cell volume and was correlated to a species-specific increase in the measured epsilon value, compared with the saturating light conditions. The largest response was in the diatom Thalassiosira weissflogii (Grun.) Fryxell et Hasle, which showed a nearly 3-fold increase between high and low light conditions (6.2-15.2parts per thousand). The smallest response was in T. pseudonana (Hustedt) Hasle et Heimdal, which showed no change in the epsilon value of approximately 5parts per thousand in both high and low light conditions. There was significant but smaller increases in the epsilon value for the diatom T. rotula Meunier (2.7-5.6parts per thousand) and the prymnesiophyte Emiliania huxleyi (Lohm.) Hay et Mohler (4.5-9.4parts per thousand) between high and low light levels. In the light:dark experiments, all three diatoms but not the prymnesiophyte exhibited an increase in epsilon. This increase was linked to the ability of diatoms to assimilate nitrate at night. The results of the these experiments suggest that the light regime influences the relative uptake, assimilation, and efflux rates of nitrate and results in differences in the expression of the isotope effect by the enzyme nitrate reductase. Therefore, variations in nitrate isotope fractionation in nature can be more accurately interpreted when the light regime and species composition are taken into consideration.