Influence of environmental factors on nitrate utilization and nitrogen isotope fractionation by marine phytoplankton

Nitrate utilization by marine phytoplankton is an important component of the marine nitrogen cycle. The 15N/14N ratio that is created during nitrate assimilation can be propagated throughout marine food webs, throughout nitrogen pools in the water column, and recorded in the organic fraction of marine sediments. Variations in the 15N/ 14N ratio are potentially useful for understanding processes of the nitrogen cycle on a variety of spatial and temporal scales.; Using laboratory batch culture experiments, the uptake and assimilation characteristics of nitrate during growth in light, temperature, and iron limiting conditions were determined as a means to investigate the variability and possible physiological mechanisms of nitrogen isotope fractionation by marine phytoplankton.; Irradiance level had the most significant impacts on nitrate assimilation and nitrogen isotope fractionation. The phytoplankton species that expressed an uncoupling between the uptake and assimilation processes under low light conditions had relatively large isotope fractionation factors. The marine diatom Thalassiosira weissflogii showed the largest isotope discrimination, with an observed epsilon value of 6.2 per mil in high light, and 15.2 per mil in low light.; Under a 12h light:dark cycle, the isotope fractionation factor was dependent on the light versus dark nitrate assimilation rates. A coccolithophorid, Emiliania huxleyi, with no dark nitrate assimilation, had the same fractionation compared to growth in 24 hours of continuous light, whereas three marine diatoms displayed higher fractionation factors during nitrate uptake in the dark, and therefore had higher fractionation factors than when grown in 24 hours of continuous light.; The growth limiting effects of temperature or iron had little influence on isotope fractionation in the two marine diatoms that were measured. Despite the lower growth rate, isotope fractionation was similar to conditions of higher temperature (18°C) and high iron concentrations in cultures of Thalassiosira weissflogii and Thalassiosira pseudonana. These results indicate that the mechanism of isotope fractionation is not altered by slow growth rates induced by temperature or iron limitation.; Lastly, the isotope fractionation mechanism of Thalassiosira weissflogii was investigated by measuring the 15N/ 14N ratio of nitrate inside and outside the cell during growth in the four growth conditions described above. The results show that the 15N/14N ratio is higher in the internal pool of nitrate than in the nitrate in the medium, and therefore the fractionation step that produces the overall phytoplankton δ15N value is a result of assimilation processes inside the cell. (Abstract shortened by UMI.)...