| Summary: | 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. Our present understanding of the processes
involved in nitrogen isotope fractionation does not allow us to confidently interpret the
¹⁵N / ¹⁴N signal in the various pools of nitrogen in the ocean.
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 lightdark 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. The ¹⁵N / ¹⁴N values of the internal nitrate
pool in the different growth conditions suggested that the overall isotope fractionation by
marine phytoplankton is a result of the isotope discrimination step during nitrate
reduction combined with the efflux of nitrate from the cytoplasm to the external medium.
A linear correlation was observed between the magnitude of isotope fractionation and the
relative efflux: influx ratio that suggests that fractionation associated with nitrate
reductase was 23 per mil.
The results and discussion in this thesis describe how isotope fractionation by
marine phytoplankton varies as a result of species composition, nutrient, and light
conditions. The variations in the magnitude of nitrogen isotope fractionation imply that
an accurate interpretation of a specific nitrogen isotope signal will depend on knowledge
of the growth parameters that control phytoplankton species composition and nitrate
utilization. === Science, Faculty of === Botany, Department of === Graduate
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