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Effects of iron on the physiology of oceanic phytoplankton from the NE subarctic Pacific Ocean Muggli, Deborah Lynn


The physiology of 2 oceanic phytoplankton species freshly isolated from the subarctic Pacific (Station P, 145°W, 50°N) was examined, focusing on the effects of iron (Fe) on various physiological parameters. The two species, a coccolithophore Emiliania huxleyi and a large diatom Actinocyclus sp., represent different taxonomic groups and size classes of indigenous phytoplankton found at Stn P. The effect that nitrogen source (NO3" vs. NH.4+) had on the physiology and metal nutrition of both E. huxleyi and Actinocyclus sp. under Fereplete and Fe-stressed conditions was examined. In general, the diatom responded as expected based on.theoretical energy and Fe requirement predictions, with NH4"*"-grown cells exhibiting a physiological advantage over NC>3"-grown cells under Fe-stressed conditions. In contrast, E. huxleyi exhibited no physiological advantage when grown on N H 4 + compared to NO3" under Fe-stressed conditions, largely due to the decrease in cell volume of NO3"-grown cells under Fe-stress. Secondly, the effect of Fe on the sinking rate of both the small oceanic coccolithophore and the larger diatom was examined. Fe conditions drastically affected the sinking rate of the diatom, with a > 5 times increase in sinking rate under Fe-replete compared to Fe-stressed conditions. Fe conditions had no effect on the oceanic coccolithophore. EDTA concentrations of 100 jtM were found to inhibit cell division rates by 30 - 50%, as well as affecting carbon metabolism in both species. As this concentration of EDTA is used in many phytoplankton/trace metal studies, caution is warranted, especially when working with oceanic species. Lastly, the ecophysiology of the 2 oceanic species in natural Stn P water (no metals or artificial chelators added) and in the same seawater to which 5 nM Fe was added was examined. Under in situ conditions, E. huxleyi grew maximally while Actinocyclus sp. was unable to divide. When 5 nM of Fe was added, Actinocyclus sp. resumed cell division and grew significantly faster than E. huxleyi. E. huxleyi's biochemical composition (chl a, C, N) did not change when Fe was added to the seawater, whereas the diatom exhibited signs of severe Fe-stress when grown on Stn P water with no additions. From the experiments conducted in this thesis, E. huxleyi appears to be better adapted to living in a low Fe environment than the diatom. These are the first laboratory data on phytoplankton species isolated from the NE subarctic Pacific (or any High Nutrient Low Chlorophyll region), and these data provide independent, physiological support for the Fe/grazing hypothesis in the NE subarctic Pacific.

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