| Summary: | This study explores two relevant questions in the realm of iron (Fe) bioavailability to phytoplankton. First, does Fe availability limit (or co-limit) growth of indigenous plankton communities in the Arctic Ocean? Second, can phytoplankton internalize ferrated siderophores with a non-reductive uptake mechanism? To address the first question, an 8-day grow out experiment was conducted in the Beaufort Sea in early September 2009, during which light, Fe, and nitrate (NO₃⁻) levels were manipulated. Bottles were sampled on days 0, 2, 4, 6, and 8 for accessory pigments, size-fractionated chlorophyll α, phytoplankton abundance and composition, nutrients, Fe quotas and uptake rates. It was found that NO₃⁻ was limiting plankton growth at the time of sampling. The community also appeared to be light limited. Additionally, co-limitation of primary production by Fe and light at light levels ≤ 10 % surface irradiance (I₀) was observed. These results have interesting implications about how the seasonality of NO₃⁻, light, and Fe availability may control primary productivity in the Beaufort Sea. To address the second question, I investigated the potential of a non-reductive Fe uptake mechanism for siderophore-bound Fe in the model diatom Thalassiosira oceanica and the in situ plankton communities along Line P in the subarctic Pacific Ocean in late summer 2010. To do this, we radiolabeled the siderophore desferrioxamine B (DFB) by methylating its terminal amine group with radioisotope ¹⁴C methyl iodide. Internalization of ⁵⁵Fe¹⁴DFB was observed both in phytoplankton cultures and field communities along Line P, suggesting the presence of a non-reductive Fe uptake mechanism in phytoplankton. However, the results are inconclusive due to the inability to purify and verify the concentration of ¹⁴DFB. The overarching goal of this investigation was to gain a better understanding on the bioavailability and acquisition of Fe by phytoplankton. This is imperative in order to predict the role of Fe in future primary productivity, and subsequently the fate of phytoplankton communities and the biological carbon pump, as our oceans respond to global warming.
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