Interactive Effects of Iron, Vitamin B12 and UV Light on Phytoplankton in the Amundsen Sea

Models suggest that ocean warming, and glacial melt will lead to enhanced ocean stratification in Antarctic coastal ecosystems. Because of reduced vertical mixing, phytoplankton populations will likely experience higher average ultraviolet (UV) fluxes in surface waters. Recent data suggest that Amundsen Sea phytoplankton populations may experience iron limitation and preliminary reports also suggest the possibility of co-limitation by Vitamin B12. We performed a bioassay incubation experiment in austral summer 2017-18 in the Amundsen Sea to test for the interactive effects of UV, iron and Vitamin B12 on various phytoplankton physiological parameters, species composition and levels of dimethylsulfoniopropionate (DMSP). DMSP is a precursor to the climatically active compound dimethylsulfide (DMS). Five treatments were incubated in +UV and -UV conditions for 5 days in triplicate. The treatments included: (1) control (no addition), (2) +Vitamin B12, (3) +iron, (4) +iron+Vitamin B12 and (5) +desferroxime-B. Photosynthetic efficiencies of PSII (Fv/Fm) were highest in the iron and iron+Vitamin B12 treatments. In addition, both iron treatments exposed to natural UV conditions resulted in approximately 20% lower Fv/Fm values relative to the minus UV treatment. Chlorophyll concentrations in the two iron addition treatments (+Fe and +Fe+B12) were approximately two-fold higher without UV compared to the treatments with UV light. UV also increased xanthophyll cycling especially in the iron treatments. Various analyses (DMSP, biogenic silica, flow cytometry) are currently being completed and will be discussed with respect to the above results.