Zinc biogeochemical cycle in the Tasman Sea : potential role for phytoplankton communities

Zinc (Zn) plays an essential role in metabolic and structural functions for marine phytoplankton, being required in nearly 300 enzymes and proteins including carbonic anhydrase and alkaline phosphatase. The concentration of total dissolved Zn in the open-ocean is typically in the nanomolar (0.1-10 nmol L\\(^{-1}\\)) range, 98 % of which is complexed by natural strong organic ligands. In January and February 2010, the Primary productivity Induced by Iron and Nitrogen in the Tasman Sea (PINTS) cruise covered a north to south transect from the oligotropic Tasman Sea to the productive waters of the subantarctic Southern Ocean. For this thesis, samples for chemical analyses and laboratory- based cultures were collected to assess the relationship between phytoplankton growth and zinc bioavailability and speciation in this region. The pennate diatom \\(Nitzschia\\) \\(closterium\\) and the coccolithophorid \\(Emiliania\\) \\(huxleyi\\) were cultured at low free Zn concentrations ([Zn\\(^{2+}\\)] = 1.5 x 10\\(^{-12}\\) M and [Zn2+] = 1.5 x 10\\(^{-14}\\) M) to mimic the range of free Zn encountered in marine systems. Both species were able to maintain their photosynthetic activity and growth at the low [Zn\\(^{2+}\\)] applied. However, while E. huxleyi grew at these low concentrations, \\(N. closterium\\) seemed to accelerate its uptake with specific and efficient transporters and access complexed Zn. In the Tasman Sea, total dissolved Zn was observed at 0.02 to 0.19 nmol L\\(^{-1}\\) (at 15 m depth) and 0.02 to 0.11 nmol L\\(^{-1}\\) (at 150 m) along the north-south transect studied, below the range reported for other open-ocean regions. Measurements with Anodic Stripping Voltammetry (ASV) on four selected profiles from the Tasman Sea established the concentrations of labile Zn (0.6 to 500 pmol L\\(^{-1}\\)) and Zn-complexing ligands (0.23 to 4.19 nmol L\\(^{-1}\\)) for the first time in the Tasman Sea. Zn speciation was dominated by complexation to organic ligands (59 to 98%) with the highest percentages in surface. These ligands were closely related to the phytoplankton assemblages found in the studied region (Chlorophytes ~ Haptophytes > Diatoms). The links between chemistry and biology were studied with a simple conceptual ZnPPZZD model. It was parameterised with information gained in the experimental phases of this work, such as the difference of uptake response observed for the two species and range of concentrations measured. The model showed the concentration of ligands and zinc speciation played an important role in the structure of the conceptual phytoplankton assemblages. Further work is required to assess if ligands are produced by the organisms to enhance their uptake or if the chemistry of Zn strongly influences the community for a specific region.