Oceanic distribution of inorganic germanium relative to silicon: Germanium discrimination by diatoms

Seventeen inorganic germanium and silicon concentration profiles collected from the Atlantic, southwest Pacific, and Southern oceans are presented. A plot of germanium concentration versus silicon concentration produced a near-linear line with a slope of 0.760 x 10(-6) (+/-0.004) and an intercept of 1.27 (+/-0.24) pmol L(-1) (r(2) = 0.993, p < 0.001). When the germanium-to-silicon ratios (Ge/Si) were plotted versus depth and/or silicon concentrations, higher values are observed in surface waters (low in silicon) and decreased with depth (high in silicon). Germanium-to-silicon ratios in diatoms (0.608-1.03 x 10(-6)) and coupled seawater samples (0.471-7.46 x 10(-6)) collected from the Southern Ocean are also presented and show clear evidence for Ge/Si fractionation between the water and opal phases. Using a 10 box model (based on PANDORA), Ge/Si fractionation was modeled using three assumptions: (1) no fractionation, (2) fractionation using a constant distribution coefficient (K(D)) between the water and solid phase, and (3) fractionation simulated using Michaelis-Menten uptake kinetics for germanium and silicon via the silicon uptake system. Model runs indicated that only Ge/Si fractionation based on differences in the Michaelis-Menten uptake kinetics for germanium and silicon can adequately describe the data. The model output using this fractionation process produced a near linear line with a slope of 0.76 x 10(-6) and an intercept of 0.92 (+/-0.28) pmol L(-1), thus reflecting the oceanic data set. This result indicates that Ge/Si fractionation in the global ocean occurs as a result of subtle differences in the uptake of germanium and silicon via diatoms in surface waters.