Effect of ultraviolet and visible radiation on iron lability in boreal and artificial waters

Abstract

Dialysis experiments were used to assess lability of Fe in boreal stream waters as well as synthetic solutions with the chelators ethylenediaminetetraacetic acid (EDTA) and 8-hydroxyquinoline in Bold Basal Medium (BBM). It was hypothesized that irradiation (UV and visible) of organic-Fe complexes would result in enhanced movement of Fe across a dialysis membrane with a 100Da molecular weight cutoff. Irradiation of boreal stream waters at pH 4 with UV plus visible radiation consistently resulted in increased Fe concentrations in the dialysis tubing containing 1.0 mM deferoxamine (a chelator)compared to the dark controls. Significant Fe lability occurred in the absence of UVB and UVA when stream water was exposed to visible wavelengths between 400 and 700 nm, but not when exposed to 550–700 nm. Irradiation of BBM media with EDTA with and without UVA under visible light increased lability of Fe and Co but not Mn, Cu or Zn, relative to dark controls indicating that EDTA binding strength is light sensitive for some metals. In contrast, Fe lability in the presence of 8-hydroxyquinoline was not significantly affected by UVA plus visible light relative to a dark control. If lability is used an indicator of biologically availability, then light intensity and spectral quality likely affects the supply rate of biologically available Fe and perhaps other metals in situ. Light conditions may also affect supply rates of biologically available metals in algal cultures, depending on the chelator used.