Phytoplankton-Mediated Redox Cycle of Iron in the Epilimnion of Lake Kinneret

Abstract

The biological-mediated redox cycle of Fe was studied in the epilimnion of Lake Kinneret (Sea of Galilee), a mesotrophic lake in Israel. Multi-annual lake water sampling and incubation experiments were carried out to study Fe(III) reduction by natural phytoplankton populations and their possible role in inhibiting Fe(II) oxidation. The reduction characteristics of the dinoflagellate Peridinium gatunense, the dominant lake alga, were further examined in the laboratory. The steady-state concentration of Fe(II) calculated from the assessed reduction and oxidation rates was compared with Fe(II) measured in the lake in order to evaluate the significance of these processes to the lake Fe redox cycle. Nanomolar concentrations of Fe(II) were measured in the oxygenated, high pH, upper water layer of the lake throughout the year. Reduction rates of Fe by natural phytoplankton assemblages ranged between 0.1 and 10 nM/h. The highest reduction rates, determined in dinoflagellate-dominated lake waters, coincided with the highest concentrations of Fe(II) measured simultaneously in the lake. Iron(II) oxidation rates calculated from the measured lake Fe(II) and the obtained reduction rates were significantly slower than published abiotic Fe(II) oxidation rates. Indeed, Fe(II) oxidation rates measured in algal-enriched lake water were 30-fold slowerthan Fe(II) oxidation rates in natural water, demonstrating the potential for Fe(II) stabilization by the lake phytoplankton.

Enhancement of the reactive iron pool by marine diatoms

“…Another possibility is a slower Fe(II) oxidation rate. Shaked et al (2002) observed slower Fe(II) oxidation rates in waters enriched with algae as compared to abiotic waters of Lake Kinneret. However, simultaneously, Shaked et al (2002) found increased Fe(III) reduction rates in the presence of the natural phytoplankton assemblage.…”

“…Shaked et al (2002) observed slower Fe(II) oxidation rates in waters enriched with algae as compared to abiotic waters of Lake Kinneret. However, simultaneously, Shaked et al (2002) found increased Fe(III) reduction rates in the presence of the natural phytoplankton assemblage. Croot et al (2001) explained the retention of Fe(II) during the iron enrichment experiment SOIREE by stabilisation of the Fe(II) by organic complexation.…”

“…However, in lakes Fe(II) maxima coincide with chlorophyll-a maxima suggesting biological (induced) reduction of Fe(III) (Emmenegger et al, 2001;Shaked et al, 2002). Various phytoplankton species seem capable of enzymatic reduction of Fe(III) at the cell surface (Maldonado and Price, 2000).…”

Chemical Speciation of Organics and of Metals at Biological Interphases

“…Fe(II), Mn(II)). Low concentrations of Fe(II) have been observed in oxygenated seawater and fresh water under the influence of light [90][91][92][93]. Reduction or oxidation reactions may also occur at the biological interphases [94,95].…”

Voltammetric determination of dissolved iron and its speciation in freshwater

“…However, there are a few studies reporting the existence of iron(II) in lake water despite the presence of oxygen (Aldrich et al 2001;Sivan et al 1998). This may be a result of photochemical reduction and biological activity (Emmenegger et al 2001;Shaked et al 2002). There is some possibility of changing the bioavailability of iron as a result of the production of iron(II).…”