Role of Natural Organic Matter in Regulating the Partitioning of Fe(II, III) in Seawater

Iron (Fe) is an essential micronutrient and plays an important role in controlling ocean productivity and carbon cycling. Fe has been shown to be mostly complexed with dissolved organic matter in seawater. However, the interaction of Fe with natural organic matter and how the quality and quantity of organic matter affect the chemical speciation of Fe in seawater remain poorly understood. Controlled laboratory experiments have been conducted to examine the partitioning of Fe(II, III) between dissolved, colloidal and particulate phases using radiotracers, model organic compounds, and ultrafiltration. In natural seawater, Fe is mostly partitioned in the colloidal and particulate phases, resulting in a logKd value of 7.3 and a logKc of 6.1, respectively. On average, about 25% of dissolved Fe-55 was found in the <1 kDa fraction, 56% in the 1-10 kDa small colloidal fraction, 11% in the 10-100 kDa medium colloidal fraction, and 8% in the large colloidal fraction (100 kDa-0.4 μm). In experimental treatments with extracellular polysaccharides, the partitioning of Fe(II, III) changed from mostly in the colloidal fraction to mostly in the <1 kDa truly dissolved fraction, with on average 67% in the <1 kDa, 13% in the 1-10 kDa, 4% in the 10-100 kDa, and 6% in the 100 kDa-0.4 μm fraction. The increase in Fe(II, III) solubility in the presence of extracellular polysaccharides is hypothesized to be the result of Fe reduction from Fe(III), a more particle-reactive form, to Fe(II), a more soluble form, during its interactions with natural organic matter in seawater. This experimental result has important implications for the biogeochemical cycling of Fe(II, III) and other redox sensitive trace elements in the ocean. While the complexation of Fe with DOM could depress the bioavailability of Fe in seawater, the resultant Fe reduction may significantly enhance its solubility and bioavailability to marine organisms.