A trace metal clean reagent to remove surface-bound iron from marine phytoplankton

Many recent studies have investigated Fe biogeochemical cycling, chemical speciation, and limitation of phytoplankton growth in the ocean. In current models of marine iron biogeochemistry, however, two critical parameters remain uncertain. These are the partitioning of particulate iron into scavenged and interior pools, and the iron quotas (Fe/C ratios) of natural plankton communities. These values have not been measured in natural samples, because the only reagent that is available to remove surface adsorbed Fe from cells and other particles (Ti(III) citrate/EDTA [Limnol Oceanogr 34 (1989) 1113]) contains substantial levels of contaminating Fe, and is therefore useful only for Fe radiotracer experiments. We developed a new reagent that differentiates between intra- and surface adsorbed Fe pools in marine phytoplankton as effectively as the Ti wash, but that is also trace metal clean, chemically stable, and harmless to cells. This reagent uses oxalate as a reductant to remove surface adsorbed Fe from phytoplankton cells and other particles. A simple cleaning protocol reduces Fe concentrations in the oxalate solution to levels suitable for trace metal clean field measurements. The oxalate reagent was used to measure scavenged and interior Fe pools in suspended particles collected at four stations in the Southern Ocean. Sixteen percent to eighty-six percent of the total Fe associated with these samples was found to be surface-adsorbed. The oxalate reagent provides a new tool to accurately measure the physical partitioning of Fe in marine particles and could be used along with appropriate corrections for lithogenic Fe to estimate the intracellular Fe quotas of natural plankton communities.