Distinguishing between cellular and Fe-oxide-associated trace elements in phytoplankton

In cultures and in nature, ferric (oxyhydro-)oxides (FeOx) precipitate and become associated with phytoplankton surfaces. Other trace elements adsorb on FeOx and it is thus difficult to differentiate between cellular- and oxide-associated concentrations of both iron and these elements. Existing techniques to selectively dissolve the FeOx associated with phytoplankton surfaces often contaminate the sample or necessitate elaborate pre-cleaning procedures and/or proceed by unknown and thus uncontrolled mechanisms.Here we examine the efficacy of various washing techniques, the mechanisms effecting FeOx dissolution, and the methods for controlling contamination from the wash solutions. Solutions containing a single chelating agent are ineffective at dissolving FeOx. A wash solution containing two types of chelating agents, oxalate and EDTA, is effective for dissolving fresh precipitates via a ligand-promoted process, in which oxalate and EDTA function synergistically. This is in contrast with a Ti–citrate–EDTA wash technique, which effectively dissolves fresh or aged FeOx by a reductive mechanism. Contaminating trace elements in the wash solutions that are complexed by excess EDTA can be effectively eliminated from filters by appropriate rinsing with a clean NaCl solution. For those elements that are not bound by excess EDTA, it is necessary to add specific chelating agents to the wash solution and, in some cases, to pre-clean the reagents.We show that under defined culture conditions, Ba and V in the diatom Thalassiosira weissflogii are mostly adsorbed on the extracellular FeOx so that their apparent cellular concentrations increase with the concentration of Fe in the culture medium. In contrast, the cellular concentrations of Cu, Zn, Co, Cd, and Mn, which are dominated by their intracellular pools, are independent of the Fe concentration in the medium and can be measured directly on rinsed filters without dissolving the FeOx.