Chromium uptake and adsorption in marine phytoplankton – Implications for the marine chromium cycle

Abstract

Using the radioisotope 51 Cr, we investigated the controls of cellular Cr accumulation in an array of marine phytoplankton grown in environmentally relevant Cr concentrations (1-10 nM). Given the affinity of Cr(III) for amorphous Fe-hydroxide mineral surfaces, and the formation of these mineral phases on the outside of phytoplankton cells, extracellular Cr was monitored in a model diatom species (Thalassiosira weissflogii) as extracellular Fe concentrations varied. Extracellular Cr in T. weissflogii increased with increasing extracellular Fe, demonstrating that Cr may be removed from seawater via extracellular adsorption to phytoplankton. Short-term Cr(VI) and Cr(III) uptake experiments performed with T. weissflogii demonstrated that Cr(III) was the primary oxidation state adsorbing to cells and being internalized by them. Cellular Cr:C ratios (<0.5 µmol Cr mol C-1) of the eight phytoplankton species surveyed were significantly lower than previously reported Cr:C ratios in marine particles with a high biogenic component (10 to 300 µmol Cr mol C-1). This indicates that Cr(III) likely accumulates in marine particles due to uptake and/or adsorption. Mass balance calculations demonstrate that surface water Cr deficits can be explained via loss of Cr(III) to exported particles, thereby providing a mechanism to account for the nutrient depth profile for Cr in modern seawater. Given the large fractionation of stable Cr isotopes during Cr(VI) reduction, Cr(III) associated with exported organic carbon is likely enriched in lighter isotopes. Most sedimentary Cr isotope studies have thus far neglected internal fractionating processes in the marine Cr cycle, but our data indicate that loss of Cr to exported particles may be traced in the sedimentary  53 Cr record.

Biological Control of Chromium Redox and Stable Isotope Composition in the Surface Ocean

“…Semeniuk et al () presented a similar calculation. Our calculation differs in two main ways: (1) Our [Cr] deficits are calculated comparing water above the winter mixed layer and the immediately underlying waters rather than comparing the mixed layer to waters at ≥500 m, and therefore our calculated Cr deficit is smaller; and (2) we use phytoplankton Cr data from the open North Pacific (Martin & Knauer, ) and exported particles in the open North Atlantic (Connelly et al, ), rather than data from North Atlantic marginal seas (Dauby et al, ), which show a much higher Cr:C (~20 μmol Cr mol/C) and may not be representative of the open ocean.…”

A systematic look at chromium isotopes in modern shells – implications for paleo-environmental reconstructions

“…The study of Farkaš et al (2018) Wang et al (2016) could be envisaged as to result from variable Cr uptake mechanisms. These authors propose that in regions with high dissolved organic matter, foraminifera could preferentially uptake Cr(III) associated with dissolved organic phases and/or organic matter, as observed for some phytoplankton (Semeniuk et al, 2016). In regions where dissolved organic concentration is low, foraminifera may switch to the reductive Cr(VI) uptake mechanism, as proposed for coral growth (Pereira et al, 2015).…”

“…Such high D Cr values observed in biogenic carbonates produced by different marine organisms point to a strong biological control over the incorporation of Cr from seawater into CaCO 3 skeletons, where it could be incorporated either as Cr(III) and/or Cr(VI) depending on species-specific redox cycling of Cr (cf. Wang et al, 2016;Semeniuk et al, 2016) and/or, as recently suggested, directly assimilated as 4916 R. Frei et al: A systematic look at chromium isotopes in modern shells organic ligand-bound Cr during biological uptake (Saad et al, 2017). It is too premature to compare the biogenic distribution coefficients with abiogenic values, simply because there is a lack of suitable modern seawater-carbonate pairs from which such values could be calculated.…”

“…Reduction processes of dissolved Cr(VI) complexes to Cr(III) species in ocean water have been used by Scheiderich et al (2015) and Paulukat et al (2016) to explain the δ 53 Cr variations in the world's oceans. Scavenging of isotopically light Cr(III) to deeper water and sediment, potentially by phytoplankton (Semeniuk et al, 2016), and subsequent release of this seawater-derived Cr(III) back into seawater, either as organic complexes with Cr(III) or after oxidation to Cr(VI), are advocated as potential processes to explain the δ 53 Cr vs.…”

“…Furthermore, due to a local redox cycling and biological uptake of Cr in the oceans (Semeniuk et al, 2016), the Cr isotope signature of present-day seawater is not globally homogeneous (Scheiderich et al, 2015;Paulukat et al, 2016). This additionally complicates the application of δ 53 Cr measurements in marine carbonate archives with respect to deducing information regarding global ocean redox, and implications thereof for climatic changes on Earth through time.…”