Iron isotope fractionation during uptake of ferrous ion by phytoplankton

The uptake of iron (Fe) by phytoplankton is an important pathway that drives the global Fe biogeochemical cycle. However, limited information is available regarding the resulting Fe isotope signatures during this metabolic processes. Here, two algal species Chlorella pyrenoidosa and Chlamydomonas reinhardtii were cultured in a medium spiked with FeSO4 to study their ability to fractionate Fe isotopes. We quantified the total cellular and intracellular Fe, and measured their isotope compositions. The amounts and isotope compositions of extracellular Fe were estimated by mass balance. We found that the intracellular Fe of algae in concentration-gradient experiments was enriched in the heavier isotopes relative to the FeSO4 solutions, up to 3% in delta Fe-56, suggesting the heavier Fe isotopes are preferably taken up by the algae. However, the intracellular Fe of algae in time-course experiments showed inconsistent fractionation patterns, either enriching or depleting heavier Fe isotopes. Extracellular Fe was isotopically variable from -2.5% to 1.9% in delta Fe-56 relative to the FeSO4 solutions, likely representing a mixture of Fe-II and Fe-III adsorbed on the cell surface. Additionally, the variation of intracellular delta Fe-56 values appears to be dependent of the intracellular Fe fractions, enriching heavier Fe isotopes at the lower intracellular Fe fractions. Our observations not only highlight the potential of using Fe isotopes as the tracer of biological Fe cycles, but also have important implications on the Fe metabolic pathways of algae.