Effect of hydroxamate ferrisiderophore complex (ferrichrome) on iron uptake and growth of a coastal marine diatom, Chaetoceros sociale

Abstract

Recent studies point out that dissolved iron in seawater is bound to strong organic complexes that have stability constants consistent with microbially produced siderophores. In the present study, the growth and iron uptake of the coastal marine diatom Chaetoceros sociale were experimentally measured in culture experiments at 10°C in media containing a terrestrial fungal hydroxamate ferrisiderophore, desferriferrichrome (DFC)‐Fe(III) (2 DFC:1 Fe and 10 DFC:1 Fe) complex with an Fe concentration of 100 nM. The DFC‐Fe(III) (2:1) medium as well as EDTA‐Fe(III) (2:1) medium in the previous study induced high cell yields and fast growth of C. sociale. The short‐term iron uptake rate in the DFC‐Fe(III) (2:1) medium aged for 9 d at 10°C was much faster than the uptake rates in the DFC‐Fe(III) (2:1 and 10:1) media aged for 1 d and in the DFC‐Fe(III) (10:1) medium aged for 9 d at 10°C. Furthermore, the DFC‐Fe(III) (2:1) medium induced the fastest iron uptake rate in the organic‐Fe(III) complexes [DFC‐Fe(III), EDTA‐Fe(III), citric‐Fe(III) and fulvic‐Fe(III)] and solid amorphous Fe(III) hydroxide [am‐Fe(III)] media in the present and previous studies. The addition of excess DFC stopped iron uptake by C. sociale. The dissociation of DFC‐Fe(III) complex in seawater was determined by simple filtration (0.025 °m). Fast Fe(III) release was observed over 6–12 d in the DFC‐Fe(III) (2:1) complex at 10°C, resulting from the thermal and microbial decomposition of the DFC complexed with Fe(III). This result is consistent with the faster iron uptake rate and growth rate of C. sociale observed in the DFC‐Fe(III) (2:1) medium at 10°C. These results show that the dissolved DFC‐Fe(III) complex in seawater supplies biologically available inorganic Fe(III) species, which may determine the iron uptake rate in culture media by providing bioavailable Fe as the DFC decomposes. The thermal, photochemical, and microbial decomposition of organic chelators complexed with Fe(III) in aquatic environments is probably one of the most important mechanisms for providing bioavailable inorganic Fe(III) species into these environments.