Citrate as a possible iron-pooling substance in the marine diatom Thalassiosira pseudonana

An iron (Fe)-binding substance (FeBS) was sought for the marine diatom Thalassiosira pseudonana, because the organism is devoid of a gene that encodes the predominant Fe storage protein ferritin. High-performance liquid chromatography-inductively coupled plasma/emission mass spectrometry (HPLC-ICP/MS) analysis of an extract of diatom cells indicated the presence of an FeBS specific to the cells grown in Fe-replete medium. Heat treatment followed by anion-exchange chromatography with monitoring of Fe(III)-dissolving activity was used to isolate the FeBS from the cell extract. Structural analysis using 1H NMR spectrometry demonstrated that the FeBS was composed of citrate. In addition, HPLC-ICP/MS together with ESI-MS demonstrated that the FeBS was likely a triferric tricitrate complex. Although the citrate content of the diatom cells was enhanced according to the Fe nutritional status of the medium, this phenomenon seemed to occur by an indirect mechanism, as a previous report demonstrated a reduced citrate content in T. pseudonana cells grown in Fe-replete medium (Bromke et al. in Plos One doi: 10.1371/journal.pone.0067340, 2013 ). It is most likely that, based on the extended Redfield ratio 121(C):16(N):1(P):0.0075(Fe), the culture used in the current study may have suffered from nutrient deficiency of a major element, such as nitrogen. Such a situation may occur frequently for this open sea diatom. As windblown dust is a prevailing Fe source for the open oceanic diatom T. pseudonana, a rapid elevation in Fe concentration in the seawater can render the cells nitrogen deficient, causing enhanced citrate synthesis and resulting in the formation of a triferric tricitrate complex as Fe storage machinery.