Iron is accumulated symplastically in kelp in a non-ferritin core that seems to be a general feature of brown algae. Microprobe studies show that Fe binding depends on tissue type. The sea is generally an iron-poor environment and brown algae were recognized in recent years for having a unique, ferritin-free iron storage system. Kelp (Laminaria digitata) and the filamentous brown alga Ectocarpus siliculosus were investigated using X-ray microprobe imaging and nanoprobe X-ray fluorescence tomography to explore the localization of iron, arsenic, strontium, and zinc, and micro-X-ray absorption near-edge structure (μXANES) to study Fe binding. Fe distribution in frozen hydrated environmental samples of both algae shows higher accumulation in the cortex with symplastic subcellular localization. This should be seen in the context of recent ultrastructural insight by cryofixation-freeze substitution that found a new type of cisternae that may have a storage function but differs from the apoplastic Fe accumulation found by conventional chemical fixation. Zn distribution co-localizes with Fe in E. siliculosus, whereas it is chiefly located in the L. digitata medulla, which is similar to As and Sr. Both As and Sr are mostly found at the cell wall of both algae. XANES spectra indicate that Fe in L. digitata is stored in a mineral non-ferritin core, due to the lack of ferritin-encoding genes. We show that the L. digitata cortex contains mostly a ferritin-like mineral, while the meristoderm may include an additional component.

Argonne National Laboratory The University of Chicago Building 434A 9700 South Cass Avenue Lemont IL 60439 USA

Czech Academy of Sciences Biology Centre Institute of Plant Molecular Biology Laboratory of Plant Biophysics and Biochemistry Branišovska 1160 31 370 05 Česke Budějovice Czech Republic

Department of Biology National and Kapodistrian University of Athens Panepistimiopolis Athens 157 84 Hellas Greece

Department of Chemistry and Biochemistry San Diego State University CA 92182 1030 USA

Department of Experimental Plant Biology University of South Bohemia Branišovská 31 1160 370 05 České Budějovice Czech Republic

ESRF The European Synchrotron Radiation Facility Beamline ID16A 71 avenue des Martyrs CS 40220 38043 Grenoble Cedex 9 France

European XFEL GmbH Holzkoppel 4 22869 Schenefeld Germany

Marine Biodiscovery Centre Department of Chemistry University of Aberdeen Aberdeen AB24 3UE UK

School of Biological Sciences University of Aberdeen Cruickshank Building St Machar Drive Aberdeen AB24 3UU UK

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