Synchrotron X-rays reveal the modes of Fe binding and trace metal storage in the brown algae Laminaria digitata and Ectocarpus siliculosus.

The sea is a generally 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 XRF tomography to explore the localization of iron, arsenic, strontium and zinc, and µXANES to study Fe binding. Fe distribution in frozen-hydrated environmental samples of both algae shows higher accumulation in the cortex with symplastic subcellular localisation. This should be seen in the context of recent ultrastructural insight by cryofixation-freeze substitution that found a new type of cisternae which may have a storage function but differs from the apoplastic Fe accumulation found by conventional chemical fixation. Zn distribution colocalizes 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 found mostly 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.