The role of the reactivity and content of iron of aerosol dust on growth rates of two Antarctic diatom species

The atmosphere is widely recognized as a major source of Fe in the form of iron-containing dust. This study provides the first experiments in which the impact of dust on the growth rates of single species of Antarctic diatoms was assessed under laboratory conditions. The dust was among others characterized by x-ray powder diffraction analysis, analysis of total and amorphous Fe content, and dissolution rates of Fe in seawater. The amount of bioavailable Fe from the dust was determined, not via the complicated chemistry of Fe in seawater but by using diatoms as bioindicators for available Fe. Cultures of two large diatom species, Actinocyclus sp. and Thalassiosira sp., were amended with potential dust aerosols from two dust-supplying regions, Namibia and Mauritania, and responses on growth rates were monitored. Apart from a difference in total Fe content, a difference in crystallinity existed in the Fe minerals of both dust types. The fraction of amorphous Fe was reflected in a higher reactivity/dissolution of Fe in seawater. The increase in growth rate upon dust addition was positively related with the amount of amorphous Fe in the dust and with the dissolution rate of Fe in seawater. However, compared with equal FeCl3 concentrations, the dissolved Fe from the dust was not completely available for the diatoms. Interestingly, the diatoms used only a small part of the dissolved Fe, demonstrating the importance of algae as bioindicators.