The study of the diversity and the function of larger protozooplankton (especially foraminifera, radiolaria and acantharia) in pelagic food webs, despite the extensive use of their mineral skeletons as proxies for palaeoceanographic reconstructions, has started only fairly recently. An important group, which comes to the fore as a biological proxy, are the acantharia. These delicate, free living, microphagic organisms form barium-enriched celestite (Ba/Sr SO4) skeletons. During the in situ iron fertilization experiments EisenEx and EIFEX acantharia showed very high abundances (up to 50 ind. L-1) already prior to the first fertilization, compared to much lower abundances of foraminifera and radiolaria, and their temporal response resembled the development of the phytoplankton bloom. Hence acantharia clearly have the capability to respond to enhanced biological productivity with population growth. Moreover, data on Ba and Sr content of individuals from EIFEX indicate that acantharia play a unique role in the barium and strontium cycle of the oceans. In this context, it has been suggested that these organisms influence barite (BaSO4) deposition in the sediments although the mechanisms leading to its formation are still under debate. The degree of barite deposition in the sediments is used as a proxy for biological productivity of the overlying water column.