Effects of Zn contamination on biomineralization processes of benthic foraminiferal tests

Benthic foraminifera are an important component of marine communities, playing a key role in ecosystem functioning and biogeochemical cycling. Their sensibility and rapid response to environmental stresses make them an efficient environmental proxy of past and present climate and environmental changes. The Ca-carbonate shells of foraminifera may incorporate trace metals present in the ocean waters through a controlled biomineralization process. In our study, we applied X-ray absorption and electron spectromicroscopy to reveal the Zn distribution at the nanoscale and its chemical speciation in selected benthic foraminifera species: Elphidium aculeatum and Quinqueloculina seminula, that were sampled from a heavy-metal polluted area of Sardinia (Italy; De Giudici et al., 2018). These species synthesise their low-Mg and high-Mg shells by extracellular and intracellular mechanism, respectively. Our analysis reveals that Zn concentration, distribution and chemical speciation differ at the micro- and nano-metric scales of the investigated species. In Q. seminula, Zn is more uniformly distributed and occurs in a less ordered environment, which suggests that it is incorporated more easily during the biologically mediated carbonation process. In addition, Zn occurs in an independent and unexpected phase, which is hydrozincite, whose formation is due to foraminiferal cellular processes. These findings offer a key for a better understanding of the processes ruling the incorporation of Zn in biominerals, their role in biogeochemical cycling and in (paleo)environmental investigations.