It is fascinating that foraminifera are single cell organisms and yet we are not able to fully understand how they precipitate their beautiful shells (also called tests). Our research efforts focuses on better understanding the biomineralization strategy of the benthic foraminiferal species Melonis barleeanus. Traditionally considered a calcareous species, our data reveal the presence of minute (~3 μm on average) sedimentary particles within the chamber walls of the examined shells (see figure below from Borrelli et al. 2018). This was an unexpected finding: these sedimentary grains were found in modern and fossil specimens collected from the Arctic Ocean and the Mediterranean Sea, and even in specimens grown under controlled conditions. In addition, the particle spatial arrangement, their chemical makeup, and consistent size range seemed to suggest a certain degree of selectivity with respect to the grains incorporated in the shell. The reasons behind this unusual biomineralization strategy (i.e., is M. barleeanus a calcareous and agglutinated species? What is the advantage of having sediment grains within a calcareous shell?) and its implications remain unclear, but we can conclude that this feature is not due to post-mortem diagenetic alteration. Considering that M. barleeanus is often used in paleoceanographic reconstructions, we are now working to increase our understanding regarding the shell structure of this foraminiferal species. Specifically, we are studying how and if these grains are strictly connected to the calcite crystals forming the shell. The results of this project will have important implications as the presence of sedimentary particles in calcareous foraminifera does not seem to be a unique characteristic of M. barleeanus. In fact, similar observations were reported for Bolivina, Cibicidoides, Stomatorbina, Uvigerina and even for planktic Globigerinoides specimens suggesting that this might be a widespread characteristic amongst Rotaliida species, including species commonly used in paleoceanographic reconstructions.
