Tracking sulfate, magnesium and amorphous phases in cultured benthic foraminifera

Abstract. In this study, we combined high resolution Transmission Electron Microscopy (TEM) bright field and High-Angle Annular Dark-Field Scanning TEM imaging with nanoscale Secondary Ion Mass Spectrometry (NanoSIMS) analyses of Rosalina like foraminifera cells cultured in 33S-labeled seawater to understand the origin of sulfur in the test and the cytoplasm, its interaction with the ultrastructure of the test and the co-distribution of sulfur with phosphorus, calcium and magnesium. Test chambers that grew in 33S-labeled seawater revealed that at least 1/3 of the sulfur incorporated is directly taken from seawater sulfate. Our observations reveal a global co-occurrence of P, S and Mg within the previous two last chambers of the test, that all appear more concentrated in two areas, mostly near the Primary Organic sheet (POS) and in the Outer Calcitic Layer (OCL) that corresponds to the calcite added during the growth of the last chamber. Less crystalline grains can be found in the P, Mg and S-richest part of the POS and within OCLs. We interpret these grains as ACC (Amorphous Calcium Carbonate), in association with organic matter and possibly ACP (Amorphous Calcium Phosphate) likely formed or assembled at the sites of calcification, revealing a complex interplay between S, Mg and P. This interplay could indicate an inorganic cause for S and P enrichments, co-occuring with Mg enrichments and amorphous phases. Finally, ~0.1 to ~3 µm Ca-rich grains are detected in the cytoplasm. The biggest ones can be interpreted as ACC vesicles, the smaller ones are Ca and P rich and can be interpreted as autophagosomes or acidocalcisomes, the latter also containing labeled sulfur. Labeled sulfur is also found within smaller vesicles in the cytoplasm and represents overall 10 to 25 % of the sulfur we observe. Though we cannot confirm the occurrence of an assimilatory pathway for sulfur in foraminifera, the clear presence of seawater sulfate in the cell demonstrates that sulfate is involved in many aspects of foraminifera biological activity, in close association with Ca-transportation, Mg and P distribution, and likely with ACC formation and its evolution to calcite.