The calcitic test growth rate of Spirillina vivipara (Foraminifera)

Abstract. Foraminiferal groups encompass vastly differing morphologies, ecological preferences and elemental/ isotopic test compositions. For some of these groups, the calcification mechanism is completely unknown, but is likely differing from that of well-studied groups. This study aimed to understand the test growth and calcification rate of Spirillina vivipara Ehrenberg, 1843 (Order Spirillinida), and to compare them to other foraminifera species. Spirillinids have a closely coiled spiral chamber like a tube, yet their calcitic microstructure is unique amongst foraminifera. Calcification observations in S. vivipara facilitate the estimation of carbonate precipitation rates during active test elongation, which are revealed to be independent of the individual’s size. We found that S. vivipara grows its test in response to food availability, suggesting that calcification directly corresponds to cell growth. Timelapse observations of S. vivipara indicate continuous growth, suggesting active growth phases and interspersed rest periods, hinting at potential biological rhythms in the growth and calcification process. We also implemented a 24-hour observation period using Calcein staining, showing calcite precipitation rates of 8.08 µm/hour in S. vivipara, which correspond to approximately 364 nmol/cm²/min i.e. 36.4mg/cm²/min. These rates are higher than those published for other foraminifera and those in most inorganic precipitation experiments. Such high rates in spirillinid foraminifera may explain their distinctive morphology, elemental composition, unusual reproduction and ecological distribution.