CALCIUM-UPTAKE AND CALCIFICATION RATE IN THE PLANKTONIC FORAMINIFER ORBULINA-UNIVERSA

We have developed a new stable isotope labeling technique to probe the uptake of Ca2+ in foraminifera shells grown via live culturing. This technique is based on the addition of the minor stable isotope 48Ca (natural abundance 0.19%) to seawater. The extent of Ca2+ uptake by foraminifera can be assessed by determination of the 48/Ca-44 of the cultured shells using plasma mass spectrometry. This new technique could also be used to assess Ca2+ uptake in other CaCO3-secreting organisms. Results from labeling experiments indicate that almost all the Ca2+ in the terminal spherical chamber of the planktonic foraminifer Orbulina universa is in isotopic equilibrium with the surrounding seawater. Only a very small fraction (<0.15 mug CaCO3 equivalent, or 0.5% of average final shell mass) is likely to come from some other pool. If our results apply generally to the precipitation of foraminifera shells, they suggest that an internal cytoplasmic Ca2+ pool probably does not provide a means by which seawater cation ratios can be fractionated. We have also applied the new labeling technique to the determination of incremental calcification rates in Orbulina universa. Our results indicate that adult calcification in individuals grown under high light is up to three times greater than for individuals grown in the dark. This difference is directly attributable to the role symbionts play in enhancing calcification. The enhancement in calcification rate is close to the calculated increase in carbonate ion due to the photosynthetic activity of the symbionts, suggesting that calcification rate in O. universa is proportional to the degree of carbonate saturation.