Abstract
The anthropogenic CO2 accumulating in the ocean is lowering seawater carbonate ion concentration and may reduce calcification rates of marine calcareous organisms. Several proxies based on test weights of planktic foraminifera have been used to evaluate the impact of ocean acidification on these organisms. Unfortunately, because of the absence of a method to evaluate the bulk density of a test, the impact of seawater carbonate chemistry on test calcification is still not fully understood. In this study, we measured bulk densities of living Globigerina bulloides (planktic foraminifera) tests with an X-ray micro-computed tomography (XMCT) scanner and compared them with ambient seawater characteristics. Results demonstrated that test bulk densities were controlled by ambient seawater carbonate ion concentrations and that changes of test bulk densities were accompanied by changes in micron to submicron scale porosity of internal ultrastructure. These results suggest that alteration of the bulk density of foraminiferal tests due to acidification of ambient seawater can be directly observed by XMCT scanning. A useful metric of calcification intensity would therefore be physical measurements of test densities with XMCT.
Highlights
Some of the CO2 gas discharged by human activities is being absorbed into ocean surface waters, where it is causing a decrease of seawater pH and concomitant reduction of the seawater carbonate ion concentration, [CO32−]
Quantitative measurements of physical characteristics of live G. bulloides test, which were sampled from seawater supersaturated with calcite (ΩCa > 1), showed which physical characteristics of the tests were most sensitive to ambient seawater [CO32−] where they sampled (Fig. 2)
The wall thickness and area density of G. bulloides tests were not correlated with ambient seawater [CO32−] (Fig. 2b,c), and results of multiple regression analysis showed that these proxies receive relatively large contribution from ambient seawater temperature
Summary
Some of the CO2 gas discharged by human activities is being absorbed into ocean surface waters, where it is causing a decrease of seawater pH and concomitant reduction of the seawater carbonate ion concentration, [CO32−]. We consider that XMCT scanning is a method well suited for quantifying test condition of foraminifera from the view point of test bulk density unaffected by test wall thickness and could supplement or even replace information on test weights in studies of ocean acidification effects. We used XMCT scanning and test weight measurements to quantify the physical characteristics of the tests (wall thickness, test bulk density, and area density) of live G. bulloides and compared them with the characteristics of ambient seawater where the samples were collected (Supplementary Table S1). Such information is expected to facilitate the understanding of sea water chemistry effect to the calcification of planktic foraminifera
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