ALBERT

Description: Porosity in planktonic foraminifers (the proportion of the shell surface covered by pores) is a conspicuous quantitative trait, well preserved in fossil shells and implicated as a source of environmental information. Despite its potential, the functional importance of porosity remains poorly understood. It is likely that pores are important in gas exchange, and differences in shell porosity among species or within species may reflect differences in metabolic rates or ambient oxygen concentration. Theoretically, porosity also affects the weight of the shell; and differences in porosity may reflect an adaptation to the specific density of the seawater or differences in allocation of resources to calcification (shell calcification intensity). Finally, there is evidence that porosity may differ between closely related cryptic species. Here we analyzed the potential role of porosity as a regulator of calcification intensity in Orbulina universa by combining biometric measurements based on sediment surface samples from the western Atlantic with a modelling approach. Specimens of O. universa were analyzed concerning their shell size, shell thickness, and shell porosity under light and scanning electron microscopy, and weighed using a microbalance. The resulting empirically derived model shows an effect size of shell thickness that is 7.5 times larger than the effect of shell porosity on the overall shell calcification intensity. This indicates that porosity is unlikely to be used by this species to regulate calcification intensity. By implementing the model on literature data which analyzed calcification intensity in O. universa, we also show that porosity differences among cryptic species in O. universa are unlikely to explain the observed differences in calcification intensity within the species. These findings indicate that functional explanations for differences in porosity in planktonic foraminifers have to be sought outside of calcification or density regulation and, conversely, that the observed differences in calcification intensity are likely driven by shell thickness and their relationship with environmental forcing can be applied without correction for porosity.

Description: Chemical signatures in the calcite of shells of polar and subpolar planktonic foraminifera have been frequently used to trace and quantify past meltwater discharge events. This approach assumes that the foraminifera can tolerate low salinity under extended periods. To obtain a first experimental constraint on salinity tolerance of Subarctic foraminifera, we carried out a culturing experiment with specimens of the subpolar species Neogloboquadrina incompta collected in the northern Norwegian Sea off Tromsø in October 2018. The foraminifera were exposed to a gradient of salinities between 35 and 25 PSU. Survival was monitored over 26 days by measuring the extent of the rhizopodial network. Although chamber growth only occurred in one of the observed specimens, likely due to the largely unknown dietary preference of the species, we observed a strong differential rhizopodial activity pattern along the gradient. The highest rhizopodial activity occurred at salinity between 35 and 31 PSU. The species is clearly able to survive long-term exposure to salinities as low as 28, but no rhizopodial activity and signs of cytoplasm degradation were observed in all specimens exposed to 25 PSU. These preliminary observations provide the first direct evidence for the salinity tolerance of N. incompta, indicating a range of salinity that could be plausibly expected to be recorded in the chemistry of fossil shells of the species.