A microsensor for carbonate ions suitable for microprofiling in freshwater and saline environments

A novel carbonate microsensor, based on the ion‐selective ionophore N,N,‐dioctyl‐3a,12 a‐bis(4‐trifluoroacetylbenzoxy)‐5β‐cholan‐24‐amide, is presented. The sensor chemistry and filling electrolyte, used previously for macrosensors, was improved for use in microsensors, and a simple calibration procedure was designed. The sensor is highly selective for carbonate, having a similar selectivity as the macrosensor, and is so insensitive to Cl− interference that it can be used in seawater. The ability to measure accurate profiles with the carbonate sensor was verified in agar gels with artificial carbonate gradients. Several environmental applications are presented, including photosynthesis and calcification measurements in freshwater stromatolites (tufas) and foraminifera. Carbonate profiles in illuminated and darkened hypersaline microbial mats were qualitatively as expected and aligned with the oxygen and pH profiles. The dissolved inorganic carbon profiles calculated from local pH and carbonate values, however, did not follow the expected trends, both in the foraminifera and the hypersaline mat. Temporal and spatial heterogeneities make perfect alignment of pH and carbonate profiles, needed for DIC calculations, unrealistic. The calculation of dissolved inorganic carbon microprofiles from pH and carbonate microprofiles is not recommended. The microsensor is highly useful in studies on calcification and decalcification, where direct concentrations of carbonate and calcium ions are needed

pH effect on the susceptibility to parasitoid infection in the marine diatom Coscinodiscus spp. (Bacillariophyceae)

The pH on the frustule of individual cells of the marine centric diatoms Coscinodiscus granii and Coscinodiscus wailesii (Bacillariophyceae) was measured with pH microsensors in culture media with increasing pH values of 8.04, 8.14, and 8.22, respectively. In 85–96% of the C. granii cells the pH on the frustule was up to 0.4 units higher than that of the medium, reaching a maximum pH 8.95. Only in 2–3% the surface pH exceeded that of the medium by up to 0.7 pH units. These results strongly suggest that diatoms in batch cultures differ, at least temporarily, in their individual photosynthetic activities. Infection experiments with the parasitoid nanoflagellate Pirsonia diadema (Stramenopile) showed that flagellates failed to infect when the culture pH was 8.8 and above. pH measurements on freshly infected C. granii showed that the prevalence of infection was higher in tendency on diatoms with low surface pH. Application of these results to parasitoid-diatom interactions in natural waters suggests that within phytoplankton populations a strong photosynthetic activity might prevent diatom cells temporarily from infection by pH-sensitive parasitoids

Calcification acidifies the microenvironment of a benthic foraminifer (Ammonia sp.)

Calcareous foraminifera are well known for their CaCO3 shells. Yet, CaCO3 precipitation acidifies the calcifying fluid. Calcification without pH regulation would therefore rapidly create a negative feedback for CaCO3 precipitation. In unicellular organisms, like foraminifera, an effective mechanism to counteract this acidification could be the externalization of H+ from the site of calcification. In this study we showthat a benthic symbiont-free foraminifer Ammonia sp. actively decreases pH within its extracellular microenvironment only while precipitating calcite. During chamber formation events the strongest pH decreases occurred in the vicinity of a newly forming chamber (range of gradient ~100 μm) with a recorded minimum of 6.31 (b10 μm from the shell) and a maximumduration of 7 h. The acidification was actively regulated by the foraminifera and correlatedwith shell diameters, indicating that the amount of protons removed during calcification is directly related to the volume of calcite precipitated. The here presented findings imply that H+ expulsion as a result of calcification may be a wider strategy for maintaining pH homeostasis in unicellular calcifying organisms