Description: Highlights • Calcification rates are stimulated by CO2 and HCO3− and inhibited by H+. • This novel substrate–inhibitor concept is tested with experimental data. • The concept enables us to reconcile conflicting results among laboratory studies. • We illustrate how this physiological concept can be included in ecological theory. • We apply the concept to discuss coccolithophore dispersal in the oceans. Abstract Coccolithophores are a group of unicellular phytoplankton species whose ability to calcify has a profound influence on biogeochemical element cycling. Calcification rates are controlled by a large variety of biotic and abiotic factors. Among these factors, carbonate chemistry has gained considerable attention during the last years as coccolithophores have been identified to be particularly sensitive to ocean acidification. Despite intense research in this area, a general concept harmonizing the numerous and sometimes (seemingly) contradictory responses of coccolithophores to changing carbonate chemistry is still lacking to date. Here, we present the “substrate–inhibitor concept” which describes the dependence of calcification rates on carbonate chemistry speciation. It is based on observations that calcification rate scales positively with bicarbonate (HCO3−), the primary substrate for calcification, and carbon dioxide (CO2), which can limit cell growth, whereas it is inhibited by protons (H+). This concept was implemented in a model equation, tested against experimental data, and then applied to understand and reconcile the diverging responses of coccolithophorid calcification rates to ocean acidification obtained in culture experiments. Furthermore, we (i) discuss how other important calcification-influencing factors (e.g. temperature and light) could be implemented in our concept and (ii) embed it in Hutchinson’s niche theory, thereby providing a framework for how carbonate chemistry-induced changes in calcification rates could be linked with changing coccolithophore abundance in the oceans. Our results suggest that the projected increase of H+ in the near future (next couple of thousand years), paralleled by only a minor increase of inorganic carbon substrate, could impede calcification rates if coccolithophores are unable to fully adapt. However, if calcium carbonate (CaCO3) sediment dissolution and terrestrial weathering begin to increase the oceans’ HCO3− and decrease its H+ concentrations in the far future (10–100 kyears), coccolithophores could find themselves in carbonate chemistry conditions which may be more favorable for calcification than they were before the Anthropocene.

Description: Coccolithophore calcite refers to the plates of calcium carbonate (CaCO3) produced by the calcifying phytoplankton, coccolithophores. The empirical study of the elemental composition has a great potential in the development of paleoproxies. However, the difficulties to separate coccolithophore carbonates from organic phases hamper the investigation of coccoliths magnesium to calcium ratios (Mg/Ca) in biogeochemical studies. Magnesium (Mg) is found in organic molecules in the cells at concentrations up to 400 times higher than in inorganically precipitated calcite in present-day seawater. The aim of this study was to optimize a reliable procedure for organic Mg removal from coccolithophore samples to ensure reproducibility in measurements of inorganic Mg in calcite. Two baseline methods comprising organic matter oxidations with (1) bleach and (2) hydrogen peroxide (H2O2) were tested on synthetic pellets, prepared by mixing reagent grade CaCO3 with organic matter from the non-calcifying marine algae Chlorella autotrophica and measured with an ICP-AES (inductively coupled plasma-atomic emission spectrometer). Our results show that treatments with a reductive solution [using hydroxylamine-hydrochloride (NH2OH·HCl + NH4OH)] followed by three consecutive oxidations (using H2O2) yielded the best cleaning efficiencies, removing 〉99% of organic Mg in 24 h. P/Ca and Fe/Ca were used as indicators for organic contamination in the treated material. The optimized protocol was tested in dried coccolithophore pellets from batch cultures of Emiliania huxleyi, Calcidiscus leptoporus and Gephyrocapsa oceanica. Mg/Ca of treated coccolithophores were 0.151 ± 0.018, 0.220 ± 0.040, and 0.064 ± 0.023 mmol/mol, respectively. Comparison with Mg/Ca literature coccolith values, suggests a tight dependence on modern seawater Mg/Ca, which changes as a consequence of different seawater origins (〈10%). The reliable determination of Mg/Ca and Sr/Ca, and the low levels of organic contamination (Fe/Ca and P/Ca) make this protocol applicable to field and laboratory studies of trace elemental composition in coccolithophore calcite

Description: Experimental setups to study modes of inorganic carbon acquisition and fixation rates by marine phytoplankton commonly make use of so-called disequilibrium techniques. The chemical or isotopic disequilibrium, either caused by phytoplankton cells taking up inorganic carbon or by a small disturbance of the isotopic equilibrium in the carbonate system, requires to account for the relatively slow chemical interconversion of carbon dioxide (CO2) to bicarbonate (HCO3−) in seawater. Because in such experiments a constant pH is a prerequisite, pH buffers are generally used. However, a possible influence of such buffers on the kinetics of the carbonate system has hitherto not been investigated. Here, a model of the carbonate system in seawater is employed to show how pH buffers are operating. Furthermore, a new approach is presented to determine the rate constants, k+ and k−, for the conversion reaction of CO2 to HCO3− and vice versa, by means of membrane inlet mass spectrometry (MIMS). For the two pH buffers tested (HEPES and BICINE) it is shown that measured rate constants are in good agreement with calculated values for k+ and k− in a pH range of 7 to 8.5 and at temperatures from 10 to 25 °C.

Description: Throughout the last similar to 900 kyr, the Late Pleistocene, Earth has experienced periods of cold glacial climate, punctuated by seven abrupt transitions to warm interglacials, the so-called terminations. Although most of glacial ice is located in the Northern Hemisphere (NH), the Southern Hemisphere (SH) seems to play a crucial role in deglaciation. Variation in the seasonal distribution of solar insolation is one candidate for the cause of these climatic shifts. But so far, no simple mechanism has been identified. Here we present a mathematical analysis of variations in midsummer insolation in both hemispheres at 65 degrees latitude. Applying this analysis to the entire Pleistocene, the last 2 Myr, we find that prior to each termination the insolation in both hemispheres increases in concert, with a SH lead. Introducing time and energy thresholds to these overlaps, calculated times for the onsets of the seven terminations by this insolation canon (exceptional overlaps meeting the two threshold prerequisites) are similar to 23, 139, 253, 345, 419, 546 and 632 kyr BP, perfectly matching the geologic record. The timing originates from the interplay between the two orbital parameters obliquity and precession, explaining why terminations occur at integer multiple of the precessional cycle. There is no such constellation between I and 2 Myr BP, the Early Pleistocene, in agreement with Earth's climate at that time. This change in orbital forcing coincides with the Mid Pleistocene Revolution, separating the Late from the Early Pleistocene. Therefore, we hypothesize that the insolation canon is the trigger for glacial terminations. (c) 2006 Elsevier B.V. All rights reserved.

Description: Iron is limiting phytoplankton productivity in large parts of today's oceans, the so-called HNLC (high nutrient low chlorophyll) areas. It is a key component in photosynthesis during which inorganic carbon fixation in most phytoplankton species is sustained by so-called carbon concentrating mechanisms (CCMs). Here we investigate CCM regulation in the coccolithophore Emiliania huxleyi in response to varying degrees of iron limitation by means of membrane-inlet mass spectrometry. Compared to iron replete conditions rates of both active CO2 and HCO-3 uptake were markedly reduced under iron limitation leading to significantly diminished growth rates. Moreover, there was a concomitant decrease in CCM efficiency, reflected in an increased CO2 loss from the cell in relation to carbon fixation. Under such conditions higher values for carbon isotope fractionation (∈P) would be expected. However, direct measurements of ∈P showed that carbon isotope fractionation was insensitive to changes in growth rates and CCM activity. This can be explained by concomitant changes in internal DIC fluxes in and out of the chloroplast as demonstrated with a simple cell model comprising two compartments. Thus, carbon isotope fractionation reflects the ability of phytoplankton to actively control their inorganic carbon acquisition depending on environmental conditions. The insensitivity of carbon isotope fractionation to changes in the availability of iron could be of interest for paleoreconstructions in the HNLC areas of today's oceans.