Description: Increasing atmospheric CO2 concentrations are expected to impact pelagic ecosystem functioning in the near future by driving ocean warming and acidification. While numerous studies have investigated impacts of rising temperature and seawater acidification on planktonic organisms separately, little is presently known on their combined effects. To test for possible synergistic effects we exposed two coccolithophore species, Emiliania huxleyi and Gephyrocapsa oceanica, to a CO2 gradient ranging from ,0.5–250 mmol kg21 (i.e. ,20–6000 matm pCO2) at three different temperatures (i.e. 10, 15, 20uC for E. huxleyi and 15, 20, 25uC for G. oceanica). Both species showed CO2-dependent optimum-curve responses for growth, photosynthesis and calcification rates at all temperatures. Increased temperature generally enhanced growth and production rates and modified sensitivities of metabolic processes to increasing CO2. CO2 optimum concentrations for growth, calcification, and organic carbon fixation rates were only marginally influenced from low to intermediate temperatures. However, there was a clear optimum shift towards higher CO2 concentrations from intermediate to high temperatures in both species. Our results demonstrate that the CO2 concentration where optimum growth, calcification and carbon fixation rates occur is modulated by temperature. Thus, the response of a coccolithophore strain to ocean acidification at a given temperature can be negative, neutral or positive depending on that strain’s temperature optimum. This emphasizes that the cellular responses of coccolithophores to ocean acidification can only be judged accurately when interpreted in the proper eco-physiological context of a given strain or species. Addressing the synergistic effects of changing carbonate chemistry and temperature is an essential step when assessing the success of coccolithophores in the future ocean.

Description: Marine bacteria are the main consumers of freshly produced organic matter. Many enzymatic processes involved in the bacterial digestion of organic compounds were shown to be pH sensitive in previous studies. Due to the continuous rise in atmospheric CO2 concentration, seawater pH is presently decreasing at a rate unprecedented during the last 300 million years but the consequences for microbial physiology, organic matter cycling and marine biogeochemistry are still unresolved. We studied the effects of elevated seawater pCO2 on a natural plankton community during a large-scale mesocosm study in a Norwegian fjord. Nine Kiel Off-Shore Mesocosms for Future Ocean Simulations (KOSMOS) were adjusted to different pCO2 levels ranging initially from ca. 280 to 3000 µatm and sampled every second day for 34 days. The first phytoplankton bloom developed around day 5. On day 14, inorganic nutrients were added to the enclosed, nutrient-poor waters to stimulate a second phytoplankton bloom, which occurred around day 20. Our results indicate that marine bacteria benefit directly and indirectly from decreasing seawater pH. During the first phytoplankton bloom, 5–10% more transparent exopolymer particles were formed in the high pCO2 mesocosms. Simultaneously, the efficiency of the protein-degrading enzyme leucine aminopeptidase increased with decreasing pH resulting in up to three times higher values in the highest pCO2/lowest pH mesocosm compared to the controls. In general, total and cell-specific aminopeptidase activities were elevated under low pH conditions. The combination of enhanced enzymatic hydrolysis of organic matter and increased availability of gel particles as substrate supported up to 28% higher bacterial abundance in the high pCO2 treatments. We conclude that ocean acidification has the potential to stimulate the bacterial community and facilitate the microbial recycling of freshly produced organic matter, thus strengthening the role of the microbial loop in the surface ocean.

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: Diatoms can occur as single cells or as chain-forming aggregates. These two strategies affect buoyancy, predator evasion, light absorption and nutrient uptake. Adjacent cells in chains establish connections through various processes that determine strength and flexibility of the bonds, and at distinct cellular locations defining colony structure. Chain length has been found to vary with temperature and nutrient availability as well as being positively correlated with growth rate. However, the potential effect of enhanced carbon dioxide (CO2) concentrations and consequent changes in seawater carbonate chemistry on chain formation is virtually unknown. Here we report on experiments with semi-continuous cultures of the freshly isolated diatom Asterionellopsis glacialis grown under increasing CO2 levels ranging from 320 to 3400 mu atm. We show that the number of cells comprising a chain, and therefore chain length, increases with rising CO2 concentrations. We also demonstrate that while cell division rate changes with CO2 concentrations, carbon, nitrogen and phosphorus cellular quotas vary proportionally, evident by unchanged organic matter ratios. Finally, beyond the optimum CO2 concentration for growth, carbon allocation changes from cellular storage to increased exudation of dissolved organic carbon. The observed structural adjustment in colony size could enable growth at high CO2 levels, since longer, spiral-shaped chains are likely to create microclimates with higher pH during the light period. Moreover increased chain length of Asterionellopsis glacialis may influence buoyancy and, consequently, affect competitive fitness as well as sinking rates. This would potentially impact the delicate balance between the microbial loop and export of organic matter, with consequences for atmospheric carbon dioxide.

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: Our present understanding of ocean acidification (OA) impacts on marine organisms caused by rapidly rising atmospheric carbon dioxide (CO2) concentration is almost entirely limited to single species responses. OA consequences for food web interactions are, however, still unknown. Indirect OA effects can be expected for consumers by changing the nutritional quality of their prey. We used a laboratory experiment to test potential OA effects on algal fatty acid (FA) composition and resulting copepod growth. We show that elevated CO2 significantly changed the FA concentration and composition of the diatom Thalassiosira pseudonana, which constrained growth and reproduction of the copepod Acartia tonsa. A significant decline in both total FAs (28.1 to 17.4 fg cell−1) and the ratio of long-chain polyunsaturated to saturated fatty acids (PUFA:SFA) of food algae cultured under elevated (750 µatm) compared to present day (380 µatm) pCO2 was directly translated to copepods. The proportion of total essential FAs declined almost tenfold in copepods and the contribution of saturated fatty acids (SFAs) tripled at high CO2. This rapid and reversible CO2-dependent shift in FA concentration and composition caused a decrease in both copepod somatic growth and egg production from 34 to 5 eggs female−1 day−1. Because the diatom-copepod link supports some of the most productive ecosystems in the world, our study demonstrates that OA can have far-reaching consequences for ocean food webs by changing the nutritional quality of essential macromolecules in primary producers that cascade up the food web.

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.