[1] Uptake of half of the fossil fuel CO 2 into the ocean causes gradual seawater acidification. This has been shown to slow down calcification of major calcifying groups, such as corals, foraminifera, and coccolithophores. Here we show that two of the most productive marine calcifying species, the coccolithophores Coccolithus pelagicus and Calcidiscus leptoporus, do not follow the CO 2 -related calcification response previously found. In batch culture experiments, particulate inorganic carbon (PIC) of C. leptoporus changes with increasing CO 2 concentration in a nonlinear relationship. A PIC optimum curve is obtained, with a maximum value at present-day surface ocean pCO 2 levels ($360 ppm CO 2 ). With particulate organic carbon (POC) remaining constant over the range of CO 2 concentrations, the PIC/POC ratio also shows an optimum curve. In the C. pelagicus cultures, neither PIC nor POC changes significantly over the CO 2 range tested, yielding a stable PIC/POC ratio. Since growth rate in both species did not change with pCO 2 , POC and PIC production show the same pattern as POC and PIC. The two investigated species respond differently to changes in the seawater carbonate chemistry, highlighting the need to consider species-specific effects when evaluating whole ecosystem responses. Changes of calcification rate (PIC production) were highly correlated to changes in coccolith morphology. Since our experimental results suggest altered coccolith morphology (at least in the case of C. leptoporus) in the geological past, coccoliths originating from sedimentary records of periods with different CO 2 levels were analyzed. Analysis of sediment samples was performed on six cores obtained from locations well above the lysocline and covering a range of latitudes throughout the Atlantic Ocean. Scanning electron micrograph analysis of coccolith morphologies did not reveal any evidence for significant numbers of incomplete or malformed coccoliths of C. pelagicus and C. leptoporus in last glacial maximum and Holocene sediments. The discrepancy between experimental and geological results might be explained by adaptation to changing carbonate chemistry.

Abstract. Four strains of the coccolithophore E. huxleyi (RCC1212, RCC1216, RCC1238, RCC1256) were grown in dilute batch culture at four CO 2 levels ranging from ∼200 µatm to ∼1200 µatm. Growth rate, particulate organic carbon content, and particulate inorganic carbon content were measured, and organic and inorganic carbon production calculated. The four strains did not show a uniform response to carbonate chemistry changes in any of the analysed parameters and none of the four strains displayed a response pattern previously described for this species. We conclude that the sensitivity of different strains of E. huxleyi to acidification differs substantially and that this likely has a genetic basis. We propose that this can explain apparently contradictory results reported in the literature.

Abstract. Two haptophyte algae, Emiliania huxleyi and Gephyrocapsa oceanica, were cultured at different temperatures and salinities to investigate the impact of these factors on the hydrogen isotopic composition of long chain alkenones synthesized by these algae. Results showed that alkenones synthesized by G. oceanica were on average depleted in D by 30‰ compared to those of E. huxleyi when grown under similar temperature and salinity conditions. The fractionation factor, α alkenones−H 2 O , ranged from 0.760 to 0.815 for E. huxleyi and from 0.741 to 0.788 for G. oceanica. There was no significant correlation of α alkenones−H 2 O with temperature but a positive linear correlation was observed between α alkenones−H 2 O and salinity with ∼3‰ change in fractionation per salinity unit and a negative correlation between α alkenones−H 2 O and growth rate. This suggests that both salinity and growth rate can have a substantial impact on the stable hydrogen isotopic composition of long chain alkenones in natural environments.

Abstract. Four strains of the coccolithophore Emiliania huxleyi (RCC1212, RCC1216, RCC1238, RCC1256) were grown in dilute batch culture at four CO2 levels ranging from ~200 μatm to ~1200 μatm. Growth rate, particulate organic carbon content, and particulate inorganic carbon content were measured, and organic and inorganic carbon production calculated. The four strains did not show a uniform response to carbonate chemistry changes in any of the analysed parameters and none of the four strains displayed a response pattern previously described for this species. We conclude that the sensitivity of different strains of E. huxleyi to acidification differs substantially and that this likely has a genetic basis. We propose that this can explain apparently contradictory results reported in the literature.

Abstract. The Mg / Ca ratio of Foraminifera calcium carbonate tests is used as proxy for seawater temperature and widely applied to reconstruct global paleo-climatic changes. However, the mechanisms involved in the carbonate biomineralization process are poorly understood. The current paradigm holds that calcium ions for the test are supplied primarily by endocytosis of seawater. Here, we combine confocal-laser scanning-microscopy observations of a membrane-impermeable fluorescent marker in the extant benthic species Ammonia aomoriensis with dynamic 44 Calabeling and NanoSIMS isotopic imaging of its test. We infer that Ca for the test in A. aomoriensis is supplied primarily via trans-membrane transport, but that a small component of passively transported (e.g., by endocytosis) seawater to the site of calcification plays a key role in defining the trace-element composition of the test. Our model accounts for the full range of Mg / Ca and Sr / Ca observed for benthic Foraminifera tests and predicts the effect of changing seawater Mg / Ca ratio. This places foram-based paleoclimatology into a strong conceptual framework.

With respect to their sensitivity to ocean acidification, calcifiers such as the coccolithophore Emiliania huxleyi have received special attention, as the process of calcification seems to be particularly sensitive to changes in the marine carbonate system. For E. huxleyi, apparently conflicting results regarding its sensitivity to ocean acidification have been published (Iglesias-Rodriguez et al., 2008a;Riebesell et al., 2000). As possible causes for discrepancies, intra-specific variability and different effects of CO 2 manipulation methods, i.e. the manipulation of total alkalinity (TA) or total dissolved inorganic carbon (DIC), have been discussed. While Langer et al. (2009) demonstrate a high degree of intra-specific variability between strains of E. huxleyi, the question whether different CO 2 manipulation methods influence the cellular responses has not been resolved yet. In this study, closed TA as well as open and closed DIC manipulation methods were compared with respect to E. huxleyi's CO 2 -dependence in growth rate, POC-and PIC-production. The differences in the carbonate chemistry between TA and DIC manipulations were shown not to cause any differences in response patterns, while the latter differed between open and closed DIC manipulation. The two strains investigated showed different sensitivities to acidification of seawater, RCC1256 being more negatively affected in growth rates and PIC production than NZEH.

Abstract. Methane (CH 4 ), an important greenhouse gas that affects radiation balance and consequently the earth's climate, still has uncertainties in its sinks and sources. The world's oceans are considered to be a source of CH 4 to the atmosphere, although the biogeochemical processes involved in its formation are not fully understood. Several recent studies provided strong evidence of CH 4 production in oxic marine and freshwaters, but its source is still a topic of debate. Studies of CH 4 dynamics in surface waters of oceans and large lakes have concluded that pelagic CH 4 supersaturation cannot be sustained either by lateral inputs from littoral or benthic inputs alone. However, regional and temporal oversaturation of surface waters occurs frequently. This comprises the observation of a CH 4 oversaturating state within the surface mixed layer, sometimes also termed the "oceanic methane paradox". In this study we considered marine algae as a possible direct source of CH 4 . Therefore, the coccolithophore Emiliania huxleyi was grown under controlled laboratory conditions and supplemented with two 13 C-labeled carbon substrates, namely bicarbonate and a position-specific 13 C-labeled methionine (R-S-13 CH 3 ). The CH 4 production was 0.7 µg particular organic carbon (POC) g −1 d −1 , or 30 ng g −1 POC h −1 . After supplementation of the cultures with the 13 C-labeled substrate, the isotope label was observed in headspace CH 4 . Moreover, the absence of methanogenic archaea within the algal culture and the oxic conditions during CH 4 formation suggest that the widespread marine algae Emiliania huxleyi might contribute to the observed spatially and temporally restricted CH 4 oversaturation in ocean surface waters.

Various protective and metabolic functions for coccolithophore calcification have been proposed such as providing a means to supply CO 2 for photosynthesis. It has also been speculated that calcification helps to dissipate excess energy under high irradiance, thereby circumventing photoinhibition. To address these questions, cells of a calcifying strain of Emiliania huxleyi were grown at three irradiances (30, 300, and 800 mmol photons m 22 s 21 ) in combination with four calcium (Ca) concentrations (0.1, 1, 2.5, and 10 mmol L 21 ) leading to different degrees of calcification in the same strain. Growth rates (m), particulate organic carbon (POC), and inorganic carbon (PIC) production as well as carbon isotope fractionation (e p ) were determined. Photosynthetic O 2 evolution and CO 2 and HCO { 3 uptake rates were measured by membrane inlet mass spectrometry (MIMS). The application of this multimethod approach provides new information on the role of calcification in E. huxleyi. Noncalcifying cells showed POC production rates as high as calcifying ones. No differences in e p were observed under different Ca concentrations. MIMS measurements indicate that noncalcifying cells can photosynthesize as efficiently as, or even more efficiently than, calcifying ones and that both use HCO { 3 as the main carbon source. The ratio of photosynthetic HCO { 3 uptake relative to net fixation did not differ among cells acclimated to 10 mmol L 21 or to 0.1 mmol L 21 Ca. These results indicate that (1) calcification is not involved in photosynthetic carbon acquisition, and (2) calcification does not provide a means of energy dissipation under high irradiances.