Description: The formation of calcareous skeletons by marine planktonic organisms and their subsequent sinking to depth generates a continuous rain of calcium carbonate to the deep ocean and underlying sediments. This is important in regulating marine carbon cycling and ocean-atmosphere CO2 exchange. The present rise in atmospheric CO2 levels causes significant changes in surface ocean pH and carbonate chemistry. Such changes have been shown to slow down calcification in corals and coralline macroalgae, but the majority of marine calcification occurs in planktonic organisms. Here we report reduced calcite production at increased CO2 concentrations in monospecific cultures of two dominant marine calcifying phytoplankton species, the coccolithophorids Emiliania huxleyi and Gephyrocapsa oceanica . This was accompanied by an increased proportion of malformed coccoliths and incomplete coccospheres. Diminished calcification led to a reduction in the ratio of calcite precipitation to organic matter production. Similar results were obtained in incubations of natural plankton assemblages from the north Pacific ocean when exposed to experimentally elevated CO2 levels. We suggest that the progressive increase in atmospheric CO2 concentrations may therefore slow down the production of calcium carbonate in the surface ocean. As the process of calcification releases CO2 to the atmosphere, the response observed here could potentially act as a negative feedback on atmospheric CO2 levels.

Description: The formation of calcareous skeletons by marine planktonic organisms and their subsequent sinking to depth generates a continuous rain of calcium carbonate to the deep ocean and underlying sediments1. This is important in regulating marine carbon cycling and ocean-atmosphere CO2 exchange2. The present rise in atmospheric CO2 levels3 causes significant changes in surface ocean pH and carbonate chemistry4. Such changes have been shown to slow down calcification in corals and coralline macroalgae5,6, but the majority of marine calcification occurs in planktonic organisms. Here we report reduced calcite production at increased CO2 concentrations in monospecific cultures of two dominant marine calcifying phytoplankton species, the coccolithophorids Emiliania huxleyi and Gephyrocapsa oceanica . This was accompanied by an increased proportion of malformed coccoliths and incomplete coccospheres. Diminished calcification led to a reduction in the ratio of calcite precipitation to organic matter production. Similar results were obtained in incubations of natural plankton assemblages from the north Pacific ocean when exposed to experimentally elevated CO2 levels. We suggest that the progressive increase in atmospheric CO2 concentrations may therefore slow down the production of calcium carbonate in the surface ocean. As the process of calcification releases CO2 to the atmosphere, the response observed here could potentially act as a negative feedback on atmospheric CO2 levels.

Description: The role of transparent exopolymer particles (TEP) and dissolved organic carbon (DOC) for organic carbon partitioning under different CO2 conditions was examined during a mesocosm experiment with the coccolithophorid Emiliania huxleyi. We designed 9 outdoor enclosures (similar to11 m(3)) to simulate CO2 concentrations of estimated 'Year 2100' (similar to710 ppm CO2), 'present' (similar to410 ppm CO2) and 'glacial' (similar to190 ppm CO2) environments, and fertilized these with nitrate and phosphate to favor bloom development. Our results showed fundamentally different TEP and DOC dynamics during the bloom. In all mesocosms, TEP concentration increased after nutrient exhaustion and accumulated steadily until the end of the study. TEP concentration was closely related to the abundance of E. huxleyi and accounted for an increase in POC concentration of 35 2 % after the onset of nutrient limitation. The production of TEP normalized to the cell Abundance of E. huxleyi was highest in the Year 2100 treatment. In contrast, DOC concentration exhibited considerable short-term fluctuations throughout the study. In all mesocosms, DOC was neither related to the abundance of E. huxleyi nor to TEP concentration. A statistically significant effect of the CO2 treatment on DOC concentration was not determined. However, during the course of the bloom, DOC concentration increased in 2 of the 3 Year 2100 mesocosms and in 1 of the present mesocosms, but in none of the glacial mesocosms. It is suggested that the observed differences between TEP and DOC were determined by their different bioavailability and that a rapid response of the microbial food web may have obscured CO2 effects on DOC production by autotrophic cells.

Description: We compared the effect of CO2 concentration ([CO2], ranging from ∼5 to ∼34 μmol l−1) at four different photon flux densities (PFD=15, 30, 80 and 150 μmol m−2 s−1) and two light/dark (L/D) cycles (16/8 and 24/0 h) on the coccolithophore Emiliania huxleyi. With increasing [CO2], a decrease in the particulate inorganic carbon to particulate organic carbon (PIC/POC) ratio was observed at all light intensities and L/D cycles tested. The individual response in cellular PIC and POC to [CO2] depended strongly on the PFD. POC production increased with rising [CO2], irrespective of the light intensity, and PIC production decreased with increasing [CO2] at a PFD of 150 μmol m−2 s−1, whereas below this light level it was unaffected by [CO2]. Cell growth rate decreased with decreasing PFD, but was largely independent of ambient [CO2]. The diurnal variation in PIC and POC content, monitored over a 38-h period (16/8 h L/D, PFD=150 μmol m−2 s−1), exceeded the difference in carbon content between cells grown at high (∼29 μmol l−1) and low (∼4 μmol l−1) [CO2]. However, consistent with the results described above, cellular POC content was higher and PIC content lower at high [CO2], compared to the values at low [CO2], and the offset was observed throughout the day. It is suggested that the observed sensitivity of POC production for ambient [CO2] may be of importance in regulating species-specific primary production and species composition

Description: In laboratory experiments with the coccolithophore species Emiliania huxleyi and Gephyrocapsa oceanica the ratio of particulate inorganic carbon (PIC) to particulate organic carbon (POC) production decreased with increasing CO2 concentration ([CO2]). This was due to both reduced PIC and enhanced POC production at elevated [CO2]. Carbon dioxide concentrations covered a range from a pre-industrial level to a value predicted for 2100 according to a "business as usual" anthropogenic CO2 emission scenario. The laboratory results were used to employ a model in which the immediate effect of a decrease in global marine calcification relative to POC production on the potential capacity for oceanic CO2 uptake was simulated. Assuming that overall marine biogenic calcification shows a similar response as obtained for E. huxleyi or G. oceanica in the present study, the model reveals a negative feedback on increasing atmospheric CO2 concentrations due to a decrease in the PIC/POC ratio. The individual response in cellular PIC and POC production of E. huxleyi to [CO2] depended strongly on the light intensity. POC production increased with increasing [CO2], irrespective of the light intensity, and PIC production decreased with increasing [CO2] at a light intensity of 150 μmol m-2 s-1, whereas below this light level it was unaffected by [CO2]. The diurnal variation in PIC and POC content, monitored over 38 h period was larger than the difference in carbon content between cells grown at high and low [CO2]. However, consistent with the results described above, cellular POC content was higher and PIC content was lower at high [CO2], respectively, compared to the values at low [CO2], and the offset was observed throughout the day. It is suggested that the observed sensitivity of POC production for ambient [CO2] may be of direct importance in regulating speciesspecific primary production and species composition. The stable carbon isotope fractionation (εp) of E. huxleyi was examined in relation to CO2 concentration and light conditions in dilute batch cultures. Er was largely independent of ambient [CO2], varying generally by less than 2 ‰ over a range of [CO2] from 5 to 34 μmol r-1. Instantaneous carbon specific growth rates (μc) and light intensity, ranging from 15 to 150 μmol m-2 s-1, positively correlated with εp. This result is inconsistent with theoretical considerations and experimental results obtained under constant light conditions, suggesting an inverse relationship between εp and μ. In the present study the effect of light intensity on εp was stronger than that of μ and thus resulted in a positive relationship between μ and εp. In addition, the light/dark cycle of 16h/8h resulted in significantly lower εp values compared to continuous light. Since the observed offset of about 8 ‰ could not be related to daylength-dependent changes in μc, this implies a direct influence of the irradiance cycle on εp. A comparison between chemostat and batch culture data corrected for the effect of the irradiance cycle on εp, suggests that a discrepancy in the εp response between E. huxleyi batch and nitrate-limited chemostat cultures may not exist and that the relationship between εp and μcl[CO2] in E. huxleyi is non-linear. The findings are best explained by invoking active carbon uptake in E. huxleyi. If representative for the natural environment, these results severely complicate the interpretation of carbon isotope data in geochemical and paleoceanographic applications. Under controlled laboratory conditions the coccolithophore E. huxleyi was grown under non-steady state conditions. Over a period of 20 days a monospecific bloom was followed, in which growth was finally limited by the nitrate concentration. The results indicate that the close correlation between cellular PIC and POC content during exponential growth is not due to an interdependence of the two parameters, but is rather a consequence of a coincident similar production rate. The dissociation of coccolith production from POC production in the stationary phase is caused by a decrease in the POC production, rather than a stimulation of calcification by the absence of a nutrient. The chemical composition of E. huxleyi showed to be highly variable during the progression of the bloom. Next to changes in the cellular PIC and POC content, both POC/PON and POP/POP ratios strongly deviate from Redfield values under N-limitation. Also the proportion of lipids to overall cell carbon is not constant. Observed enhanced alkenone production relative to POC production during exponential growth points to a membrane or some other structural function of alkenones in the cell. The offset in the stable carbon isotopic signal between POC and lipids changes during exponential growth. Furthermore, calculated temperatures using the uk'37 - temperature relationship strongly deviate from the actual temperature during exponential growth. These findings may compromise the use of biomarkers like alkenones as proxies for paleo-CO2 and temperature, and a much better understanding of the relevant physiological factors and processes involved in the build-up of the signal are required for a sensible application for ancient environmental reconstruction.