Zondervan, Ingrid (2001) Influence of carbonate chemistry and other environmental factors on the chemical and isotopic composition of coccolithophores, with emphasis on calcification and photosynthetic carbon fixation. (PhD/ Doctoral thesis), Universität Bremen, Bremen, Bremen, Germany, 169 pp.

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Abstract

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.

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