Key words: acid-base metabolism, BCECF (2¢,7¢-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein), bicarbonate transport, dissolved inorganic carbon (DIC), DIDS (4,4¢-diisothiocyanatostilbene-2,2¢-disulfonic acid), Emiliania huxleyi, nigericin, pH homeostasis. Summary• To understand the influence of changing surface ocean pH and carbonate chemistry on the coccolithophore Emiliania huxleyi, it is necessary to characterize mechanisms involved in pH homeostasis and ion transport.• Here, we measured effects of changes in seawater carbonate chemistry on the fluorescence emission ratio of BCECF (2¢,7¢-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein) as a measure of intracellular pH (pH i ). Out of equilibrium solutions were used to differentiate between membrane permeation pathways for H + , CO 2 and HCO 3) .• Changes in fluorescence ratio were calibrated in single cells, resulting in a ratio change of 0.78 per pH i unit. pH i acutely followed the pH of seawater (pH e ) in a linear fashion between pH e values of 6.5 and 9 with a slope of 0.44 per pH e unit. pH i was nearly insensitive to changes in seawater CO 2 at constant pH e and HCO 3 ) . An increase in extracellular HCO 3 ) resulted in a slight intracellular acidification. In the presence of DIDS (4,4¢-diisothiocyanatostilbene-2,2¢-disulfonic acid), a broad-spectrum inhibitor of anion exchangers, E. huxleyi acidified irreversibly. DIDS slightly reduced the effect of pH e on pH i .• The data for the first time show the occurrence of a proton permeation pathway in E. huxleyi plasma membrane. pH i homeostasis involves a DIDS-sensitive mechanism.

Abstract. Microzooplankton grazing and algae growth responses to increasing pCO 2 levels (350, 700 and 1050 µatm) were investigated in nitrate and phosphate fertilized mesocosms during the PeECE III experiment 2005. Grazing and growth rates were estimated by the dilution technique combined with taxon specific HPLC pigment analysis. Microzooplankton composition was determined by light microscopy. Despite a range of up to 3 times the present CO 2 levels, there were no clear differences in any measured parameter between the different CO 2 treatments. During days 3-9 of the experiment the algae community standing stock, measured as chlorophyll a (Chl-a), showed the highest instantaneous grow rates (k=0.37-0.99 d −1 ) and increased from ca. 2-3 to 6-12 µg l −1 , in all mesocosms. Afterwards the phytoplankton standing stock decreased in all mesocosms until the end of the experiment. The microzooplankton standing stock, that was mainly constituted by dinoflagellates and ciliates, varied between 23 and 130 µg C l −1 (corresponding to 1.9 and 10.8 µmol C l −1 ), peaking on day 13-15, apparently responding to the phytoplankton development. Instantaneous Chl-a growth rates were generally higher than the grazing rates, indicating only a limited overall effect of microzooplankton grazing on the most dominant phytoplankton. Diatoms and prymnesiophytes were significantly grazed (12-43% of the standing stock d −1 ) only in the prebloom phase when they were in low numbers, and in the post-bloom phase when they were already affected by low Correspondence to: K. Suffrian

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Abstract. Within the frame of the Pelagic Ecosystem CO2 Enrichment (PeECE III) experiment, reproduction and feeding of the copepod Calanus finmarchicus was monitored in relation to phytoplankton development in two mesocosms, at present 1× (350 μatm) and ca 3× present (1050 μatm) CO2 concentrations, respectively. Both mesocosms showed rapid phytoplankton growth after the initial nutrient additions and reached maximum chlorophyll (Chl) a concentrations around day 10. Flow-cytometry and specific pigment analysis (HPLC-CHEMTAX), showed that diatoms and prymnesiophyceae (Emiliania huxleyi (Ehux) and other nanoplankton) dominated the biomass. Feeding and egg production rates of C. finmarchicus developed similarly in both mesocosms, and were positively correlated with Chla, Ehux, diatom and prymnesiophyceae concentrations. Although the total number of copepod nauplii recruited during the experiment was similar in 1× and 3×, significantly less nauplii were recruited in 3× during the peak of the bloom compared to in 1×. We conclude that the algae responsible for the higher biomass in 3× during the peak of the bloom (diatoms and Ehux), may have been relatively inferior food for C. finmarchicus naupliar recruitment, possibly due to a high C:N ratio (>8). Nevertheless, the 3 fold increase in CO2 concentration did not show any clear overall effect on bulk phytoplankton or zooplankton development over the whole experiment, suggesting a more complex coupling between increased CO2 and the nutritional status of the system.

Abstract. Microzooplankton grazing and algae growth responses to increasing pCO2 levels (350, 700 and 1050 μatm) were investigated in nitrate and phosphate fertilized mesocosms during the PeECE III experiment 2005. Grazing and growth rates were estimated by the dilution technique combined with taxon specific HPLC pigment analysis. Phytoplankton and microzooplankton composition were determined by light microscopy. Despite a range up to 3 times the present CO2 levels, there were no clear differences in any measured parameter between the different CO2 treatments. Thus, during the first 9 days of the experiment the algae community standing stock (SS), measured as chlorophyll a (Chl a), showed the highest instantaneous grow rates (0.02–0.99 d-1) and increased from ca 2–3 to 6–12 μg l−1, in all mesocosms. Afterwards the phytoplankton SS decreased in all mesocosms until the end of the experiment. The microzooplankton SS, that was mainly dinoflagellates and ciliates varied between 23 and 130 μg C l−1, peaking on day 13–15, apparently responding to the phytoplankton development. Instantaneous Chl a growth rates were generally higher than the grazing rates, indicating only a limited overall effect of microzooplankton grazing on the most dominant phytoplankton. Diatoms and prymnesiophytes were significantly grazed (14–43% of the SS d-1) only in the pre-bloom phase when they were in low numbers and in the post-bloom phase when they were already limited by low nutrients and/or virus lysis. The cyanobacteria populations appeared more effected by microzooplankton grazing, generally removing 20–65% of the SS d−1.