Despite the fact that ocean acidification is considered to be especially pronounced in the Southern Ocean, little is known about CO 2 -dependent physiological processes and the interactions of Antarctic phytoplankton key species. We therefore studied the effects of CO 2 partial pressure (P CO 2 ) (16.2, 39.5, and 101.3 Pa) on growth and photosynthetic carbon acquisition in the bloom-forming species Chaetoceros debilis, Pseudo-nitzschia subcurvata, Fragilariopsis kerguelensis, and Phaeocystis antarctica. Using membrane-inlet mass spectrometry, photosynthetic O 2 evolution and inorganic carbon (C i ) fluxes were determined as a function of CO 2 concentration. Only the growth of C. debilis was enhanced under high P CO 2 . Analysis of the carbon concentrating mechanism (CCM) revealed the operation of very efficient CCMs (i.e., high C i affinities) in all species, but there were species-specific differences in CO 2 -dependent regulation of individual CCM components (i.e., CO 2 and HCO { 3 uptake kinetics, carbonic anhydrase activities). Gross CO 2 uptake rates appear to increase with the cell surface area to volume ratios. Species competition experiments with C. debilis and P. subcurvata under different P CO 2 levels confirmed the CO 2 -stimulated growth of C. debilis observed in monospecific incubations, also in the presence of P. subcurvata. Independent of P CO 2 , high initial cell abundances of P. subcurvata led to reduced growth rates of C. debilis. For a better understanding of future changes in phytoplankton communities, CO 2 -sensitive physiological processes need to be identified, but also species interactions must be taken into account because their interplay determines the success of a species.The Southern Ocean (SO) is a high-nutrient lowchlorophyll region. Compared with most other regions of the World oceans, the concentrations of nitrate and phosphate are high. The reason for this phenomenon is that the biological production is limited by the trace metal iron, which is essential for photosynthesis (Martin et al. 1990). Most of the primary production in the SO is achieved by sporadic bloom events, which mainly occur along the continental margins and only extend offshore when iron and other nutrient concentrations are high due to upwelling. These blooms are usually dominated by medium-sized diatoms and the flagellate Phaeocystis antarctica (Smetacek et al. 2004). Light is also a major factor controlling phytoplankton growth and productivity in the SO due to the occurrence of strong and frequent winds, causing pronounced deep mixing and therefore low mean and highly varying light levels (Tilzer et al. 1985). Deeply mixed layers were associated with a predominant occurrence of P. antarctica, while diatoms such Fragilariopsis cylindrus seem to favor shallow mixed layers (Kropuenske et al. 2010).Varying CO 2 concentrations were found to also influence SO phytoplankton assemblages and growth (Tortell et al. 2008b;Feng et al. 2010). During winter time, the presence of sea ice prevents gas exchange betw...

The coccolithophore Emiliania huxleyi (PML B92/11) was grown in batch culture under nitrogen (N) as well as phosphorus (P) limitation. Growth rate, particulate inorganic carbon (PIC), particulate organic carbon (POC), particulate organic nitrogen (PON), and particulate organic phosphorus (POP) production were determined. While PON production decreased by 96% under N-limitation and POP production decreased by 85% under P-limitation, growth rate decreased by 31% under N-and by 26% under P-limitation. POC production increased by a factor of 1.5 under N-limitation and by a factor of 3.3 under P-limitation. PIC production increased by a factor of 1.2 under N-limitation and did not change under P-limitation. It is concluded that the decrease in PON production under N-limitation and the decrease in POP production under P-limitation represent a physiological response of the cells while the increase in particulate carbon production represents a methodological artefact. The latter conclusion is based on a direct comparison of this strain's responses to nutrient limitation in different experimental setups, i.e., batch-, semi-continuous-, and continuous cultures.

This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues.Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. The coccolithophore Calcidiscus leptoporus was grown in batch culture under nitrogen (N) as well as phosphorus (P) limitation. Growth rate, particulate inorganic carbon (PIC), particulate organic carbon (POC), particulate organic nitrogen (PON), and particulate organic phosphorus (POP) production were determined and coccolith morphology was analysed. While PON production decreased by 70% under N-limitation and POP production decreased by 65% under P-limitation, growth rate decreased by 33% under N-as well as P-limitation. POC as well as PIC production (calcification rate) increased by 27% relative to the control under P-limitation, and did not change under N-limitation. Coccolith morphology did not change in response to either P or N limitation. While these findings, supported by a literature survey, suggest that coccolith morphogenesis is not hampered by either P or N limitation, calcification rate might be. The latter conclusion is in apparent contradiction to our data. We discuss the reasons for this inference.

To better understand the impact of ocean acidification (OA) and changes in light availability on Southern Ocean phytoplankton physiology, we investigated the effects of pCO2 (380 and 800 µatm) in combination with low and high irradiance (20 or 50 and 200 µmol photons m−2 s−1) on growth, particulate organic carbon (POC) fixation and photophysiology in the three ecologically relevant species Chaetoceros debilis, Fragilariopsis kerguelensis and Phaeocystis antarctica. Irrespective of the light scenario, neither growth nor POC per cell was stimulated by OA in any of the tested species and the two diatoms even displayed negative responses in growth (e.g. C. debilis) or POC content (e.g. F. kerguelensis) under OA in conjunction with high light. For both diatoms, also maximum quantum yields of photosystem II (Fv/Fm) were decreased under these conditions, indicating lowered photochemical efficiencies. To counteract the negative effects by OA and high light, the two diatoms showed diverging photoacclimation strategies. While cellular chlorophyll a (Chl a) and fucoxanthin contents were enhanced in C. debilis to potentially maximize light absorption, F. kerguelensis exhibited reduced Chl a per cell, increased disconnection of antennae from photosystem II reaction centers and strongly lowered absolute electron transport rates (ETR). The decline in ETRs in F. kerguelensis might be explained in terms of different species‐specific strategies for tuning the available flux of adenosine triphosphate and nicotinamide adenine dinucleotide phosphate. Overall, our results revealed that P. antarctica was more tolerant to OA and changes in irradiance than the two diatoms, which may have important implications for biogeochemical cycling.

1000 µatm by the end of this century (RCP6.0 scenario; IPCC 2014). The dissolution of CO 2 in seawater alters its chemistry by increasing the dissolved CO 2 concentration and lowering pH (called 'ocean acidifi-cation', OA). Since the beginning of the industrial revolution, the ocean has absorbed about a third of the anthropogenic CO 2 emissions. Among the world oceans, the Southern Ocean (SO) sequesters a disproportionally large share of anthropogenic CO 2 , accounting for about 40% of the global oceanic uptake of anthropogenic CO 2 (Sabine et al. 2004, Landschützer et al. 2015). However, in this region the biological sequestration potential is constrained by iron (Fe) input (Martin et al. 1990, Boyd et al. 2007,