Description: An indoor mesocosm experiment was carried out to investigate the combined effects of ocean acidification and warming on the species composition and biogeochemical element cycling during a winter/spring bloom with a natural phytoplankton assemblage from the Kiel fjord, Germany. The experimental setup consisted of a "Control" (ambient temperature of similar to 4.8 degrees C and similar to 535 +/- 25 mu atm pCO(2)), a "High-CO2" treatment (ambient temperature and initially 1020 +/- 45 mu atm pCO(2)) and a "Greenhouse" treatment (similar to 8.5 degrees C and initially 990 +/- 60 mu atm pCO(2)). Nutrient replete conditions prevailed at the beginning of the experiment and light was provided at in situ levels upon reaching pCO(2) target levels. A diatom-dominated bloom developed in all treatments with Skeletonema costatum as the dominant species but with an increased abundance and biomass contribution of larger diatom species in the Greenhouse treatment. Conditions in the Greenhouse treatment accelerated bloom development with faster utilization of inorganic nutrients and an earlier peak in phytoplankton biomass compared to the Control and High CO2 but no difference in maximum concentration of particulate organic matter (POM) between treatments. Loss of POM in the Greenhouse treatment, however, was twice as high as in the Control and High CO2 treatment at the end of the experiment, most likely due to an increased proportion of larger diatom species in that treatment. We hypothesize that the combination of warming and acidification can induce shifts in diatom species composition with potential feedbacks on biogeochemical element cycling.

Description: Global change leads to a multitude of simultaneous modifications in the marine realm among which shoaling of the upper mixed layer, leading to enhanced surface layer light intensities, as well as increased carbon dioxide (CO2) concentration are some of the most critical environmental alterations for phytoplankton. In this study, we investigated the responses of growth, photosynthetic carbon fixation and calcification of the coccolithophore Gephyrocapsa oceanica to elevated inline image (51 Pa, 105 Pa, and 152 Pa) (1 Pa ≈ 10 μatm) at a variety of light intensities (50–800 μmol photons m−2 s−1). By fitting the light response curve, our results showed that rising inline image reduced the maximum rates for growth, photosynthetic carbon fixation and calcification. Increasing light intensity enhanced the sensitivity of these rate responses to inline image, and shifted the inline image optima toward lower levels. Combining the results of this and a previous study (Sett et al. 2014) on the same strain indicates that both limiting low inline image and inhibiting high inline image levels (this study) induce similar responses, reducing growth, carbon fixation and calcification rates of G. oceanica. At limiting low light intensities the inline image optima for maximum growth, carbon fixation and calcification are shifted toward higher levels. Interacting effects of simultaneously occurring environmental changes, such as increasing light intensity and ocean acidification, need to be considered when trying to assess metabolic rates of marine phytoplankton under future ocean scenarios.

Description: A mesocosm experiment was conducted to investigate the impact of rising fCO2 on the build-up and decline of organic matter during coastal phytoplankton blooms. Five mesocosms (∼38 m³ each) were deployed in the Baltic Sea during spring (2009) and enriched with CO2 to yield a gradient of 355–862 µatm. Mesocosms were nutrient fertilized initially to induce phytoplankton bloom development. Changes in particulate and dissolved organic matter concentrations, including dissolved high-molecular weight (〉1 kDa) combined carbohydrates, dissolved free and combined amino acids as well as transparent exopolymer particles (TEP), were monitored over 21 days together with bacterial abundance, and hydrolytic extracellular enzyme activities. Overall, organic matter followed well-known bloom dynamics in all CO2 treatments alike. At high fCO2, higher ΔPOC:ΔPON during bloom rise, and higher TEP concentrations during bloom peak, suggested preferential accumulation of carbon-rich components. TEP concentration at bloom peak was significantly related to subsequent sedimentation of particulate organic matter. Bacterial abundance increased during the bloom and was highest at high fCO2. We conclude that increasing fCO2 supports production and exudation of carbon-rich components, enhancing particle aggregation and settling, but also providing substrate and attachment sites for bacteria. More labile organic carbon and higher bacterial abundance can increase rates of oxygen consumption and may intensify the already high risk of oxygen depletion in coastal seas in the future.

Description: Thermal reaction norms for growth rates of six Emiliania huxleyi isolates originating from the central Atlantic (Azores, Portugal) and five isolates from the coastal North Atlantic (Bergen, Norway) were assessed. We used the template mode of variation model to decompose variations in growth rates into modes of biological interest: vertical shift, horizontal shift, and generalist–specialist variation. In line with the actual habitat conditions, isolates from Bergen (Bergen population) grew well at lower temperatures, and isolates from the Azores (Azores population) performed better at higher temperatures. The optimum growth temperature of the Azores population was significantly higher than that of the Bergen population. Neutral genetic differentiation was found between populations by microsatellite analysis. These findings indicate that E. huxleyi populations are adapted to local temperature regimes. Next to between-population variation, we also found variation within populations. Genotype-by-environment interactions resulted in the most pronounced phenotypic differences when isolates were exposed to temperatures outside the range they naturally encounter. Variation in thermal reaction norms between and within populations emphasizes the importance of using more than one isolate when studying the consequences of global change on marine phytoplankton. Phenotypic plasticity and standing genetic variation will be important in determining the potential of natural E. huxleyi populations to cope with global climate change.

Description: The coccolithophore Emiliania huxleyi was cultured under a broad range of carbonate chemistry conditions to distinguish the effects of individual carbonate system parameters on growth, primary production, and calcification. In the first experiment, alkalinity was kept constant and the fugacity of CO2 (fCO2) varied from 2 to 600 Pa (1 Pa ≈ 10 µatm). In the second experiment, pH was kept constant (pHfree = 8) with fCO2 varying from 4 to 370 Pa. Results of the constant-alkalinity approach revealed physiological optima for growth, calcification, and organic carbon production at fCO2 values of ∼ 20 Pa, ∼ 40 Pa, and ∼ 80 Pa, respectively. Comparing this with the constant-pH approach showed that growth and organic carbon production increased similarly from low to intermediate CO2 levels but started to diverge towards higher CO2 levels. In the high CO2 range, growth rates and organic carbon production decreased steadily with declining pH at constant alkalinity while remaining consistently higher at constant pH. This suggests that growth and organic carbon production rates are directly related to CO2 at low (sub-saturating) concentrations, whereas towards higher CO2 levels they are adversely affected by the associated decrease in pH. A pH dependence at high fCO2 is also indicated for calcification rates, while the key carbonate system parameter determining calcification at low fCO2 remains unclear. These results imply that key metabolic processes in coccolithophores have their optima at different carbonate chemistry conditions and are influenced by different parameters of the carbonate system at both sides of the optimum.

Description: It is currently under debate whether organisms that regulate their acid–base status under environmental hypercapnia demand additional energy. This could impair animal fitness, but might be compensated for via increased ingestion rates when food is available. No data are yet available for dominant Calanus spp. from boreal and Arctic waters. To fill this gap, we incubated Calanus glacialis at 390, 1120, and 3000 µatm for 16 d with Thalassiosira weissflogii (diatom) as food source on-board RV Polarstern in Fram Strait in 2012. Every 4 d copepods were subsampled from all CO2 treatments and clearance and ingestion rates were determined. During the SOPRAN mesocosm experiment in Bergen, Norway, 2011, we weekly collected Calanus finmarchicus from mesocosms initially adjusted to 390 and 3000 µatm CO2 and measured grazing at low and high pCO2. In addition, copepods were deep frozen for body mass analyses. Elevated pCO2 did not directly affect grazing activities and body mass, suggesting that the copepods did not have additional energy demands for coping with acidification, neither during long-term exposure nor after immediate changes in pCO2. Shifts in seawater pH thus do not seem to challenge these copepod species.

Description: Environmental context. Approximately 25 % of CO2 released to the atmosphere by human activities has been absorbed by the oceans, resulting in ocean acidification. We investigate the acidification effects on marine phytoplankton and subsequent production of the trace gas dimethylsulfide, a major route for sulfur transfer from the oceans to the atmosphere. Increasing surface water CO2 partial pressure (pCO2) affects the growth of phytoplankton groups to different degrees, resulting in varying responses in community production of dimethylsulfide. Abstract. The human-induced rise in atmospheric carbon dioxide since the industrial revolution has led to increasing oceanic carbon uptake and changes in seawater carbonate chemistry, resulting in lowering of surface water pH. In this study we investigated the effect of increasing CO2 partial pressure (pCO2) on concentrations of volatile biogenic dimethylsulfide (DMS) and its precursor dimethylsulfoniopropionate (DMSP), through monoculture studies and community pCO2 perturbation. DMS is a climatically important gas produced by many marine algae: it transfers sulfur into the atmosphere and is a major influence on biogeochemical climate regulation through breakdown to sulfate and formation of subsequent cloud condensation nuclei (CCN). Overall, production of DMS and DMSP by the coccolithophore Emiliania huxleyi strain RCC1229 was unaffected by growth at 900 μatm pCO2, but DMSP production normalised to cell volume was 12 % lower at the higher pCO2 treatment. These cultures were compared with community DMS and DMSP production during an elevated pCO2 mesocosm experiment with the aim of studying E. huxleyi in the natural environment. Results contrasted with the culture experiments and showed reductions in community DMS and DMSP concentrations of up to 60 and 32 % respectively at pCO2 up to 3000 μatm, with changes attributed to poorer growth of DMSP-producing nanophytoplankton species, including E. huxleyi, and potentially increased microbial consumption of DMS and dissolved DMSP at higher pCO2. DMS and DMSP production differences between culture and community likely arise from pH affecting the inter-species responses between microbial producers and consumers.