Beschreibung: 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.

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: Carbon acquisition in relation to CO2 supply was investigated in three marine bloom-forming microalgae, the diatom Skeletonema costatum, the flagellate Phaeocystis globosa, and the coccolithophorid Emiliania huxleyi. In vivo activities of extracellular (eCA) and intracellular (iCA) carbonic anhydrase activity, photosynthetic O2 evolution, CO2 and HCO uptake rates were measured by membrane inlet mass spectrometry in cells acclimated to pCO2 levels of 36, 180, 360, and 1,800 ppmv. Large differences were obtained between species both with regard to the efficiency and regulation of carbon acquisition. While eCA activity increased with decreasing CO2 concentration in S. costatum and P. globosa, consistently low values were obtained for E. huxleyi. No clear trends with pCO2 were observed in iCA activity for any of the species tested. Half saturation concentrations (K1/2) for photosynthetic O2 evolution, which were highest for E. huxleyi and lowest for S. costatum, generally decreased with decreasing CO2 concentration. In contrast, K1/2 values for P. globosa remained unaffected by pCO2 of the incubation. CO2 and HCO3- were taken up simultaneously by all species. The relative contribution of HCO3- to total carbon uptake generally increased with decreasing CO2, yet strongly differed between species. Whereas K1/2 for CO2 and HCO3- uptake was lowest at the lowest pCO2 for S. costatum and E. huxleyi, it did not change as a function of pCO2 in P. globosa. The observed taxon-specific differences in CO2 sensitivity, if representative for the natural environment, suggest that changes in CO2 availability may influence phytoplankton species succession and distribution. By modifying the relative contribution of different functional groups, e.g., diatomaceous versus calcareous phytoplankton, to the overall primary production this could potentially affect marine biogeochemical cycling and air-sea gas exchange.

Beschreibung: To date no study exists that directly addresses changes in dynamics of heterotrophic bacteria in surface waters in relation to partial pressure of CO2 (pCO2). Therefore, we studied the effect of changes in pCO2 on bacterial abundance and activities by using mesocosms with different pCO2 levels (~190, ~370, and ~700 ppmV, representing past, present-day, and future atmospheric pCO2, respectively). Abundance of total bacteria did not differ with increasing pCO2 throughout the whole study period, whereas bacterial protein production (BPP) was highest at highest pCO2. This effect was even more pronounced for cell-specific production rates, especially those of attached bacteria, which were up to 25 times higher than those of free bacteria. During the breakdown of the bloom, however, the abundance of both free and attached bacteria was significantly increased with pCO2. Differences in bacterial growth rate (µ) were smaller than those of BPP, but both µ and BPP of attached bacteria were elevated under high pCO2. Averages of total protease as well as α- and α -glucosidase activities were highest at elevated pCO2 levels, but a statistically significant dependence on pCO2 was only evident for protease activity. There is a measurable but indirect effect of changes in pCO2 on bacterial activities that are mainly linked to phytoplankton and presumably particle dynamics

Beschreibung: Throughout Earth’s history, the ocean has played a crucial role in modulating atmospheric carbon dioxide through a variety of physical, chemical and biological processes. The same processes are involved in the ocean’s response to anthropogenic perturbations of the global carbon cycle. A key process responsible for about three quarters of the surface to deep-ocean gradient in dissolved inorganic carbon (DIC) is the biological carbon pump. This transports carbon bound by photosynthesis from the sunlit surface layer to the deep ocean. Integrated over the global ocean, the bioticallydriven surface to deep-ocean DIC gradient corresponds to a carbon pool 3.5 times larger than the total amount of atmospheric carbon dioxide. Hence, small changes in this pool, for example, caused by biological responses to ocean change, would have a strong affect on atmospheric CO2.

Beschreibung: Diatoms have evolved a multitude of morphologies, including highly elongated cells and cell chains. Elongation and chain formation have many possible functions, such as grazing protection or effects on sinking. Here, a model of diffusive and advective nutrient transport is used to predict impacts of cell shape and chain length on potential nutrient supply and uptake in a turbulent environment. Rigid, contiguous, prolate spheroids thereby represent the shapes of simple chains and solitary cells. Ar scales larger than a few centimeters, turbulent water motions produce a more or less homogeneous nutrient distribution. At the much smaller scale of diatom cells, however, turbulence creates a roughly linear shear and nutrients can locally become strongly depleted because of nutrient uptake by phytoplankton cells. The potential diffusive nutrient supply is greater for elongated than for spherically shaped cells of similar volume but lower for chains than for solitary cells. Although the relative increase in nutrient transport due to turbulence is greater for chains, single cells still enjoy a greater total nutrient supply in turbulent environments, Only chains with specialized structures, such as spaces between the cells, can overcome this disadvantage and even obtain a higher nutrient supply than do solitary cells. The model results are compared to laboratory measurements of nutrient uptake under turbulent conditions and to effects of sinking.

Beschreibung: The formation of large marine snow macroflocs and stringers was monitored at a fixed station in the southern North Sea. During an 18-month investigation, large marine snow floes occurred in the water column on nine occasions for periods lasting between 5 and 16 d. Their formation coincided with both high phytoplankton biomass and low windspeeds. Retention of marine snow in the surface layer was related to slow sinking, neutral buoyancy, and rising of the large floes. Applying a specially developed instrument showed buoyancy to be caused by gas bubbles incorporated in the floes. Marine snow flotation coincided with periods of oxygen supersaturation in the water column, suggesting that oxygen production could be responsible for gas bubble formation within the floes. Disappearance of the large marine snow floes from the surface layer coincided with drastic increases in windspeed, indicating that enhanced wind-induced turbulent mixing in the surface layer could have resulted in the breakup of these floes. Formation and disappearance of marine snow had a marked influence on the light regime in the water.

Beschreibung: A microcosm experiment was conducted to investigate the interactive effects of rising sea-surface temperature and altered nutrient stoichiometry on the biogeochemical cycling of organic matter in a pelagic algal–bacterial assemblage. Natural seawater, containing a mixed bacterial community, was inoculated with an axenic culture of the bloom-forming diatom species Skeletonema costatum. A factorial combination of three temperatures, simulating weak to strong warming as projected for the end of the 21st century, and either nitrogen (N)-replete or -deficient growth conditions were applied. Depending on the type of nutrient limitation, the mixed algal–bacterial communities displayed pronounced differences in the accumulation and microbial utilization of organic matter in response to warming. Under N-deficient conditions, the build-up of organic matter occurred, irrespective of temperature, dominantly in the particulate pool, and only small amounts of dissolved material accumulated. The subsequent bacterial consumption of organic matter was low, as indicated by measurements of bacterial secondary production and extracellular enzyme activities, and remained also largely unaffected by an increase in temperature from 4°C up to 12°C. Contrastingly, warming resulted in a distinct temperature-dependent increase in the accumulation of dissolved organic carbon compounds under N-replete growth conditions. Moreover, rising temperature notably stimulated the bacterial activity, indicating an enhanced flow of organic matter through the microbial loop. These findings suggest that there will be strong shifts in the biogeochemical cycling of organic matter in the upper ocean in response to increased temperature and nutrient loading that will affect pelagic food-web structures and the biological sequestration of organic matter.