Biogeochemical response of Emiliania huxleyi (PML B92/11) to elevated CO2 and temperature under phosphorous limitation: A chemostat study
Highlights
► E. huxleyi cell density and POC concentration were higher at elevated CO2 and T. ► Carbon-cell-quotas were similar under P-limitation for all CO2 and T conditions. ► E. huxleyi shows a large plasticity of the P-cell-quota while grown at high CO2. ► Cell sizes were significantly decreased at high CO2 under severe P-stress. ► High C:N:P ratios suggest an enhanced carbon export in the greenhouse ocean.
Introduction
During the anthropocene, atmospheric CO2 increased from a concentration of ~ 280 μatm at the beginning, to 380 μatm in the year 2008, and is predicted to rise further up to 750 μatm (Houghton et al., 2001) or even > 1000 μatm by the end of this century (Meehl et al., 2007, Raupach et al., 2007, Raven et al., 2005). Dissolution of CO2 in the ocean will lead to a lowering of pH in surface waters on the order of 0.5 units over the next 100 years (Caldeira and Wickett, 2003). This acidification of the ocean is projected to be accompanied by an increase in sea surface temperature (SST) ranging between 1.1 and 6.4 °C as a consequence of climate change (Meehl et al., 2007). The increase in temperature will induce complex environmental changes such as surface seawater freshening due to sea-ice-melt, stronger water-column stratification and rising irradiance levels in surface waters (Boyd and Doney, 2002, Sarmiento et al., 2004). Responses of individual plankton species or natural communities to rising pCO2 and temperature have been investigated in several perturbation experiments accomplished with a variety of experimental approaches concerning CO2 manipulation, e.g. addition of HCl/NaOH (Riebesell et al., 2000), or aeration with gas of a defined CO2 concentration (Sciandra et al., 2003), and the type of cultivation, e.g. batch cultures (Iglesias-Rodriguez et al., 2008), mesocosms (Delille et al., 2005, Engel et al., 2005, Riebesell et al., 2007), semi-continuous/dilute batch cultures (Feng et al., 2008, Riebesell et al., 2000) or chemostats (Leonardos and Geider, 2005, Sciandra et al., 2003). These studies indicate that physiological processes such as growth (Feng et al., 2008), primary production (Egge et al., 2009), calcification (Delille et al., 2005), the efficiency and regulation of carbon concentration mechanisms (CCM) (Rost et al., 2003) and the production of extracellular organic matter (Engel, 2002) are affected by changes in pCO2.
Coccolithophores play a major role in the global carbon cycle and are known to be sensitive to rising pCO2 (Paasche, 2002, Thierstein and Young, 2004). The expected changes in the ocean carbonate chemistry will thus likely affect the performance of coccolithophores, and may change global biogeochemical cycling in the future (Gattuso and Buddemeier, 2000). Emiliania huxleyi, a prominent cosmopolitan species of coccolithophores, was investigated in field, batch and chemostat studies under a variety of growth rates and nutrient concentrations. E. huxleyi has a low affinity for CO2 and a low efficient CCM and is therefore suggested to be carbon limited in the present day ocean (Paasche, 2002, Rost et al., 2003). Under nutrient replete conditions, E. huxleyi generally increases photosynthetic rates and concentrations of produced particulate organic carbon (POC) while grown at high pCO2 (Riebesell et al., 2000, Rost et al., 2003, Zondervan et al., 2001). At nitrogen limitation however, elevated pCO2 (700 μatm) led to decreased photosynthetic rates and C cell quotas (Sciandra et al., 2003), while a non-calcifying strain of E. huxleyi grown at phosphorous limitation was found to exhibit higher cellular POC at high pCO2 concentrations of 2000 μatm (Leonardos and Geider, 2005). Continuous culture experiments with E. huxleyi revealed that sole nutrient limitation generally increases cell quotas for POC, especially under phosphorous control (Paasche, 1998, Riegman et al., 2000). With respect to calcification in coccolithophores, increasing pCO2 was found to either decrease (Berry et al., 2002, Riebesell et al., 2000, Rost et al., 2003, Sciandra et al., 2003, Zondervan et al., 2002) or increase the concentration of biogenic calcite produced (Iglesias-Rodriguez et al., 2008) or induce complex responses (Langer et al., 2006, Langer et al., 2009).
In order to better estimate effects of global change on E. huxleyi, and potential consequences for biogeochemical cycling in the ocean, a better understanding of individual and combined effects of pCO2, temperature and growth conditions, and other environmental factors, such as nutrients and light, is required (Engel, 2010, Riebesell et al., 2010, Rost et al., 2008). Therefore, we used a chemostat set-up to address combined pCO2 and temperature effects on E. huxleyi under two controlled levels of phosphorous depletion. We further give a detailed description of the CO2–aeration system used in this study and advocate that pCO2- and temperature-controlled continuous culture facilities are likely to be an important tool for future ocean research. Earlier studies with E. huxleyi (PML B92/11) revealed increased POC production at elevated CO2 (Riebesell et al., 2000) and temperature (Langer et al., 2007) while grown at nutrient replete conditions. In the future, the rise in CO2 and temperature will occur simultaneously and is likely to be accompanied by changed nutrient conditions. Therefore, we tested for the combined effect of elevated CO2 and temperature on inter alia POC production under phosphorous limiting conditions in order to investigate a more realistic greenhouse ocean scenario.
Section snippets
The chemostat
Chemostats allow for the full control of growing conditions during the cultivation of plankton organisms and were originally developed for the investigation of bacterial physiology by Monod, 1950, Novick and Szilard, 1950. In chemostats, the cell yield is controlled by the concentration of nutrients, while the growth rate μ (d− 1) is balanced to the dilution rate D (d− 1), which is defined aswith F (mL d− 1) for the rate of inflow of nutrient media, and V (mL) for total volume of the incubator.
Evaluation of temperature control and CO2 regulation
The pre-tests showed good stability of CO2 concentrations in gas streams obtained by the CO2 regulation system. Concentrations of 200, 370 and 760 μatm CO2 in airstreams were stable over a period of 7 d within standard deviations of ± 0.7, ± 1.7 and ± 1.6%, respectively. Equilibration of gas with preset CO2 concentrations in artificial seawater was reached after a maximum of 60 h, observed by the development of pH (Fig. 4, Table 1). As temperature control was set before slowly filling the incubator
CO2- and temperature-control in chemostats
A simultaneous pCO2 and temperature control to chemostats allows for the investigation of co-effects, i.e. with nutrient availability over a prolonged period of time. In comparison to other cultivation methods, the continuous culture is the only approach that enables the full control of growth at any given rate within minimum and maximum growth rate of investigated organisms. Therefore, chemostats have the great advantage to investigate distinct responses induced by environmental factors, such
Acknowledgements
We thank the AWI-workshop-team of Erich Dunker for technical support, Klaus-Uwe Richter for helpful discussions on the CO2 aeration system and Christiane Lorenzen for assistance during the C/N-analysis. Judith Piontek and Gerald Langer are gratefully acknowledged for supportive discussions improving this manuscript. This research was supported by the Helmholtz Association contract no HZ-NG-102 and contributes to the Belgian Federal Science Policy Office PEACE project (contract no. SD/CS/03A/B).
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2017, Journal of Experimental Marine Biology and EcologyCitation Excerpt :Despite this, very few OA studies report on cell size. For most that do, their reported changes in E. huxleyi volume cannot be uncoupled from concomitant OA-induced changes in population growth rate (e.g. Avgoustidi et al., 2012; Barcelos e Ramos et al., 2010; De Bodt et al., 2010; Fiorini et al., 2011; Gao et al., 2009; Iglesias-Rodriguez et al., 2008; Jones et al., 2013; McCarthy et al., 2012; Müller et al., 2015; Pedrotti et al., 2012), increased organic C, N and P (Rouco et al., 2013), or decoupled from the effect of additional stressors, e.g. phosphorous limitation (Borchard et al., 2011) and temperature (e.g. Arnold et al., 2013; Borchard et al., 2011; De Bodt et al., 2010; Feng et al., 2008). Cell size and other phytoplankton morphological characteristics are generally assumed to have evolved in response to selective pressures involving diffusion limitation of resource supply rates (e.g. Chisholm, 1992; Irwin et al., 2006), and sinking from regions with adequate light for photosynthesis.
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2015, Journal of Experimental Marine Biology and EcologyCitation Excerpt :In five out of six Mediterranean E. huxleyi strains the PIC/POC ratio decreased in response to P-limitation, while one strain showed no change (Oviedo et al., 2014). Although these authors used the batch approach, other studies show that strain-specificity can also be observed in continuous cultures (Borchard et al., 2011; Riegman et al., 2000). There is a fundamental difference between E. huxleyi and C. pelagicus in that the former can overcalcify, producing multiple layers of coccoliths which are shed into the surrounding waters (Holligan et al., 1983; Paasche, 1998), whereas C. pelagicus covers its cell with a single coccolith layer.
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2015, Marine ChemistryCitation Excerpt :PIC and POC cell quotas increased with decreasing growth rate as the amount of calcite per cell increased. Further information on biogeochemical results of the chemostat study are given in Borchard et al. (2011). TEPcolor concentrations are expressed in xanthan gum equivalents per liter (μg XG eq. L− 1) and were normalized to cell abundance (μg XG eq. cell− 1), ranged from below detection to ~ 830 μg XG eq. L− 1 (or b.d. to ~ 3.2 pg XG eq. cell− 1) and were higher at lower growth rate (Table 2).