Temporal biomass dynamics of an Arctic plankton bloom in response to increasing levels of atmospheric carbon dioxide

Ocean acidification and carbonation, driven by anthropogenic emissions of carbon dioxide (CO2), have been shown to affect a variety of marine organisms and are likely to change ecosystem functioning. High latitudes, especially the Arctic, will be the first to encounter profound changes in carbonate chemistry speciation at a large scale, namely the under-saturation of surface waters with respect to aragonite, a calcium carbonate polymorph produced by several organisms in this region. During a CO2 perturbation study in Kongsfjorden on the west coast of Spitsbergen (Norway), in the framework of the EU-funded project EPOCA, the temporal dynamics of a plankton bloom was followed in nine mesocosms, manipulated for CO2 levels ranging initially from about 185 to 1420 μatm. Dissolved inorganic nutrients were added halfway through the experiment. Autotrophic biomass, as identified by chlorophyll a standing stocks (Chl a), peaked three times in all mesocosms. However, while absolute Chl a concentrations were similar in all mesocosms during the first phase of the experiment, higher autotrophic biomass was measured as high in comparison to low CO2 during the second phase, right after dissolved inorganic nutrient addition. This trend then reversed in the third phase. There were several statistically significant CO2 effects on a variety of parameters measured in certain phases, such as nutrient utilization, standing stocks of particulate organic matter, and phytoplankton species composition. Interestingly, CO2 effects developed slowly but steadily, becoming more and more statistically significant with time. The observed CO2-related shifts in nutrient flow into different phytoplankton groups (mainly dinoflagellates, prasinophytes and haptophytes) could have consequences for future organic matter flow to higher trophic levels and export production, with consequences for ecosystem productivity and atmospheric CO2.publishedVersio

Temperature Modulates Coccolithophorid Sensitivity of Growth, Photosynthesis and Calcification to Increasing Seawater pCO2

Increasing atmospheric CO2 concentrations are expected to impact pelagic ecosystem functioning in the near future by driving ocean warming and acidification. While numerous studies have investigated impacts of rising temperature and seawater acidification on planktonic organisms separately, little is presently known on their combined effects. To test for possible synergistic effects we exposed two coccolithophore species, Emiliania huxleyi and Gephyrocapsa oceanica, to a CO2 gradient ranging from ,0.5–250 mmol kg21 (i.e. ,20–6000 matm pCO2) at three different temperatures (i.e. 10, 15, 20uC for E. huxleyi and 15, 20, 25uC for G. oceanica). Both species showed CO2-dependent optimum-curve responses for growth, photosynthesis and calcification rates at all temperatures. Increased temperature generally enhanced growth and production rates and modified sensitivities of metabolic processes to increasing CO2. CO2 optimum concentrations for growth, calcification, and organic carbon fixation rates were only marginally influenced from low to intermediate temperatures. However, there was a clear optimum shift towards higher CO2 concentrations from intermediate to high temperatures in both species. Our results demonstrate that the CO2 concentration where optimum growth, calcification and carbon fixation rates occur is modulated by temperature. Thus, the response of a coccolithophore strain to ocean acidification at a given temperature can be negative, neutral or positive depending on that strain’s temperature optimum. This emphasizes that the cellular responses of coccolithophores to ocean acidification can only be judged accurately when interpreted in the proper eco-physiological context of a given strain or species. Addressing the synergistic effects of changing carbonate chemistry and temperature is an essential step when assessing the success of coccolithophores in the future ocean

Crassulacean acid metabolism contributes significantly to the in situ carbon budget in a population of the invasive aquatic macrophyte Crassula helmsii

1. The ecophysiological significance of Crassulacean Acid Metabolism (CAM) in the invasive aquatic macrophyte Crassula helmsii (T. Kirk) Cockayne was studied in an English soft-water lake. The extent and the contribution of CAM to the carbon budget was examined in spring (April) and summer (July) along a depth gradient (0.5-2.2 m), covering the growth range of C. helmsii in the lake. 2. Significant in situ CAM activity (30-80 meq kg-1 FW) was present in all specimens, although it decreased with depth and hence correlated with the decline in photon irradiance. Potential CAM activity (60-161 meq kg-1 FW), measured after exposure to low concentrations of CO2 in the day and high concentrations at night, were on average 2.7-times greater than in situ CAM activity. Overall CAM activity increased from April to July, which is consistent with higher potential carbon limitation caused by increased temperature and light availability. 3. CAM activity in C. helmsii appeared to be carbon-limited at night because night-time carbon-fixation increased at elevated, compared to ambient, concentrations of CO2. 4. The high in situ CAM activity in C. helmsii was reflected in the contribution of CAM to the total carbon budget which, independent of depth and season, ranged from 18 to 42 %. The amount of CO2 taken up in the night via CAM was 0.74- to 2.94-times the amount of CO2 lost in respiration, thus emphasizing the importance of CAM in refixation of potentially lost respiratory CO2. 5. The onset of decarboxylation in the morning appeared to be under circadian control as there was a delay of up to 5.5 hours between the start of the light period and a decline in cell acidity level. 6. There was little variation in δ13C content (-21.69-23.49 ‰) with season or depth suggesting, along with the estimated contribution to the carbon-budget, that CAM is highly important for the whole population of C. helmsii. CAM may confer a competitive advantage in relation to growth, which may be one of the reasons for the invasiveness of this species

Physiological response of <i>G. oceanica</i> and <i>E. huxleyi</i> to increasing CO₂ and temperature.

<p>Response of growth, POC production, calcification rates and PIC:POC to increasing CO₂ and temperature of <i>G. oceanica</i> (left, open symbols) and <i>E. huxleyi</i> (right, closed symbols). Horizontal bars indicate change of CO₂ from beginning to end of experiment. In some cases the changes were small and thus appear absent. Shaded areas represent OA relevant ranges (∼280–1000 µatm <i>p</i>CO₂). Note that the investigated CO₂ range (x-axis) is only half as broad for experiments with <i>G.oceanica</i> compared to the one of <i>E.huxleyi</i>.</p

Calcification rate of <i>E. huxleyi</i> in response to elevated CO₂ at different temperatures.

<p>Depending on the growth temperature the rate of calcification can decrease strongly or moderately or even increase with rising CO₂ levels. The “low”, “intermediate” and “high” refers to experimental temperature of 10, 15 and 20°C, respectively. The slope of a tangent at [CO₂] of 18 µmol kg⁻¹ in the 10°C treatment of <i>E. huxleyi</i> is almost 0 which means that the optimum curve has reached a plateau in the OA relevant CO₂ range. At 20°C there is a positive slope which means that cells have not yet reached the optimum CO₂ for calcification at 18 µmol kg⁻¹ in this temperature.</p

Parameters from fit equation (4) for <i>G. oceanica</i>.

<p>Parameters from fit <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0088308#pone.0088308.e004" target="_blank">equation (4</a>) for <i>G. oceanica</i>.</p

Parameters from fit equation (4) for <i>E. huxleyi</i>.

<p>Parameters from fit <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0088308#pone.0088308.e004" target="_blank">equation (4</a>) for <i>E. huxleyi</i>.</p

Effect of light and CO2 on inorganic carbon uptake in the invasive aquatic CAM-plant Crassula helmsii

Crassula helmsii (T. Kirk) Cockayne is an invasive aquatic plant in Europe that can suppress many native species because it can grow at a large range of dissolved inorganic carbon concentrations and light levels. One reason for its ecological success may be the possession of a regulated Crassulacean Acid Metabolism (CAM), which allows aquatic macrophytes to take upCO2 in the night in addition to the daytime. The effect of light andCO2 on the regulation ofCAMand photosynthesis in C. helmsii was investigated to characterise how physiological acclimation may confer this ecological flexibility. After 3 weeks of growth at high light (230 mmol photonm–2 s–1), C. helmsii displayed 2.8 times higher CAM at low compared with highCO2 (22 v. 230 mmolm–3).CAMwas absent in plants grown at low light (23 mmol photonm–2 s–1) at both CO2 concentrations. The observed regulation patterns are consistent with CAM acting as a carbon conserving mechanism. For C. helmsii grown at high light and low CO2, mean photosynthetic rates were relatively high at low concentrations of CO2 and were on average 80 and 102 mmol O2 g–1DWh–1 at CO2 concentrations of 3 and 22 mmolm–3 CO2, which, together with meanfinalpHvalues of 9.01 in thepHdrift, indicate a lowCO2 compensation point (<3 mmolm–3) but do not indicate use of bicarbonate as an additional source of exogenous inorganic carbon. The relatively high photosynthetic rates during the entire daytime were caused by internally derived CAM-CO2 and uptake from the external medium. During decarboxylation, CO2 generated from CAM contributed up to 29% to photosynthesis, whereas over a day the contribution to the carbon balance was 13%. The flexible adjustment of CAM and the ability to maintain photosynthesis at very low external CO2 concentrations, partly by making use of internally generated CO2 via CAM, may contribute to the broad ecological niche of C. helmsii

Crassulacean acid metabolism in the context of other carbon-concentrating mechanisms in freshwater plants: a review

Inorganic carbon can be in short-supply in freshwater relative to that needed by freshwater plants for photosynthesis because of a large external transport limitation coupled with frequent depleted concentrations of CO2 and elevated concentrations of O2. Freshwater plants have evolved a host of avoidance, exploitation and amelioration strategies to cope with the low and variable supply of inorganic carbon in water. Avoidance strategies rely on the spatial variation in CO2 concentrations within and among lakes. Exploitation strategies involve anatomical and morphological features that take advantage of sources of CO2 outside of the water column such as the atmosphere or sediment. Amelioration strategies involve carbon concentrating mechanisms (CCM) based on uptake of bicarbonate, which is widespread, C4-fixation which is infrequent and Crassulacean Acid Metabolism (CAM) which is of intermediate frequency. CAM enables aquatic plants to take up inorganic carbon in the night. Furthermore, daytime inorganic carbon uptake is generally not inhibited and therefore CAM is considered to be a carbon conserving mechanism. CAM in aquatic plants is a plastic mechanism regulated by environmental variables and is generally down-regulated when inorganic carbon does not limit photosynthesis. CAM is regulated in the long term (acclimation during growth), but is also affected by environmental conditions in the short term (response on a daily basis). In aquatic plants CAM appears to be an ecologically important mechanism for increasing inorganic carbon uptake, since the in situ contribution from CAM to the C-budget generally is high (18-55%)