CO2 perturbation experiments: similarities and differences between dissolved inorganic carbon and total alkalinity manipulations

Increasing atmospheric carbon dioxide (CO2) through human activities and invasion of anthropogenic CO2 into the surface ocean alters the seawater carbonate chemistry, increasing CO2 and bicarbonate (HCO3−) at the expense of carbonate ion (CO32−) concentrations. This redistribution in the dissolved inorganic carbon (DIC) pool decreases pH and carbonate saturation state (Ω). Several components of the carbonate system are considered potential key variables influencing for instance calcium carbonate precipitation in marine calcifiers such as coccolithophores, foraminifera, corals, mollusks and echinoderms. Unravelling the sensitivities of marine organisms and ecosystems to CO2 induced ocean acidification (OA) requires well-controlled experimental setups and accurate carbonate system manipulations. Here we describe and analyse the chemical changes involved in the two basic approaches for carbonate chemistry manipulation, i.e. changing DIC at constant total alkalinity (TA) and changing TA at constant DIC. Furthermore, we briefly introduce several methods to experimentally manipulate DIC and TA. Finally, we examine responses obtained with both approaches using published results for the coccolithophore Emiliania huxleyi. We conclude that under most experimental conditions in the context of ocean acidification DIC and TA manipulations yield similar changes in all parameters of the carbonate system, which implies direct comparability of data obtained with the two basic approaches for CO2 perturbation

Short-term response of the coccolithophore Emiliania huxleyi to an abrupt change in seawater carbon dioxide concentrations

The response of the coccolithophore Emiliania huxleyi to rising CO2 concentrations is well documented for acclimated cultures where cells are exposed to the CO2 treatments for several generations prior to the experiment. The exact number of generations required for acclimation to CO2-induced changes in seawater carbonate chemistry, however, is unknown. Here we show that Emiliania huxleyi's short-term response (26 h) after cultures (grown at 500 μatm) were abruptly exposed to changed CO2 concentrations (~190, 410, 800 and 1500 μatm) is similar to that obtained with acclimated cultures under comparable conditions in earlier studies. Most importantly, from the lower CO2 levels (190 and 410 μatm) to 750 and 1500 μatm calcification decreased and organic carbon fixation increased within the first 8 to 14 h after exposing the cultures to changes in carbonate chemistry. This suggests that Emiliania huxleyi rapidly alters the rates of essential metabolical processes in response to changes in seawater carbonate chemistry, establishing a new physiological "state" (acclimation) within a matter of hours. If this relatively rapid response applies to other phytoplankton species, it may simplify interpretation of studies with natural communities (e.g. mesocosm studies and ship-board incubations), where often it is not feasible to allow for a pre-conditioning phase before starting experimental incubations

Einfluß von Kohlendioxid und Licht auf ausgewählte Arten des marinen Phytoplanktons

Atmospheric carbon dioxide (CO2) concentrations have been increasing since the industrial revolution and are expected to almost triple from pre-industrial values by the year 2100. CO2 enters the ocean by atmosphere-surface ocean gas exchange, decreasing carbonate ion (CO32-) concentrations and pH (ocean acidification). Additionally, the rise of CO2 concentrations and other green-house gases in the atmosphere, increase global average temperatures in the air and, consequently, in the surface ocean. This strengthens thermal stratification, decreasing mixed layer depth and changing light availability. The overarching goal of this thesis was to investigate the effects of global change, namely of increasing CO2 and light, on selected species of marine phytoplankton (cyanobacteria, coccolithophores and diatoms). At first, the focus was put on investigating the response of the evolutionarily oldest phytoplankton group (cyanobacteria) to changing CO2 concentrations. From further work with cyanobacteria during mesocosm experiments in the Baltic Sea emerged the idea of determining the time necessary for phytoplankton cells to acclimate to the changed conditions. To assess this question the best studied phytoplankton species in the context of changing carbonate chemistry, Emiliania huxleyi, was chosen. Other cellular adjustments of high interest in a future ocean are those to sudden light variation, since organisms might be exposed to an average higher light intensity and more frequent high light events. It could be possible that diatoms such as Phaeodactylum tricornutum and coccolithophores like Emiliania huxleyi differ in their ability to cope with these changes. Nitrogen fixing cyanobacteria (diazotrophs) are responsible for the input of new nitrogen into large areas of the ocean. Until the beginning of this thesis it was unknown whether and how they would respond to the expected changes in the ocean’s carbonate chemistry. The important non-heterocystous diazotroph Trichodesmium strongly responded to rising CO2 from 140 to 750 μatm, increasing cell division rate, nitrogen fixation rate per unit of phosphorus utilization and carbon fixation (publication I). In the heterocystous Nodularia spumigena (co-authored manuscript IV) and Anabaena sp. (co-authored manuscript V), however, nitrogen fixation was found to decrease with increasing CO2, potentially resulting from decreasing pH and not the CO2 concentration itself. Together these results hint to fundamental differences between heterocystous and non-heterocystous cyanobacteria. Hence, depending on their distribution, some regions could see increasing nitrogen fixation in the future while others not. This could influence regional primary productivity and possibly carbon sequestration. Globally the distribution and abundance of non-heterocystous cyanobacteria in the oceans is more significant than that of heterocystous species, so the overall feedback will be determined by the former. While the effect of increasing CO2 concentrations on cyanobacteria only started to be analyzed very recently, other phytoplankton groups, especially coccolithophores, have already been considered for a longer period of time. Often studies were performed after the cells were pre-exposed to experimental CO2 concentrations for about 9 to 12 generations. However, it is unknown how much time is actually required for cells to reach a new physiological “equilibrium” (acclimation). Hence, the frequently studied Emiliania huxleyi was exposed to abrupt variations in carbonate chemistry and its response followed over 26 hours. Cells acclimated within about 8 hours, increasing photosynthesis and decreasing calcification under elevated CO2 concentrations, similar to cells pre-exposed to those conditions (manuscript II). If such a rapid acclimation is a general phenomenon within phytoplankton species it simplifies the interpretation of short-term results for several experimental setups such as mesocosm and ship-board incubations, since in these situations it is not feasible to pre-expose the communities to the experimental conditions. As stated above, the expected rise of CO2 concentrations might indirectly change the light supply for plankton. Phytoplankton species respond differently to dramatic (abrupt and strong) changes in light intensity, influencing their competitive fitness for a certain ecological niche. Exactly how cells photoacclimate to light changes and whether there are differences between species is still not completely clear. Emiliania huxleyi and Phaeodactylum tricornutum dissipated extra energy after an abrupt rise in light intensity as heat, fluorescence and photochemistry (manuscript III). However, there were differences between the species in both magnitude and timing of their individual responses. Additionally, the coccolithophore was found to use an additional dissipation valve, calcification. Species-specific responses to dramatic increases in light intensity as those found here might be important defining competitive fitness and therefore, community composition. Results of this doctoral thesis point out the importance of the response of diazotrophs in marine feedbacks to global change. The strong increase in nitrogen fixation with rising CO2 observed for the globally important Trichodesmium might provide a negative feedback, depending on the magnitude of the effect on other changing factors, such as temperature and light. Moreover, changes in light intensity might influence community composition due to species-specific differences in response time, with potential consequences for the biological carbon pump.Die Konzentration von Kohlendioxid (CO2) in der Atmosphäre steigt seit Beginn der industriellen Revolution kontinuierlich an und wird sich Prognosen zu Folge bis zum Jahr 2100 fast verdreifachen. Durch Gasaustausch zwischen Atmosphäre und Ozean gelangt CO2 auch in den Oberflächenozean, wo es die Konzentrationen von Karbonationen (CO32-) und den pH Wert verringert - ein Vorgang, der als Ozeanversauerung bezeichnet wird. Desweiteren steigen durch die Zunahme von CO2 und anderen Treibhausgasen in der Atmosphäre auch globale Mitteltemperaturen über Land und im Oberflächenozean. Dies verstärkt die Dichteschichtung der oberen Wassermassen und reduziert damit die Tiefe der durchmischten Schicht und verändert somit die Verfügbarkeit von Licht. Ziel dieser Arbeit war es nun, die Auswirkungen des globalen Klimawandels, im Besonderen den Anstieg von CO2 und Lichtverfügbarkeit, auf ausgewählte Arten des marinen Phytoplanktons (Cyanobakterien, Coccolithophoriden und Diatomeen) zu untersuchen. Dazu wurde als erstes der Einfluß von CO2 auf die älteste der drei Gruppen, die Cyanobakterien, untersucht. In weiterführenden Mesokosmosexperimenten mit Cyanobakterien aus der Ostsee wurde dann die Idee geboren, die tatsächliche Zeit zu bestimmen, welche Phytoplankton braucht, um sich an neue, veränderte Bedinungen anzupassen. Für diese Fragestellung wurde eine der im Kontext von Ozeanversauerung am häufigsten untersuchten Arten, die Coccolithophoride Emiliania huxleyi, ausgewählt. Neben der Anpassung an die CO2 Verfügbarkeit ist auch die an variable Lichtbedingungen von großer Bedeutung. Da im Ozean der Zukunft ausgeprägtere Dichteschichtungen und verringerte Durchmischungstiefen erwartet werden, wäre es denkbar, daß die Lebewesen in der euphotischen Zone höheren Lichtintensitäten und Lichtvariationen ausgesetzt würden. Darauf könnten etwa Diatomeen wie Phaeodactylum tricornutum und Coccolithophoriden wie Emiliania huxleyi unterschiedlich reagieren, da sie eventuell unterschiedlich gut mit diesen Änderungen zu Recht kommen. In weiten Bereichen des Ozeans ist der von Cyanobakterien fixierte atmosphärische Stickstoff ein wichtiger Nährstoff im marinen Nahrungsnetz. Ob und wie diese Schlüsselorganismen auf zu erwartende Änderungen der Seewasserkarbonatchemie reagieren werden, war bis zu Beginn dieser Doktorarbeit jedoch vollkommen unklar. Hier konnte nun gezeigt werden, daß mit zunehmenden CO2 Konzentrationen im Meerwasser (von 140 auf 750 μatm) die Zellteilungs-, und die Phosphat normalisierten Stickstoff und Kohlenstoff Fixierungsraten eines wichtigen Stickstoff Fixierers, Trichodesmium sp., deutlich ansteigen (Publikation I). In den Heterocysten bildenden Arten Nodularia spumigena (Co-Autorenschaft IV) und Anabaena sp. (Co-Autorenschaft V) hingegen nahmen die Stickstoffixierungsraten jedoch tendenziell ab. Es scheint also in dieser Hinsicht fundamentale Unterschiede zwischen Heterocysten bildenden Cyanobakterien und solchen ohne Heterocysten zu geben. Je nach Vorkommen könnte so in der Zukunft die Stickstoffixierung in einigen Regionen zunehmen, in anderen jedoch abnehmen. Das könnte sich dann auch auf die Fähigkeit des Ozeans Kohlenstoff aufzunehmen auswirken. Während die CO2 bedingten Effekte auf Stickstoff fixierende Cyanobakterien erst am Anfang ihrer Aufklärung stehen, so zieht eine andere Phytoplanktongruppe schon seit längerem reges Interesse auf sich. Zahlreiche Studien wurden an Coccolithophoriden durchgeführt, viele von ihnen an Kulturen, in denen die Zellen für etwa zwölf Generationen an die neuen CO2 Bedingungen akklimiert wurden. Es ist jedoch unklar wie viel Zeit tatsächlich erforderlich ist, bis die Zellen ein neues physiologisches Equilibrium erreichen und somit akklimiert sind. Deshalb wurde die gut untersuchte Coccolithophoride Emiliania huxleyi einer abrupten Änderung der CO2 Konzentration ausgestzt und ihre Reaktion darauf 26 Stunden lang verfolgt (Manuskript II). Nach nur etwa acht Stunden unter erhöhten CO2 Bedingungen stieg die organische Kohlenstoffixierung an während zeitgleich die anorganische absank. Dies ist die typische Reaktion, welche für längere Zeit akklimierte Zellen charakteristisch ist. Daß Akklimationszeiten auch in anderen Phytoplanktonarten derart schnell verlaufen, ist eine wichtige Vorraussetzung für zahlreiche Experimente wie zum Beispiel solche mit natürlichen Planktongemeinschaften in Mesokosmen oder Deck-Inkubationen, bei denen es unmöglich ist, eine Akklimationsphase vor Beginn des eigentlichen Experimentes durchzuführen. Neben direkten CO2 Effekten wird Phytoplankton zukünftig auch indirekten ausgesetzt sein. Der prognostizierte Anstieg atmosphärischer CO2 Konzentrationen wird als Folge wärmere Temperaturen im globalen Mittel mit sich bringen, welche die Dichteschichtung im Oberflächenozean verstärken und damit die durchmischte Schicht reduzieren könnten. Dies würde die Lichtverfügbarkeit für das marine Phytoplankton verändern. Unterschiedliche Arten sind unterschiedlich gut an variable Lichtverhältnisse angepaßt, was ihre kompetitive Fitness für ihre spezifische ökologische Nische beeinflußt. Wie genau Photoakklimation in Reaktion auf abrupte Änderungen in der Lichtintensität bei verschiedenen Arten abläuft ist noch nicht eindeutig charakterisiert worden. Hier konnte nun gezeigt werden, daß die Coccolithophoride Emiliania huxleyi und die Diatomee Phaeodactylum tricornutum der zusätzlichen Energie einer abrupten Lichtintensitätserhöhung teilweise durch eine Reduktion von Lichtsammel- und verstärkter Produktion von Lichtschutzpigmenten begegneten (Manuskript III). Unterschiede zwischen den beiden untersuchten Arten zeigten sich hingegen hinsichtlich der Reaktionsgeschwindigkeit und der tatsächlichen Nutzung der zusätzlich verfügbaren Energie. Neben verstärkter Fixierung von Kohlenstoff in organische Materie bei beiden Arten, stieg auch die in anorganische durch Kalzifizierung bei Emiliania huxleyi an. Somit fungierte die Kalzifizierung als zusätzliche Energiesenke. In Bezug auf marine Rückkopplungsprozess im Klimasystem Erde heben die hier erzielten Ergebnisse die Bedeutung von Stickstoff fixierenden Cyanobakterien deutlich hervor. Der prominente Astieg der Fixierungsraten der weit verbreiteten Art Trichodesmium mit zunehmenden CO2 Konzentrationen könnte so einen negativen Rückkopplungsmeschanismus auf atmosphärisches CO2 darstellen. Hier sind natürlich mögliche weitere Effekte, hervorgerufen durch Temperatur- und Lichtveränderungen, noch zu berücksichtigen. Schließlich könnten letztere die Zusammensetzung der Planktongemeinschaft durch Art spezifische Unterschiede in zellulären Reaktionszeiten beeinflussen. Es ist nicht auszuschließen, daß dies dann Auswirkungen auf die biologische Kohlenstoffpumpe hat

Effect of rising atmospheric carbon dioxide on the marine nitrogen fixer Trichodesmium

Diazotrophic (N2-fixing) cyanobacteria provide the biological source of new nitrogen for large parts of the ocean. However, little is known about their sensitivity to global change. Here we show that the single most important nitrogen fixer in today's ocean, Trichodesmium, is strongly affected by changes in CO2 concentrations. Cell division rate doubled with rising CO2 (glacial to projected year 2100 levels) prompting lower carbon, nitrogen and phosphorus cellular contents, and reduced cell dimensions. N2 fixation rates per unit of phosphorus utilization as well as C:P and N:P ratios more than doubled at high CO2, with no change in C:N ratios. This could enhance the productivity of N-limited oligotrophic oceans, drive some of these areas into P limitation, and increase biological carbon sequestration in the ocean. The observed CO2 sensitivity of Trichodesmium could thereby provide a strong negative feedback to atmospheric CO2 increase

Effects of Increasing Seawater Carbon Dioxide Concentrations on Chain Formation of the Diatom Asterionellopsis glacialis

Diatoms can occur as single cells or as chain-forming aggregates. These two strategies affect buoyancy, predator evasion, light absorption and nutrient uptake. Adjacent cells in chains establish connections through various processes that determine strength and flexibility of the bonds, and at distinct cellular locations defining colony structure. Chain length has been found to vary with temperature and nutrient availability as well as being positively correlated with growth rate. However, the potential effect of enhanced carbon dioxide (CO2) concentrations and consequent changes in seawater carbonate chemistry on chain formation is virtually unknown. Here we report on experiments with semi-continuous cultures of the freshly isolated diatom Asterionellopsis glacialis grown under increasing CO2 levels ranging from 320 to 3400 mu atm. We show that the number of cells comprising a chain, and therefore chain length, increases with rising CO2 concentrations. We also demonstrate that while cell division rate changes with CO2 concentrations, carbon, nitrogen and phosphorus cellular quotas vary proportionally, evident by unchanged organic matter ratios. Finally, beyond the optimum CO2 concentration for growth, carbon allocation changes from cellular storage to increased exudation of dissolved organic carbon. The observed structural adjustment in colony size could enable growth at high CO2 levels, since longer, spiral-shaped chains are likely to create microclimates with higher pH during the light period. Moreover increased chain length of Asterionellopsis glacialis may influence buoyancy and, consequently, affect competitive fitness as well as sinking rates. This would potentially impact the delicate balance between the microbial loop and export of organic matter, with consequences for atmospheric carbon dioxide

Phytoplankton calcification as an effective mechanism to prevent cellular calcium poisoning

Marine phytoplankton have developed the remarkable ability to tightly regulate the concentration of free calcium ions in the intracellular cytosol at a level of ~ 0.1 μmol L−1 in the presence of seawater Ca2+ concentrations of 10 mmol L−1. The low cytosolic calcium ion concentration is of utmost importance for proper cell signalling function. While the regulatory mechanisms responsible for the tight control of intracellular Ca2+ concentration are not completely understood, phytoplankton taxonomic groups appear to have evolved different strategies, which may affect their ability to cope with changes in seawater Ca2+ concentrations in their environment on geological timescales. For example, the Cretaceous (145 to 66 Ma), an era known for the high abundance of coccolithophores and the production of enormous calcium carbonate deposits, exhibited seawater calcium concentrations up to 4 times present-day levels. We show that calcifying coccolithophore species (Emiliania huxleyi, Gephyrocapsa oceanica and Coccolithus braarudii) are able to maintain their relative fitness (in terms of growth rate and photosynthesis) at simulated Cretaceous seawater calcium concentrations, whereas these rates are severely reduced under these conditions in some non-calcareous phytoplankton species (Chaetoceros sp., Ceratoneis closterium and Heterosigma akashiwo). Most notably, this also applies to a non-calcifying strain of E. huxleyi which displays a calcium sensitivity similar to the non-calcareous species. We hypothesize that the process of calcification in coccolithophores provides an efficient mechanism to alleviate cellular calcium poisoning and thereby offered a potential key evolutionary advantage, responsible for the proliferation of coccolithophores during times of high seawater calcium concentrations. The exact function of calcification and the reason behind the highly ornate physical structures of coccoliths remain elusive

Influence of temperature and CO₂ on the strontium and magnesium composition of coccolithophore calcite

Marine calcareous sediments provide a fundamental basis for paleoceanographic studies aiming to reconstruct past oceanic conditions and understand key biogeochemical element cycles. Calcifying unicellular phytoplankton (coccolithophores) are a major contributor to both carbon and calcium cycling by photosynthesis and the production of calcite (coccoliths) in the euphotic zone and the subsequent long-term deposition and burial into marine sediments. Here we present data from controlled laboratory experiments on four coccolithophore species and elucidate the relation between the divalent cation (Sr, Mg and Ca) partitioning in coccoliths and cellular physiology (growth, calcification and photosynthesis). Coccolithophores were cultured under different seawater temperature and carbonate chemistry conditions. The partition coefficient of strontium (DSr) was positively correlated with both carbon dioxide (pCO2) and temperature but displayed no coherent relation to particulate organic and inorganic carbon production rates. Furthermore, DSr correlated positively with cellular growth rates when driven by temperature but no correlation was present when changes in growth rates were pCO2-induced. The results demonstrate the complex interaction between environmental forcing and physiological control on the strontium partitioning in coccolithophore calcite. The partition coefficient of magnesium (DMg) displayed species-specific differences and elevated values under nutrient limitation. No conclusive correlation between coccolith DMg and temperature was observed but pCO2 induced a rising trend in coccolith DMg. Interestingly, the best correlation was found between coccolith DMg and chlorophyll a production suggesting that chlorophyll a and calcite associated Mg originate from the same intracellular pool. These results give an extended insight into the driving factors that lead to variations in the coccolith Mg / Ca ratio and can be used for Sr / Ca and Mg / Ca paleoproxy calibration

Simulated 21st century's increase in oceanic suboxia by CO2-enhanced biotic carbon export

The primary impacts of anthropogenic CO2 emissions on marine biogeochemical cycles predicted so far include ocean acidification, global warming induced shifts in biogeographical provinces, and a possible negative feedback on atmospheric CO2 levels by CO2‐fertilized biological production. Here we report a new potentially significant impact on the oxygen‐minimum zones of the tropical oceans. Using a model of global climate, ocean circulation, and biogeochemical cycling, we extrapolate mesocosm‐derived experimental findings of a pCO2‐sensitive increase in biotic carbon‐to‐nitrogen drawdown to the global ocean. For a simulation run from the onset of the industrial revolution until A.D. 2100 under a “business‐as‐usual” scenario for anthropogenic CO2 emissions, our model predicts a negative feedback on atmospheric CO2 levels, which amounts to 34 Gt C by the end of this century. While this represents a small alteration of the anthropogenic perturbation of the carbon cycle, the model results reveal a dramatic 50% increase in the suboxic water volume by the end of this century in response to the respiration of excess organic carbon formed at higher CO2 levels. This is a significant expansion of the marine “dead zones” with severe implications not only for all higher life forms but also for oxygen‐sensitive nutrient recycling and, hence, for oceanic nutrient inventories

Portuguese recommendations for the use of methotrexate in rheumatic diseases – 2016 update

info:eu-repo/semantics/publishedVersio

Inorganic carbon and pH dependency of Trichodesmium's photosynthetic rates.

We established the relationship between photosynthetic carbon fixation rates and pH, CO2 and HCO3- concentrations in the diazotroph Trichodesmium erythraeum IMS101. Inorganic 14C-assimilation was measured in TRIS-buffered ASW medium where the absolute and relative concentrations of CO2, pH and HCO3- were manipulated. First, we varied the total dissolved inorganic carbon concentration (TIC) (< 0 to ~ 5 mM) at constant pH, so ratios of CO2 and HCO3- remained relatively constant. Second, we varied pH (~ 8.54 to 7.52) at constant TIC, so CO2 increased whilst HCO3- declined. We found that 14C-assimilation could be described by the same function of CO2 for both approaches but showed different dependencies on HCO3- when pH was varied at constant TIC than when TIC was varied at constant pH. A numerical model of Trichodesmium's CCM showed carboxylation rates are modulated by HCO3- and pH. The decrease in Ci assimilation at low CO2, when TIC was varied, is due to HCO3- uptake limitation of the carboxylation rate. Conversely, when pH was varied, Ci assimilation declined due to a high-pH mediated increase in HCO3- and CO2 leakage rates, potentially coupled to other processes (uncharacterised within the CCM model) that restrict Ci assimilation rates under high-pH conditions