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

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

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

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

Global variability in seawater Mg:Ca and Sr:Ca ratios in the modern ocean

Seawater Mg:Ca and Sr:Ca ratios are biogeochemical parameters reflecting the Earth–ocean–atmosphere dynamic exchange of elements. The ratios’ dependence on the environment and organisms' biology facilitates their application in marine sciences. Here, we present a measured single-laboratory dataset, combined with previous data, to test the assumption of limited seawater Mg:Ca and Sr:Ca variability across marine environments globally. High variability was found in open-ocean upwelling and polar regions, shelves/neritic and river-influenced areas, where seawater Mg:Ca and Sr:Ca ratios range from ∼4.40 to 6.40 mmol:mol and ∼6.95 to 9.80 mmol:mol, respectively. Open-ocean seawater Mg:Ca is semiconservative (∼4.90 to 5.30 mol:mol), while Sr:Ca is more variable and nonconservative (∼7.70 to 8.80 mmol:mol); both ratios are nonconservative in coastal seas. Further, the Ca, Mg, and Sr elemental fluxes are connected to large total alkalinity deviations from International Association for the Physical Sciences of the Oceans (IAPSO) standard values. Because there is significant modern seawater Mg:Ca and Sr:Ca ratios variability across marine environments we cannot absolutely assume that fossil archives using taxa-specific proxies reflect true global seawater chemistry but rather taxa- and process-specific ecosystem variations, reflecting regional conditions. This variability could reconcile secular seawater Mg:Ca and Sr:Ca ratio reconstructions using different taxa and techniques by assuming an error of 1 to 1.50 mol:mol, and 1 to 1.90 mmol:mol, respectively. The modern ratios’ variability is similar to the reconstructed rise over 20 Ma (Neogene Period), nurturing the question of seminonconservative behavior of Ca, Mg, and Sr over modern Earth geological history with an overlooked environmental effect

Correction for Lebrato et al., Global variability in seawater Mg:Ca and Sr:Ca ratios in the modern ocean

4 pages, 5 figures.-- Correction Global variability in seawater Mg:Ca and Sr:Ca ratios in the modern ocean; Proceedings of the National Academy of Sciences of the USA 117(36): 22281-22292 (2020); doi: 10.1073/pnas.1918943117; http://hdl.handle.net/10261/221953The authors wish to note the following: “This study’s seawater Sr:Ca values were systematically low as a consequence of normalization to another published low value for the International Association for the Physical Sciences of the Oceans (IAPSO) (1). IAPSO has been used at the Ocean Drilling Program, Texas A&M University (ODP-TAMU) (http://www-odp.tamu.edu/), and is still being used as the primary standard for elemental composition of seawater/interstitial water. Consequently, our seawater value of Sr:Ca = 8.28 mmol:mol was systematically low by approx. 3.70%, if we accept seawater Sr:Ca 8.60 mmol:mol as the recommended value for IAPSO North Atlantic surface water salinity standard. The uncertainty budget should be expanded including the uncertainty of IAPSO composition. The largest contribution to expanded uncertainty of our data comes from the uncertainty of the IAPSO reference composition, which is 3.29% using all published values. This will result in 3.30% (1 SD) expanded uncertainty for seawater Sr:Ca (and 0.5%, for seawater Mg:Ca) of the entire data set with respect to accuracy. We have corrected all seawater Sr:Ca values with a factor of 1.0243 in all our tables (e.g., SI Appendix, Table S1 averages) and in the figures (Fig. 4, Fig. 5), where a ratio was used. Note that the seawater Sr:Ca % changes are small, thus changes are hardly noticeable on large displays (e.g., Figures), but they can be seen in the tables and averages/SD calculations. Seawater Sr:Ca ratios are also corrected in the main text where relevantPeer reviewe

Influence of elevated CO2 concentrations on cell division and nitrogen fixation rates in the bloom-forming cyanobacterium Nodularia spumigena

The surface ocean absorbs large quantities of the CO2 emitted to the atmosphere from human activities. As this CO2 dissolves in seawater, it reacts to form carbonic acid. While this phenomenon, called ocean acidification, has been found to adversely affect many calcifying organisms, some photosynthetic organisms appear to benefit from increasing [CO2]. Among these is the cyanobacterium Trichodesmium, a predominant diazotroph (nitrogen-fixing) in large parts of the oligotrophic oceans, which responded with increased carbon and nitrogen fixation at elevated pCO2. With the mechanism underlying this CO2 stimulation still unknown, the question arises whether this is a common response of diazotrophic cyanobacteria. In this study we therefore investigate the physiological response of Nodularia spumigena, a heterocystous bloom-forming diazotroph of the Baltic Sea, to CO2-induced changes in seawater carbonate chemistry. N. spumigena reacted to seawater acidification/carbonation with reduced cell division rates and nitrogen fixation rates, accompanied by significant changes in carbon and phosphorus quota and elemental composition of the formed biomass. Possible explanations for the contrasting physiological responses of Nodularia compared to Trichodesmium may be found in the different ecological strategies of non-heterocystous (Trichodesmium) and heterocystous (Nodularia) cyanobacteria

Seawater carbonate chemistry and processes during experiments with cyanobacterium Nodularia spumigena, 2009

The surface ocean absorbs large quantities of the CO2 emitted to the atmosphere from human activities. As this CO2 dissolves in seawater, it reacts to form carbonic acid. While this phenomenon, called ocean acidification, has been found to adversely affect many calcifying organisms, some photosynthetic organisms appear to benefit from increasing [CO2]. Among these is the cyanobacterium Trichodesmium, a predominant diazotroph (nitrogen-fixing) in large parts of the oligotrophic oceans, which responded with increased carbon and nitrogen fixation at elevated pCO2. With the mechanism underlying this CO2 stimulation still unknown, the question arises whether this is a common response of diazotrophic cyanobacteria. In this study we therefore investigate the physiological response of Nodularia spumigena, a heterocystous bloom-forming diazotroph of the Baltic Sea, to CO2-induced changes in seawater carbonate chemistry. N. spumigena reacted to seawater acidification/carbonation with reduced cell division rates and nitrogen fixation rates, accompanied by significant changes in carbon and phosphorus quota and elemental composition of the formed biomass. Possible explanations for the contrasting physiological responses of Nodularia compared to Trichodesmium may be found in the different ecological strategies of non-heterocystous (Trichodesmium) and heterocystous (Nodularia) cyanobacteria