Growth response of Emiliania huxleyi to ocean alkalinity enhancement

The CO2 concentration of air has increased over the last two centuries and recently surpassed 400 ppm. Carbon cycle models project CO2 concentrations of 720 to 1000 ppm for the IPCC intermediate scenario (RCP 6.0), resulting in an increase in global mean temperature of ~ 2.6 °C and a decrease in seawater pH of ~ 0.3. Together, global warming and ocean acidification are often referred to as the “evil twins” of climate change, potentially inducing severe threats in the near future. In this paper, our discussion is focused on the response of two major calcifiers, foraminifera and corals, which contribute much to the global carbonate burial rate. Photosymbiosis is regarded as an adaptive ecology for living in warm and oligotrophic oceans, especially for reef-building corals and larger reef-dwelling benthic foraminifera. As a consequence of global warming, bleaching may be a global threat to algal symbiont-bearing marine calcifying organisms under conditions of high temperature and light intensity. If CO2 is dissolved in seawater, the partial pressure of CO2 in seawater (pCO2) and dissolved inorganic carbon (DIC) increases while pH and the saturation state of carbonate minerals decreases without any change in total alkalinity. Generally, marine calcifying organisms show decreases in calcification rates in response to acidified seawater. However, the response often differs depending on situations, species, and life-cycle stage. Some benthic foraminifera showed a positive response to low pH conditions. The Acropora digitifera coral calcification of adult branches was not reduced markedly at higher pCO2 conditions, although calcification tended to decrease versus pCO2 in both aposymbiotic and symbiotic polyps. New analytical technologies help identify important constraints on calcification processes. Based upon Ca isotopes, the transport path of Ca2+ and the degree of its activity would predominantly control the carbonate precipitation rate. Visualization of the extracellular pH distribution shows that proton pumping produces the high internal pH and large internal–external pH gap in association with foraminiferal calcification. From the perspective of a long-term change in the Earth’s surface environment, foraminifera seem to be more adaptive and robust than corals in coping with ocean warming and acidification but it is necessary to further understand the mechanisms underlying variations in sensitivity to heat stress and acidified seawater for future prediction. Since CO2 is more soluble in lower temperature seawater, ocean acidification is more critical in the polar and high-latitude regions. Additionally, older deep-water has enhanced acidity owing to the addition of CO2 from the degradation of organic matter via a synergistic effect with high pressure. With current ocean acidification, pH and the saturation state of carbonate minerals are decreasing without any change in total alkalinity. However, in the Earth’s history, it is well known that alkalinity has fluctuated significantly. Therefore, it is necessary to quantitatively reconstruct alkalinity, which is another key factor determining the saturation state of carbonate minerals. The rapid release of anthropogenic CO2 (in the present day and at the Paleocene/Eocene boundary) induces severe ocean acidification, whereas in the Cretaceous, slow environmental change, even at high levels of pCO2, could raise alkalinity, thereby neutralizing ocean acidification.

<p>The alkalinity of seawater sets the overall capacity of the ocean to hold carbon dioxide in dissolved forms. Variations in past alkalinity, related to changing weathering or carbonate compensation, may have played an important role in moderating or controlling past variations of atmospheric <em>p</em>CO<sub>2</sub>.  Future manipulation of ocean alkalinity by direct addition of suitable chemicals to seawater, or through enhanced weathering on land, has also been suggested as one possible route to intentionally draw CO<sub>2</sub> from the modern atmosphere and mitigate the impacts of future climate change [1]. Although we know an increasing amount about how biological species and ecosystems respond to changes in pH, we know much less about their response to changes in alkalinity. Calcifying plankton play a crucial role in modulating the surface ocean carbonate system and its buffering of alkalinity perturbations [2]. Here we investigate the growth and calcification response of both coccolithophores and foraminifera to elevated ocean alkalinity and potential CO<sub>2</sub> limitation [3] through a series of carefully designed batch culture laboratory experiments. Alkalinity is raised by two different methods during the experiments: by (i) addition of NaHCO<sub>3</sub> and (ii) addition of Na<sub>2</sub>CO<sub>3</sub> and CaCl<sub>2</sub>. The reason for two differing elevated alkalinity treatments is that they allow us to constrain how physiology and calcification respond to two different modes of alkalinity manipulation; both of which provide simple laboratory analogues for probable real-world scenarios.</p><p>I will present results from experiments with two species of coccolithophores: <em>Emiliania huxleyi</em> and <em>Coccolithus braarudii</em>, as well as two species of planktonic foraminifera: <em>Gloigerinoides ruber </em>and <em>Globigerinella siphonifera</em>. We have found that the main bloom-forming coccolithophore, <em>Emiliania huxleyi</em>, may increase its calcification and growth rate in response to enhanced alkalinity up to Total Alkalinity (TA) = 4000µmol/kg. Whereas <em>Coccolithus braarudii</em>, a much larger and relatively less abundant coccolithophore, shows only a hint of increased calcification in enhanced alkalinity, with negligible changes in growth rate in enhanced alkalinity up to a threshold of Total Alkalinity (TA) = 3500µmol/kg. However, at TA = 4000µmol/kg, <em>C. braarudii’</em>s growth is significantly suppressed/delayed compared to control conditions. In contrast, planktonic foraminifera’s gametogenic success rate alters with enhanced alkalinity, and they may live longer in enhanced alkalinity before undergoing gametogenesis, but with no concurrent measurable increase in calcification. These results from two major groups of calcifiers have implications for future experiments on biotic response to ocean alkalinity enhancement (OAE) schemes, as well as implications for the design implementation of OAE schemes. </p><p>[1] Renforth, P., Henderson, G., 2017. Assessing ocean alkalinity for carbon sequestration. <em>Rev. Geophys.</em> [2] Boudreau, B.P., Middelburg, J.J., Luo, Y., 2018. The role of calcification in carbonate compensation. <em>Nat. Geosci. </em>11, 894. [3] Bach, L. T., Gill, S. J., Rickaby, R. E. M., Gore, S., Renforth, P., 2019. CO2 Removal With Enhanced Weathering and Ocean Alkalinity Enhancement: Potential Risks and Co-Benefits for Marine Pelagic Ecosystems. <em>Frontiers in Climate</em> 1.</p>

There is concern that the increasing pCO2 following the Industrial Revolution Period might lead to ocean acidification, which could affect calcifying organisms in the oceans. Recently, negative emission technology has been attracting attention as an effective countermeasure for greenhouse gas emissions. In the ocean, ocean alkalinization technology is proposed to neutralize acidified oceans and enhance the absorption capacity of CO2 in the oceans. The potential effectiveness of ocean alkalinization technology is also suggested by the history of the Earth. During the Cretaceous period, when pCO2 is interpreted >1,000 ppm, calcifying organisms thrived in the Cretaceous oceans. It is hypothesized that it was due to the total alkalinity (TA) of the seawater being maintained higher, thereby kept the calcium carbonate saturation state at necessary. In this study, we examined this hypothesis as well as attempted to predict the effects of the application of current alkalinization techniques in the ocean on calcifying organisms.Clonal populations of large benthic foraminifers were cultured in highly alkalinized seawater under high pCO2 conditions, and amounts of calcification (weight and volume) were measured (Group 1: high TA and high pCO2). Specimens taken from same clonal population were kept in modern surface seawater (Group 2: low TA and low pCO2) as a control treatment. The same experiments were also conducted as Group 3 (low TA and high pCO2) to simulate future ocean acidification conditions, and as Group 4 (high TA and low pCO2) to simulate alkalinized ocean under a low pCO2 environment. It was showed significant differences in the amount of calcification in each of the Groups after three months cultivation. The amount of calcification in Group 1 was almost the same as that in the control treatment, confirming the possibility of maintaining the growth of calcifying organisms by alkalinization. Calcification amount in Group 3 was the smallest among all groups, indicating that future ocean acidification may inhibit calcification of large benthic foraminifers. In addition, the calcification rate was the greatest in Group 4, it is indicated that ocean alkalinization may enhance the calcification of the organisms. Finally, these results suggest that the calcium carbonate saturation state of seawater is an important parameter for calcification.

The negative emissions technology, artificial ocean alkalinization(AOA), aims to store atmospheric carbon dioxide (CO2) in theocean by increasing total alkalinity (TA). Calcium carbonate saturation state(ΩCaCO3) and pH would also increase meaning that AOAcould alleviate sensitive regions and ecosystems from ocean acidification. However,AOA could raise pH and ΩCaCO3 well above modern-day levels,and very little is known about the environmental and biological impact of this.After treating a red calcifying algae (Corallinaspp.) to elevated TA seawater, carbonate production increased by 60% over a control.This has implication for carbon cycling in the past, but also constrains theenvironmental impact and efficiency of AOA. Carbonate production could reduce theefficiency of CO2 removal. Increasing TA, however, did notsignificantly influence Corallina spp. primaryproductivity, respiration, or photophysiology. These results show that AOA may notbe intrinsically detrimental for Corallina spp.and that AOA has the potential to lessen the impacts of ocean acidification.However, the experiment tested a single species within a controlled environment toconstrain a specific unknown, the rate change of calcification, and additional workis required to understand the impact of AOA on other organisms, whole ecosystems,and the global carbon cycle.

The Intergovernmental Panel on Climate Change reports indicate that the global mean temperature is about one-degree Celsius higher than pre-industrial levels, that this increase is anthropogenic, and that there is a causal relationship between this higher temperature and an increase in frequency and magnitude of extreme weather events. This causal relationship seems at odds with common sense, and may be difficult to explain to non-experts. Thus to appreciate the significance of a one-degree increase in global mean temperature, we perform back-of-the-envelope calculations relying on simple physics. We estimate the excess thermal energy trapped in the climate system (oceans, land, atmosphere) from a one-degree Celsius increase in global mean temperature, and show that it is thousands of times larger than the estimated energy required to form and maintain a hurricane. Our estimates show that global warming is forming a very large pool of excess energy that could in principle power heatwaves, heavy precipitation, droughts, and hurricanes. The arguments presented here are sufficiently simple to be presented in introductory physics classes, and can serve as plausibility arguments showing that even a seemingly small increase in global mean temperature can potentially lead to extreme weather events.

The Intergovernmental Panel on Climate Change reports indicate that the global mean temperature is about 1 °C higher than pre-industrial levels, that this increase is anthropogenic, and that there is a causal relationship between this higher temperature and an increase in frequency and magnitude of extreme weather events. This causal relationship seems at odds with common sense, and may be difficult to explain to non-experts. Thus to appreciate the significance of a one degree increase in global mean temperature, we perform back-of-the-envelope calculations relying on simple physics. We estimate the excess thermal energy trapped in the climate system (oceans, land, atmosphere) from a 1 °C increase in global mean temperature, and show that it is thousands of times larger than the estimated energy required to form and maintain a hurricane. Our estimates show that global warming is forming a very large pool of excess energy that could in principle power heatwaves, heavy precipitation, droughts, and hurricanes. The arguments presented here are sufficiently simple to be presented in introductory physics classes, and can serve as plausibility arguments showing that even a seemingly small increase in global mean temperature can potentially lead to extreme weather events.

Responses of marine primary production to a changing climate are determined by a concert of multiple environmental changes, for example in temperature, light, pCO2 , nutrients, and grazing. To make robust projections of future global marine primary production, it is crucial to understand multiple driver effects on phytoplankton. This meta-analysis quantifies individual and interactive effects of dual driver combinations on marine phytoplankton growth rates. Almost 50% of the single-species laboratory studies were excluded because central data and metadata (growth rates, carbonate system, experimental treatments) were insufficiently reported. The remaining data (42 studies) allowed for the analysis of interactions of pCO2 with temperature, light, and nutrients, respectively. Growth rates mostly respond non-additively, whereby the interaction with increased pCO2 profusely dampens growth-enhancing effects of high temperature and high light. Multiple and single driver effects on coccolithophores differ from other phytoplankton groups, especially in their high sensitivity to increasing pCO2 . Polar species decrease their growth rate in response to high pCO2 , while temperate and tropical species benefit under these conditions. Based on the observed interactions and projected changes, we anticipate primary productivity to: (a) first increase but eventually decrease in the Arctic Ocean once nutrient limitation outweighs the benefits of higher light availability; (b) decrease in the tropics and mid-latitudes due to intensifying nutrient limitation, possibly amplified by elevated pCO2 ; and (c) increase in the Southern Ocean in view of higher nutrient availability and synergistic interaction with increasing pCO2 . Growth-enhancing effect of high light and warming to coccolithophores, mainly Emiliania huxleyi, might increase their relative abundance as long as not offset by acidification. Dinoflagellates are expected to increase their relative abundance due to their positive growth response to increasing pCO2 and light levels. Our analysis reveals gaps in the knowledge on multiple driver responses and provides recommendations for future work on phytoplankton.

Ocean Alkalinity Enhancement (OAE) is a proposed mechanism of atmospheric CO2 removal, or negative emission technology. The addition of mineral particles to natural waters is one potential method of achieving OAE, yet there are substantial uncertainties concerning the efficiency of this process in terms of total alkalinity (TA) generation under natural conditions. Laboratory incubations are generally conducted under standardized conditions with filtered, deionized or sterile water which, whilst necessary to conduct reproducible mechanistic studies, is clearly not representative of most natural waters in which OAE might be deployed. In order to assess how variation in natural water properties affects the dissolution of minerals proposed as OAE agents, here we test the conversion of Mg(OH)2 to total alkalinity (TA) in river water, estuarine water, coastal seawater and offshore seawater. We found that when added as milk of magnesia, a 10 g/L (final concentration) Mg(OH)2 dose was efficiently converted to TA (>95% efficiency) in river water with low initial TA (mean TA = 239.44), river water with medium initial TA (mean TA = 1636.13), high salinity estuarine water (salinity 27), low salinity estuarine water (salinity 5.3), and seawater. However, when Mg(OH)2 was applied to high TA river water as a single dose (10 mg/L), the TA increase was only 60% of the calculated addition. The effect of multiple small doses (2.5 mg/L) was also tested, with no significant difference in the TA conversion in most cases. Dry additions of Mg(OH)2, rather than pre-mixed suspensions of milk of magnesia, were found to be inefficient TA sources, sometimes leading to negative TA changes- especially when using small incubation bottles (2 L). Overall it was demonstrated that Mg(OH)2 can be used as an efficient TA source in most natural waters for final doses in the range 2.5-10 mg/L.

Estimating global impacts from climate change

A pronounced fall in the CaCO3 saturation state and the total alkalinity of the surface ocean during the Mid Mesozoic

Total alkalinity (TA) is a popularly measured carbonate system parameter and is widely used in calculations of key carbonate system descriptors such as the calcium carbonate saturation state, an important indicator of ocean acidification. Organic alkalinity (OrgAlk) is recognised as a contributor to TA in coastal waters, with this having implications on the use of TA to calculate key carbonate chemistry descriptors. As titratable charge groups of OrgAlk can act as unknown acid-base species, the inclusion of the total concentration and apparent dissociation constants of OrgAlk in carbonate calculations involving TA is required to minimise uncertainty in computed speciation. Here we present an investigation of the prevalence and properties of OrgAlk as well as the impact of OrgAlk on carbonate chemistry calculations in a transitional waterbody. Water samples were collected during low and high tide over a 5-week period in Rogerstown Estuary, Dublin Ireland. TA and OrgAlk were measured using modified Global Ocean Acidification Observing Network (GOA-ON) titration apparatus in conjunction with OrgAlkCalc, an open-source Python based computational programme. pH was measured on the total scale using meta-cresol purple (mCP) as the indicator dye. Dissolved inorganic carbon (DIC), the partial pressure of CO2 (pCO2), in situ pH on the total scale (pHT) and the saturation state of aragonite (∆ΩA) were calculated using pH and both OrgAlk adjusted TA and measured TA as the input parameters. Optical analysis of DOM was conducted to compliment OrgAlk characterisations and to further elucidate OrgAlk sources and dynamics. OrgAlk charge groups concentrations ranged from 35,198 μmol·kg−1, with the highest concentrations observed in more marine waters. Two apparent charge groups were associated with OrgAlk, with pK values of 4.38 ± 0.27 and 6.95 ± 0.43. Differences between calculated carbonate system parameters when using OrgAlk adjusted TA and non-OrgAlk adjusted TA ranged from 88 to 254 μmol·kg−1 DIC, −98–67 μatm pCO2, −0.02–0.12 pHT and 0.02–0.64 ∆ΩA. Variability in the differences in calculated carbonate systems was largely a factor of OrgAlk charge group concentration and pK. This work highlights the importance of considering OrgAlk if using TA as an input parameter in carbonate system investigations of coastal waters.

<strong class="journal-contentHeaderColor">Abstract.</strong> The Arctic Ocean is subject to high rates of ocean warming and acidification, with critical implications for marine organisms as well as ecosystems and the services they provide. Carbonate system data in the Arctic realm are spotty in space and time and, until recently, there was no time-series station measuring the carbonate chemistry at high frequency in this region, particularly in coastal waters. We report here on the first high-frequency (1 h), multi-year (5 years) dataset of salinity, temperature, dissolved inorganic carbon, total alkalinity, CO₂ partial pressure (pCO₂) and pH at a coastal site (11 m) in a high-Arctic fjord (Kongsfjorden, Svalbard). We show that (1) the choice of formulations for calculating the dissociation constants of the carbonic acid remains unsettled for Arctic waters, (2) the water column is generally somewhat stratified despite the shallow depth, (3) the saturation state of calcium carbonate is subject to large seasonal changes but never reaches undersaturation (&Omega;_a ranges between 1.4 and 3.0) and (4) pCO₂ is lower than atmospheric CO₂ at all seasons, making this site a sink for atmospheric CO₂ (16.8 mol CO₂ m^&minus;2 yr^&minus;1).

Coastal ocean is more vulnerable to ocean acidification (OA) than open ocean due to high inputs of nutrients from land, large biological production, and various human activities. We started coastal acidification monitoring at three coastal (Busan, Jeju, and Ulleung) and one offshore (3km away from the Busan site) sites in Korea to examine long-term trends of the OA and their effects on coastal ocean environment. Discrete surface seawater samples for measurement of total dissolved inorganic carbon and total alkalinity were collected at the Busan, the Jeju and the Ulleung, and the offshore sites on a weekly, biweekly, and monthly basis, respectively. Here, we report changes in pH and saturation state of seawater with respect to aragonite (&amp;#937;) at the four sites for the period of 2019&amp;#8211;2022. At the Busan and the Ulleung sites, both pH and &amp;#937; showed significant decreases, but there were no trends at the other two sites. The change rates of deseasonalized pH and &amp;#937; (-0.011 &amp;#177; 0.005 yr-1 for pH and -0.049 &amp;#177; 0.024 yr-1 for &amp;#937;) found at the Busan site were similar to those (-0.010 &amp;#177; 0.005 yr-1 for pH and -0.058 &amp;#177; 0.025 yr-1 for &amp;#937;) of the Ulleung site. These rates are about six times greater than the global long-term mean rates (-0.016 per decade for pH and -0.07 per decade for &amp;#937;). Sea surface temperature and salinity did not show any significant trends for the same period. Continuous monitoring of carbonate parameters at these sites is necessary to get robust long-term OA trends and understand coastal OA processes by finding their drivers.

<strong class="journal-contentHeaderColor">Abstract.</strong> The Arctic Ocean is subject to high rates of ocean warming and acidification, with critical implications for marine organisms as well as ecosystems and the services they provide. Carbonate system data in the Arctic realm are spotty in space and time and, until recently, there was no time-series station measuring the carbonate chemistry at high frequency in this region, particularly in coastal waters. We report here on the first high-frequency (1 h), multi-year (5 years) dataset of salinity, temperature, dissolved inorganic carbon, total alkalinity, CO₂ partial pressure (pCO₂) and pH at a coastal site (11 m) in a high-Arctic fjord (Kongsfjorden, Svalbard). We show that (1) the choice of formulations for calculating the dissociation constants of the carbonic acid remains unsettled for Arctic waters, (2) the water column is generally somewhat stratified despite the shallow depth, (3) the saturation state of calcium carbonate is subject to large seasonal changes but never reaches undersaturation (&Omega;_a ranges between 1.4 and 3.0) and (4) pCO₂ is lower than atmospheric CO₂ at all seasons, making this site a sink for atmospheric CO₂ (16.8 mol CO₂ m^&minus;2 yr^&minus;1).

Abstract. The Arctic Ocean is subject to high rates of ocean warming and acidification, with critical implications for marine organisms as well as ecosystems and the services they provide. Carbonate system data in the Arctic realm are spotty in space and time, and, until recently, there was no time-series station measuring the carbonate chemistry at high frequency in this region, particularly in coastal waters. We report here on the first high-frequency (1 h), multi-year (5 years) dataset of salinity, temperature, CO2 partial pressure (pCO2) and pH at a coastal site (bottom depth of 12 m) in a high-Arctic fjord (Kongsfjorden, Svalbard). Discrete measurements of dissolved inorganic carbon and total alkalinity were also performed. We show that (1) the choice of formulations for calculating the dissociation constants of the carbonic acid remains unsettled for polar waters, (2) the water column is generally somewhat stratified despite the shallow depth, (3) the saturation state of calcium carbonate is subject to large seasonal changes but never reaches undersaturation (Ωa ranges between 1.4 and 3.0) and (4) pCO2 is lower than atmospheric CO2 at all seasons, making this site a sink for atmospheric CO2 (−9 to −16.8 molCO2m-2yr-1, depending on the parameterisation of the gas transfer velocity). Data are available on PANGAEA: https://doi.org/10.1594/PANGAEA.960131 (Gattuso et al., 2023a).