Good tidings for red tides? Responses of toxic and calcareous dinoflagellates to global change

Atmospheric CO2 partial pressure (pCO2) rises at a yet unprecedented rate, which enhances the uptake of CO2 by the surface ocean and concomitantly lowers the pH. Due to the latter, these changes are often referred to as ocean acidification (OA). In the last decades, consequences of OA on marine phytoplankton have been intensively studied from cellular to ecosystem level. These investigations have, however, largely focused on coccolithophores, diatoms and cyanobacteria. Little is known about the responses of dinoflagellates to OA, even though they represent an important component of phytoplankton assemblages. Moreover, owing to their type II RubisCO, a carboxylating enzyme with very low affinities for its substrate CO2, dinoflagellates may be particularly sensitive to changes in CO2 concentrations. In my first publication, I therefore investigated the impact of OA on two dinoflagellate species, the calcareous Scrippsiella trochoidea and the paralytic shellfish poisoning (PSP) toxin producing Alexandrium fundyense (previously A. tamarense). The results show that, besides species-specific differences, growth characteristics remained largely unaltered with rising pCO2 (Publication I). To understand these responses, several aspects of inorganic carbon (Ci) acquisition were investigated, revealing effective yet differently expressed carbon concentrating mechanisms (CCMs). These CCMs were moreover adjusted to the respective CO2 environment, which enabled both species to keep their growth rates relatively unaffected over a broad range of pCO2. In addition to OA, rising CO2 causes global warming, which in turn will lead to a rise in sea surface temperatures. Consequences will be an enhanced thermal stratification and a lowered nutrient resupply from nutrient-rich deep waters. Nutrient limitation may alter the response of dinoflagellates towards elevated pCO2. In Publication II, I therefore investigated the effects of rising CO2 and nitrogen (N) limitation on S. trochoidea and A. fundyense. The findings indicate a close coupling between C and N assimilation and showed a CO2-dependent increase in N assimilation in both species. Although N-rich compounds per cell were highest at high pCO2, this came at the expense of higher N requirements and lower N affinities, which will reduce the competitive ability of both species that potentially translate to changes in the phytoplankton community composition in a future ocean. To test the effect of OA on the productivity of phytoplankton in a natural community, a five months mesocosm study was conducted at the coast of the Swedish North Sea (Publication III). Besides early spring blooms of diatoms, dinoflagellate blooms often occur in these waters in late summer. During the experimental phase from March until July, we observed two major phytoplankton bloom events, which were both dominated by diatoms. Dinoflagellates usually overwinter as resting cysts in the sediment and as the applied mesocosms were closed in early spring, the initial inoculum of dinoflagellates was low. Weekly attempts to introduce seed populations of dinoflagellates to the mesocosms were not effective enough for species to subsist in these systems. Concerning the overall phytoplankton community, impacts of OA on primary production were generally small, though total primary production increased during the second phytoplankton bloom when nutrients were depleted to very low concentrations. In conclusion, OA seems to have an effect on the photosynthetic activity of marine dinoflagellates, and furthermore cause changes in various physiological processes also related to nutrient acquisition. Even though these changes may appear a smalla , at least when compared to OA-responses of other taxa, they can nonetheless influence the competitive abilities of species, especially when being exposed to nutrient limitation. On an ecosystem level, OA therefore has the potential to stimulate primary production and alter the phytoplankton community structure in coastal waters, especially at times when the availability of nutrients is limited

Inhibitors of dihydroorotate dehydrogenase cooperate with molnupiravir and N4-hydroxycytidine to suppress SARS-CoV-2 replication

The nucleoside analog N4-hydroxycytidine (NHC) is the active metabolite of the prodrug molnupiravir, which has been approved for the treatment of COVID-19. SARS-CoV-2 incorporates NHC into its RNA, resulting in defective virus genomes. Likewise, inhibitors of dihydroorotate dehydrogenase (DHODH) reduce virus yield upon infection, by suppressing the cellular synthesis of pyrimidines. Here, we show that NHC and DHODH inhibitors strongly synergize in the inhibition of SARS-CoV-2 replication in vitro. We propose that the lack of available pyrimidine nucleotides upon DHODH inhibition increases the incorporation of NHC into nascent viral RNA. This concept is supported by the rescue of virus replication upon addition of pyrimidine nucleosides to the media. DHODH inhibitors increased the antiviral efficiency of molnupiravir not only in organoids of human lung, but also in Syrian Gold hamsters and in K18-hACE2 mice. Combining molnupiravir with DHODH inhibitors may thus improve available therapy options for COVID-19

Inhibitors of dihydroorotate dehydrogenase cooperate with molnupiravir and N4-hydroxycytidine to suppress SARS-CoV-2 replication

Funding Information: We thank Thorsten Wolff, Daniel Bourquain, Jessica Schulz, and Christian Mache from the Robert-Koch Institute and Martin Beer from the Friedrich Loeffler Institute (FLI) for providing isolates of SARS-CoV-2 variants. We thank Anna Kraft and Gabriele Czerwinski (both FLI) for support in the preparation of samples for pathology, and Catherine Hambly (University of Aberdeen) for help with daily energy expenditure measurements. We would like to thank Cathrin Bierwirth (University Medical Center Göttingen), Isabell Schulz, Anne-Kathrin Donner, and Frank-Thorben Peters for excellent technician assistance and Jasmin Fertey and Alexandra Rockstroh for providing the virus stocks for the mice experiment (Fraunhofer Institute IZI Leipzig). We acknowledge support by the Open Access Publication Funds of the Göttingen University. KMS was a member of the Göttingen Graduate School GGNB during this work. This work was funded by the COVID-19 Forschungsnetzwerk Niedersachsen (COFONI) to MD, by the Federal Ministry of Education and Research Germany ( Bundesministerium für Bildung und Forschung; BMBF ; OrganSARS , 01KI2058 ) to SP and TM, and by a grant of the Max Planck Foundation to DG. Declaration of interests AS, HK, EP, and DV are employees of Immunic AG and own shares and/or stock-options of the parent company of Immunic AG, Immunic Inc. Some of the Immunic AG employees also hold patents for the Immunic compounds described in this manuscript (WO2012/001,148, WO03006425). KMS, AD, and MD are employees of University Medical Center Göttingen, which has signed a License Agreement with Immunic AG covering the combination of DHODH inhibitors and nucleoside analogs to treat viral infections, including COVID-19 (inventors: MD, KMS, and AD). The other authors declare no conflict of interest.Peer reviewedPublisher PD

American Step-Up and Step-Down Default Swaps under Levy Models

This paper studies the valuation of a class of default swaps with the embedded option to switch to a different premium and notional principal anytime prior to a credit event. These are early exercisable contracts that give the protection buyer or seller the right to step-up, step-down, or cancel the swap position. The pricing problem is formulated under a structural credit risk model based on Levy processes. This leads to the analytic and numerical studies of several optimal stopping problems subject to early termination due to default. In a general spectrally negative Levy model, we rigorously derive the optimal exercise strategy. This allows for instant computation of the credit spread under various specifications. Numerical examples are provided to examine the impacts of default risk and contractual features on the credit spread and exercise strategy.Comment: 35 pages, 5 figure

Influence of Ocean Acidification on a Natural Winter-to-Summer Plankton Succession : First Insights from a Long-Term Mesocosm Study Draw Attention to Periods of Low Nutrient Concentrations

Every year, the oceans absorb about 30% of anthropogenic carbon dioxide (CO2) leading to a re-equilibration of the marine carbonate system and decreasing seawater pH. Today, there is increasing awareness that these changes-summarized by the term ocean acidification (OA)-could differentially affect the competitive ability of marine organisms, thereby provoking a restructuring of marine ecosystems and biogeochemical element cycles. In winter 2013, we deployed ten pelagic mesocosms in the Gullmar Fjord at the Swedish west coast in order to study the effect of OA on plankton ecology and biogeochemistry under close to natural conditions. Five of the ten mesocosms were left unperturbed and served as controls (similar to 380 mu atm pCO(2)), whereas the others were enriched with CO2-saturated water to simulate realistic end-of-the-century carbonate chemistry conditions (mu 760 mu atm pCO(2)). We ran the experiment for 113 days which allowed us to study the influence of high CO2 on an entire winter-to-summer plankton succession and to investigate the potential of some plankton organisms for evolutionary adaptation to OA in their natural environment. This paper is the first in a PLOS collection and provides a detailed overview on the experimental design, important events, and the key complexities of such a "long-term mesocosm" approach. Furthermore, we analyzed whether simulated end-of-the-century carbonate chemistry conditions could lead to a significant restructuring of the plankton community in the course of the succession. At the level of detail analyzed in this overview paper we found that CO2-induced differences in plankton community composition were non-detectable during most of the succession except for a period where a phytoplankton bloom was fueled by remineralized nutrients. These results indicate: (1) Long-term studies with pelagic ecosystems are necessary to uncover OA-sensitive stages of succession. (2) Plankton communities fueled by regenerated nutrients may be more responsive to changing carbonate chemistry than those having access to high inorganic nutrient concentrations and may deserve particular attention in future studies.Peer reviewe

Gute Gezeiten für Rote Fluten? Auswirkungen des Klimawandels auf toxische und kalzifizierende Dinoflagellaten

Atmospheric CO2 partial pressure (pCO2) rises at a yet unprecedented rate, which enhances the uptake of CO2 by the surface ocean and concomitantly lowers the pH. Due to the latter, these changes are often referred to as "ocean acidification" (OA). In the last decades, consequences of OA on marine phytoplankton have been intensively studied from cellular to ecosystem level. These investigations have, however, largely focused on coccolithophores, diatoms and cyanobacteria. Little is known about the responses of dinoflagellates to OA, even though they represent an important component of phytoplankton assemblages. Moreover, owing to their type II RubisCO, a carboxylating enzyme with very low affinities for its substrate CO2, dinoflagellates may be particularly sensitive to changes in CO2 concentrations. In my first publication, I therefore investigated the impact of OA on two dinoflagellate species, the calcareous Scrippsiella trochoidea and the paralytic shellfish poisoning (PSP) toxin producing Alexandrium fundyense (previously A. tamarense). The results show that, besides species-specific differences, growth characteristics remained largely unaltered with rising pCO2 (Publication I). To understand these responses, several aspects of inorganic carbon (Ci) acquisition were investigated, revealing effective yet differently expressed carbon concentrating mechanisms (CCMs). These CCMs were moreover adjusted to the respective CO2 environment, which enabled both species to keep their growth rates relatively unaffected over a broad range of pCO2. In addition to OA, rising CO2 causes global warming, which in turn will lead to a rise in sea surface temperatures. Consequences will be an enhanced thermal stratification and a lowered nutrient resupply from nutrient-rich deep waters. Nutrient limitation may alter the response of dinoflagellates towards elevated pCO2. In Publication II, I therefore investigated the effects of rising CO2 and nitrogen (N) limitation on S. trochoidea and A. fundyense. The findings indicate a close coupling between C and N assimilation and showed a CO2-dependent increase in N assimilation in both species. Although N-rich compounds per cell were highest at high pCO2, this came at the expense of higher N requirements and lower N affinities, which will reduce the competitive ability of both species that potentially translate to changes in the phytoplankton community composition in a future ocean. To test the effect of OA on the productivity of phytoplankton in a natural community, a five months mesocosm study was conducted at the coast of the Swedish North Sea (Publication III). Besides early spring blooms of diatoms, dinoflagellate blooms often occur in these waters in late summer. During the experimental phase from March until July, we observed two major phytoplankton bloom events, which were both dominated by diatoms. Dinoflagellates usually overwinter as resting cysts in the sediment and as the applied mesocosms were closed in early spring, the initial inoculum of dinoflagellates was low. Weekly attempts to introduce seed populations of dinoflagellates to the mesocosms were not effective enough for species to subsist in these systems. Concerning the overall phytoplankton community, impacts of OA on primary production were generally small, though total primary production increased during the second phytoplankton bloom when nutrients were depleted to very low concentrations. In conclusion, OA seems to have an effect on the photosynthetic activity of marine dinoflagellates, and furthermore cause changes in various physiological processes also related to nutrient acquisition. Even though these changes may appear a smalla , at least when compared to OA-responses of other taxa, they can nonetheless influence the competitive abilities of species, especially when being exposed to nutrient limitation. On an ecosystem level, OA therefore has the potential to stimulate primary production and alter the phytoplankton community structure in coastal waters, especially at times when the availability of nutrients is limited

Differential effects of ocean acidification on carbon acquisition in two bloom-forming dinoflagellate species

Dinoflagellates represent a cosmopolitan group of phytoplankton with the^ability to form harmful algal blooms. Featuring a Ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) with very low CO2 affinities, photosynthesis of this group may be particularly prone to carbon limitation and thusv benefit from rising atmospheric CO2 partial pressure (pCO2) under ocean acidification (OA). Here, we investigated the consequences of OA on two bloom-forming dinoflagellate species, the calcareous Scrippsiella trochoidea and the toxic Alexandrium tamarense. Using dilute batch incubations, we assessed growth characteristics over a range of pCO2 (i.e. 180–1200 atm). To understand the underlying physiology, several aspects of inorganic carbon acquisition were investigated by membrane-inlet mass spectrometry. Our results show that both species kept growth rates constant over the tested pCO2 range, but we observed a number of species-specific responses. For instance, biomass production and cell size decreased in S. trochoidea, while A. tamarense was not responsive to OA in these measures. In terms of oxygen fluxes, rates of photosynthesis and respiration remained unaltered in S. trochoidea whereas respiration increased in A. tamarense under OA. Both species featured efficient carbon concentrating mechanisms (CCMs) with a CO2-dependent contribution of HCO3− uptake. In S. trochoidea, the CCM was further facilitated by exceptionally high and CO2-independent carbonic anhydrase activity. Comparing both species, a general trade-off between maximum rates of photosynthesis and respective affinities is indicated. In conclusion, our results demonstrate effective CCMs in both species, yet very different strategies to adjust their carbon acquisition. This regulation in CCMs enables both species to maintain growth over a wide range of ecologically relevant pCO2

CO2-dependent carbon isotope fractionation in dinoflagellates relates to their inorganic carbon fluxes

Carbon isotope fractionation (εp) between the inorganic carbon source and organic matter has been proposed to be a function of pCO2. To understand the CO2-dependency of εp and species-specific differences therein, inorganic carbon fluxes in the four dinoflagellate species Alexandrium fundyense, Scrippsiella trochoidea, Gonyaulax spinifera and Protoceratium reticulatum have been measured by means of membrane-inlet mass spectrometry. In-vivo assays were carried out at different CO2 concentrations, representing a range of pCO2 from 180 to 1200 μatm. The relative bicarbonate contribution (i.e. the ratio of bicarbonate uptake to total inorganic carbon uptake) and leakage (i.e. the ratio of CO2 efflux to total inorganic carbon uptake) varied from 0.2 to 0.5 and 0.4 to 0.7, respectively, and differed significantly between species. These ratios were fed into a single-compartment model, and εp values were calculated and compared to carbon isotope fractionation measured under the same conditions. For all investigated species, modeled and measured εp values were comparable (A. fundyense, S. trochoidea, P. reticulatum) and/or showed similar trends with pCO2 (A. fundyense, G. spinifera, P. reticulatum). Offsets are attributed to biases in inorganic flux measurements, an overestimated fractionation factor for the CO2-fixing enzyme RubisCO, or the fact that intracellular inorganic carbon fluxes were not taken into account in the model. This study demonstrates that CO2-dependency in εp can largely be explained by the inorganic carbon fluxes of the individual dinoflagellates