Seawater carbonate chemistry and physiological responses of three temperate coralline algae in a laboratory experiment

Coralline algae are major calcifiers of significant ecological importance in marine habitats but are among the most sensitive calcifying organisms to ocean acidification. The elevated pCO2 effects were examined in three coralline algal species living in contrasting habitats from intertidal to subtidal zones on the north-western coast of Brittany, France: (i) Corallina elongata, a branched alga found in tidal rock pools, (ii) Lithophyllum incrustans, a crustose coralline alga from the low intertidal zone, and (iii) Lithothamnion corallioides (maerl), a free-living form inhabiting the subtidal zone. Metabolic rates were assessed on specimens grown for one month at varying pCO2: 380 (current pCO2), 550, 750 and 1000 µatm (elevated pCO2). There was no pCO2 effect on gross production in C. elongata and L. incrustans but L. incrustans respiration strongly increased with elevated pCO2. L. corallioides gross production slightly increased at 1000 µatm, while respiration remained unaffected. Calcification rates decreased with pCO2 in L. incrustans (both in the light and dark) and L. corallioides (only in the light), while C. elongata calcification was unaffected. This was consistent with the lower skeletal mMg/Ca ratio of C. elongata (0.17) relative to the two other species (0.20). L. incrustans had a higher occurrence of bleaching that increased with increasing pCO2. pCO2 could indirectly impact this coralline species physiology making them more sensitive to other stresses such as diseases or pathogens. These results underlined that the physiological response of coralline algae to near-future ocean acidification is species-specific and that species experiencing naturally strong pH variations were not necessarily more resistant to elevated pCO2 than species from more stable environment.

Physiological responses of the marine diatom Thalassiosira pseudonana to increased pCO2 and seawater acidity

We studied the effects of elevated CO2 concentration and seawater acidity on inorganic carbon acquisition, photoinhibition and photoprotection as well as growth and respiration in the marine diatom Thalassiosira pseudonana. After having grown under the elevated CO2 level (1000 µatm, pH 7.83) at sub-saturating photosynthetically active radiation (PAR, 75 µmol photons/m**2/s) for 20 generations, photosynthesis and dark respiration of the alga increased by 25% (14.69 ± 2.55 fmol C/cell/h) and by 35% (4.42 ± 0.98 fmol O2/cell/h), respectively, compared to that grown under the ambient CO2 level (390 µatm, pH 8.16), leading to insignificant effects on growth (1.09 ± 0.08 (1/d))v 1.04 ± 0.07 (1/d)). The photosynthetic affinity for CO2 was lowered in the high-CO2 grown cells, reflecting a down-regulation of the CO2 concentrating mechanism (CCM). When exposed to an excessively high level of PAR, photochemical and non-photochemical quenching responded similarly in the low- and high-CO2 grown cells, reflecting that photoinhibition was not influenced by the enriched level of CO2. In T. pseudonana, it appeared that the energy saved due to the down-regulated CCM did not contribute to any additional light stress as previously found in another diatom Phaeodactylum tricornutum, indicating differential physiological responses to ocean acidification between these two diatom species.

Assessing the physiological responses of the gastropod Crepidula fornicata to predicted ocean acidification and warming

Organisms inhabiting coastal waters naturally experience diel and seasonal physico-chemical variations. According to various assumptions, coastal species are either considered to be highly tolerant to environmental changes or, conversely, living at the thresholds of their physiological performance. Therefore, these species are either more resistant or more sensitive, respectively, to ocean acidification and warming. Here, we focused on Crepidula fornicata, an invasive gastropod that colonized bays and estuaries on northwestern European coasts during the 20th century. Small (<3 cm in length) and large (>4.5 cm in length), sexually mature individuals of C. fornicata were raised for 6 months in three different pCO2 conditions (390 µatm, 750 µatm, and 1400 µatm) at four successive temperature levels (10°C, 13°C, 16°C, and 19°C). At each temperature level and in each pCO2 condition, we assessed the physiological rates of respiration, ammonia excretion, filtration and calcification on small and large individuals. Results show that, in general, temperature positively influenced respiration, excretion and filtration rates in both small and large individuals. Conversely, increasing pCO2 negatively affected calcification rates, leading to net dissolution in the most drastic pCO2 condition (1400 µatm) but did not affect the other physiological rates. Overall, our results indicate that C. fornicata can tolerate ocean acidification, particularly in the intermediate pCO2 scenario. Moreover, in this eurythermal species, moderate warming may play a buffering role in the future responses of organisms to ocean acidification.

Experiment: Physiological responses of the calcifying rhodophyte, Corallina officinalis (L.), to future CO2 levels

Future atmospheric CO2 levels will most likely have complex consequences for marine organisms, particulary photosynthetic calcifying organisms. Corallina officinalis L. is an erect calcifying macroalga found in the inter- and subtidal regions of temperate rocky coastlines and provides important substrate and refugia for marine meiofauna. The main goal of the current study was to determine the physiological responses of C. officinalis to increased CO2 concentrations expected to occur within the next century and beyond. Our results show that growth and production of inorganic material decreased under high CO2 levels, while carbonic anhydrase activity was stimulated and negatively correlated to algal inorganic content. Photosynthetic efficiency based on oxygen evolution was also negatively affected by increased CO2. The results of this study indicate that C. officinalis may become less competitive under future CO2 levels, which could result in structural changes in future temperate intertidal communities.

Experiment: Ultraviolet radiation modulates the physiological responses of the calcified rhodophyte Corallina officinalis to elevated CO2

Ocean acidification reduces the concentration of carbonate ions and increases those of bicarbonate ions in seawater compared with the present oceanic conditions. This altered composition of inorganic carbon species may, by interacting with ultraviolet radiation (UVR), affect the physiology of macroalgal species. However, very little is known about how calcareous algae respond to UVR and ocean acidification. Therefore, we conducted an experiment to determine the effects of UVR and ocean acidification on the calcified rhodophyte Corallina officinalis using CO2-enriched cultures with and without UVR exposure. Low pH increased the relative electron transport rates (rETR) but decreased the CaCO3 content and had a miniscule effect on growth. However, UVA (4.25 W m-2) and a moderate level of UVB (0.5 W m-2) increased the rETR and growth rates in C. officinalis, and there was a significant interactive effect of pH and UVR on UVR-absorbing compound concentrations. Thus, at low irradiance, pH and UVR interact in a way that affects the multiple physiological responses of C. officinalis differently. In particular, changes in the skeletal content induced by low pH may affect how C. officinalis absorbs and uses light. Therefore, the light quality used in ocean acidification experiments will affect the predictions of how calcified macroalgae will respond to elevated CO2.

Seawater carbonate chemistry and combined physiological effects of CO2 and light on the N2-fixing cyanobacterium Trichodesmium IMS101 during experiments, 2010

Recent studies on the diazotrophic cyanobacterium Trichodesmium erythraeum(IMS101) showed that increasing CO2 partial pressure (pCO2) enhances N2 fixation and growth. Significant uncertainties remain as to the degree of the sensitivity to pCO2, its modification by other environmental factors, and underlying processes causing these responses. To address these questions, we examined the responses ofTrichodesmium IMS101 grown under a matrix of low and high levels of pCO2 (150 and 900 µatm) and irradiance (50 and 200 µmol photons m-2 s-1). Growth rates as well as cellular carbon and nitrogen contents increased with increasing pCO2 and light levels in the cultures. The pCO2-dependent stimulation in organic carbon and nitrogen production was highest under low light. High pCO2 stimulated rates of N2fixation and prolonged the duration, while high light affected maximum rates only. Gross photosynthesis increased with light but did not change with pCO2. HCO3- was identified as the predominant carbon source taken up in all treatments. Inorganic carbon uptake increased with light, but only gross CO2 uptake was enhanced under high pCO2. A comparison between carbon fluxes in vivo and those derived from 13C fractionation indicates high internal carbon cycling, especially in the low-pCO2treatment under high light. Light-dependent oxygen uptake was only detected underlow pCO2 combined with high light or when low-light-acclimated cells were exposed to high light, indicating that the Mehler reaction functions also as a photoprotective mechanism in Trichodesmium. Our data confirm the pronounced pCO2 effect on N2fixation and growth in Trichodesmium and further show a strong modulation of these effects by light intensity. We attribute these responses to changes in the allocation of photosynthetic energy between carbon acquisition and the assimilation of carbon and nitrogen under elevated pCO2. These findings are supported by a complementarystudy looking at photosynthetic fluorescence parameters of photosystem II, photosynthetic unit stoichiometry (photosystem I:photosystem II), and pool sizes of key proteins in carbon and nitrogen acquisition.

Carbon and oxygen isotopic compositions of Cibicidoides spp. and fine fraction separates (<10 µm) from ODP Holes 113-689B, 119-738B, and 120-748B

A prominent middle Eocene warming event is identified in Southern Ocean deep-sea cores, indicating that long-term cooling through the middle and late Eocene was not monotonic. At sites on Maud Rise and the Kerguelen Plateau, a distinct negative shift in d18O values (~1.0 per mil) is observed ca. 41.5 Ma. This excursion is interpreted as primarily a temperature signal, with a transient warming of 4°C over 600 k.y. affecting both surface and middle-bathyal deep waters in the Indian-Atlantic region of the Southern Ocean. This isotopic event is designated as the middle Eocene climatic optimum, and is interpreted to represent a significant climatic reversal in the midst of middle to late Eocene deep-sea cooling. The lack of a significant negative carbon isotope excursion, as observed during the Paleocene-Eocene thermal maximum, and the gradual rate of high-latitude warming suggest that this event was not triggered by methane hydrate dissociation. Rather, a transient rise in pCO2 levels is suspected, possibly as a result of metamorphic decarbonation in the Himalayan orogen or increased ridge/arc volcanism during the late middle Eocene.