Effects of elevated pCO2 on the metabolism of a temperate rhodolith Lithothamnion corallioides grown under different temperatures

Coralline algae are considered among the most sensitive species to near future ocean acidification. We tested the effects of elevated pCO₂ on the metabolism of the free‐living coralline alga Lithothamnion corallioides (“maerl”) and the interactions with changes in temperature. Specimens were collected in North Brittany (France) and grown for 3 months at pCO₂ of 380 (ambient pCO₂), 550, 750, and 1000 μatm (elevated pCO₂) and at successive temperatures of 10°C (ambient temperature in winter), 16°C (ambient temperature in summer), and 19°C (ambient temperature in summer +3°C). At each temperature, gross primary production, respiration (oxygen flux), and calcification (alkalinity flux) rates were assessed in the light and dark. Pigments were determined by HPLC. Chl a, carotene, and zeaxanthin were the three major pigments found in L. corallioides thalli. Elevated pCO₂ did not affect pigment content while temperature slightly decreased zeaxanthin and carotene content at 10°C. Gross production was not affected by temperature but was significantly affected by pCO₂ with an increase between 380 and 550 μatm. Light, dark, and diel (24 h) calcification rates strongly decreased with increasing pCO₂ regardless of the temperature. Although elevated pCO₂ only slightly affected gross production in L. corallioides, diel net calcification was reduced by up to 80% under the 1,000 μatm treatment. Our findings suggested that near future levels of CO₂ will have profound consequences for carbon and carbonate budgets in rhodolith beds and for the sustainability of these habitats.     

One‐year experiment on the physiological response of the Mediterranean crustose coralline alga,Lithophyllum cabiochae,to elevated pCO2 and temperature

The response of respiration, photosynthesis, and calcification to elevated pCO₂ and temperature was investigated in isolation and in combination in the Mediterranean crustose coralline alga Lithophyllum cabiochae. Algae were maintained in aquaria during 1 year at near‐ambient conditions of irradiance, at ambient or elevated temperature (+3°C), and at ambient (ca. 400 μatm) or elevated pCO₂ (ca. 700 μatm). Respiration, photosynthesis, and net calcification showed a strong seasonal pattern following the seasonal variations of temperature and irradiance, with higher rates in summer than in winter. Respiration was unaffected by pCO₂ but showed a general trend of increase at elevated temperature at all seasons, except in summer under elevated pCO₂. Conversely, photosynthesis was strongly affected by pCO₂ with a decline under elevated pCO₂ in summer, autumn, and winter. In particular, photosynthetic efficiency was reduced under elevated pCO₂. Net calcification showed different responses depending on the season. In summer, net calcification increased with rising temperature under ambient pCO₂ but decreased with rising temperature under elevated pCO₂. Surprisingly, the highest rates in summer were found under elevated pCO₂ and ambient temperature. In autumn, winter, and spring, net calcification exhibited a positive or no response at elevated temperature but was unaffected by pCO₂. The rate of calcification of L. cabiochae was thus maintained or even enhanced under increased pCO₂. However, there is likely a trade‐off with other physiological processes. For example, photosynthesis declines in response to increased pCO₂ under ambient irradiance. The present study reports only on the physiological response of healthy specimens to ocean warming and acidification, however, these environmental changes may affect the vulnerability of coralline algae to other stresses such as pathogens and necroses that can cause major dissolution, which would have critical consequence for the sustainability of coralligenous habitats and the budgets of carbon and calcium carbonate in coastal Mediterranean ecosystems.     

Species‐specific responses to climate change and community composition determine future calcification rates of Florida Keys reefs

Anthropogenic climate change compromises reef growth as a result of increasing temperatures and ocean acidification. Scleractinian corals vary in their sensitivity to these variables, suggesting species composition will influence how reef communities respond to future climate change. Because data are lacking for many species, most studies that model future reef growth rely on uniform scleractinian calcification sensitivities to temperature and ocean acidification. To address this knowledge gap, calcification of twelve common and understudied Caribbean coral species was measured for two months under crossed temperatures (27, 30.3 °C) and CO₂ partial pressures (pCO₂) (400, 900, 1300 μatm). Mixed‐effects models of calcification for each species were then used to project community‐level scleractinian calcification using Florida Keys reef composition data and IPCC AR5 ensemble climate model data. Three of the four most abundant species, Orbicella faveolata, Montastraea cavernosa, and Porites astreoides, had negative calcification responses to both elevated temperature and pCO₂. In the business‐as‐usual CO₂ emissions scenario, reefs with high abundances of these species had projected end‐of‐century declines in scleractinian calcification of >50% relative to present‐day rates. Siderastrea siderea, the other most common species, was insensitive to both temperature and pCO₂ within the levels tested here. Reefs dominated by this species had the most stable end‐of‐century growth. Under more optimistic scenarios of reduced CO₂ emissions, calcification rates throughout the Florida Keys declined <20% by 2100. Under the most extreme emissions scenario, projected declines were highly variable among reefs, ranging 10–100%. Without considering bleaching, reef growth will likely decline on most reefs, especially where resistant species like S. siderea are not already dominant. This study demonstrates how species composition influences reef community responses to climate change and how reduced CO₂ emissions can limit future declines in reef calcification.     

Interacting effects of CO2 partial pressure and temperature on photosynthesis and calcification in a scleractinian coral

We show here that CO₂ partial pressure (pCO₂) and temperature significantly interact on coral physiology. The effects of increased pCO₂ and temperature on photosynthesis, respiration and calcification rates were investigated in the scleractinian coral Stylophora pistillata. Cuttings were exposed to temperatures of 25°C or 28°C and to pCO₂ values of ca. 460 or 760 μatm for 5 weeks. The contents of chlorophyll c₂ and protein remained constant throughout the experiment, while the chlorophyll a content was significantly affected by temperature, and was higher under the ‘high‐temperature–high‐pCO₂’ condition. The cell‐specific density was higher at ‘high pCO₂’ than at ‘normal pCO₂’ (1.7 vs. 1.4). The net photosynthesis normalized per unit protein was affected by both temperature and pCO₂, whereas respiration was not affected by the treatments. Calcification decreased by 50% when temperature and pCO₂ were both elevated. Calcification under normal temperature did not change in response to an increased pCO₂. This is not in agreement with numerous published papers that describe a negative relationship between marine calcification and CO₂. The confounding effect of temperature has the potential to explain a large portion of the variability of the relationship between calcification and pCO₂ reported in the literature, and warrants a re‐evaluation of the projected decrease of marine calcification by the year 2100.     

Down-regulation of Rubisco activity under combined increases of CO<Subscript>2</Subscript> and temperature minimized by changes in Rubisco k<Subscript>cat</Subscript> in wheat

Increases in growth temperature have been observed to affect photosynthesis differently under long-term exposure to ambient- and twice ambient-air CO₂ concentrations. This study investigates the causes of this interaction in wheat (Triticum aestivum L.) grown in the field over two consecutive years under temperature gradient chambers in ambient (370 μmol mol⁻¹) or elevated (700 μmol mol⁻¹) atmospheric CO₂ concentrations and at ambient or ambient +4°C temperatures, with either a low or a high nitrogen supply. The photosynthesis-internal CO₂ response curves and the activity, activation state, k_cat and amount of Ribulose-1, 5-bisphosphate carboxylase/oxygenase (Rubisco) were measured, as well as the soluble protein concentration in flag leaves at ear emergence and 8–15 days after anthesis. A high nitrogen supply increased V_cmax, the Rubisco amount and activity and soluble protein contents, but did not significantly change the Rubisco k_cat. Both elevated CO₂ and above ambient temperatures had negative effects on V_cmax and Rubisco activity, but at elevated CO₂, an increase in temperature did not decrease V_cmax or Rubisco activity in relation to ambient temperature. The amounts of Rubisco and soluble protein decreased with elevated CO₂ and temperature. The negative impact of elevated CO₂ on Rubisco properties was somewhat counteracted at elevated temperatures by an increase in k_cat. This effect can diminish the detrimental effects on photosynthesis of combined increases of CO₂ and temperature.     

Relative sensitivity of five Hawaiian coral species to high temperature under high-pCO2 conditions

Coral reef ecosystems are presently undergoing decline due to anthropogenic climate change. The chief detrimental factors are increased temperature and increased pCO₂. The purpose of this study was to evaluate the effect of these two stressors operating independently and in unison on the biological response of common Hawaiian reef corals. Manipulative experiments were performed using five species (Porites compressa, Pocillopora damicornis, Fungia scutaria, Montipora capitata, and Leptastrea purpurea) in a continuous-flow mesocosm system under natural sunlight conditions. Corals were grown together as a community under treatments of high temperature (2 °C above normal maximum summer temperature), high pCO₂ (twice present-day conditions), and with both factors acting in unison. Control corals were grown under present-day pCO₂ and at normal summer temperatures. Leptastrea purpurea proved to be an extremely hardy coral. No change in calcification or mortality occurred under treatments of high temperature, high pCO₂, or combined high temperature–high pCO₂. The remaining four species showed reduced calcification in the high-temperature treatment. Two species (L. purpurea and M. capitata) showed no response to increased pCO₂. Also, high pCO₂ ameliorated the negative effect of high temperature on the calcification rates of P. damicornis. Mortality was driven primarily by high temperature, with a negative synergistic effect in P. compressa only in the high-pCO₂–high-temperature treatment. Results support the observation that biological response to temperature and pCO₂ elevation is highly species-specific, so generalizations based on response of a single species might not apply to a diverse and complex coral reef community.

     

Larvae of the coral eating crown‐of‐thorns starfish,Acanthaster planci in a warmer‐high CO2 ocean

Outbreaks of crown‐of‐thorns starfish (COTS), Acanthaster planci, contribute to major declines of coral reef ecosystems throughout the Indo‐Pacific. As the oceans warm and decrease in pH due to increased anthropogenic CO₂ production_, coral reefs are also susceptible to bleaching, disease and reduced calcification. The impacts of ocean acidification and warming may be exacerbated by COTS predation, but it is not known how this major predator will fare in a changing ocean. Because larval success is a key driver of population outbreaks, we investigated the sensitivities of larval A. planci to increased temperature (2–4 °C above ambient) and acidification (0.3–0.5 pH units below ambient) in flow‐through cross‐factorial experiments (3 temperature × 3 pH/pCO₂ levels). There was no effect of increased temperature or acidification on fertilization or very early development. Larvae reared in the optimal temperature (28 °C) were the largest across all pH treatments. Development to advanced larva was negatively affected by the high temperature treatment (30 °C) and by both experimental pH levels (pH 7.6, 7.8). Thus, planktonic life stages of A. planci may be negatively impacted by near‐future global change. Increased temperature and reduced pH had an additive negative effect on reducing larval size. The 30 °C treatment exceeded larval tolerance regardless of pH. As 30 °C sea surface temperatures may become the norm in low latitude tropical regions, poleward migration of A. planci may be expected as they follow optimal isotherms. In the absence of acclimation or adaptation, declines in low latitude populations may occur. Poleward migration will be facilitated by strong western boundary currents, with possible negative flow‐on effects on high latitude coral reefs. The contrasting responses of the larvae of A. planci and those of its coral prey to ocean acidification and warming are considered in context with potential future change in tropical reef ecosystems.     

Ocean warming has a greater effect than acidification on the early life history development and swimming performance of a large circumglobal pelagic fish

Ocean warming and acidification are serious threats to marine life; however, their individual and combined effects on large pelagic and predatory fishes are poorly understood. We determined the effects of projected future temperature and carbon dioxide (CO₂) levels on survival, growth, morphological development and swimming performance on the early life stages of a large circumglobal pelagic fish, the yellowtail kingfish Seriola lalandi. Eggs, larvae and juveniles were reared in cross‐factored treatments of temperature (21 and 25°C) and pCO₂ (500 and 985 μatm) from fertilisation to 25 days post hatching (dph). Temperature had the greatest effect on survival, growth and development. Survivorship was lower, but growth and morphological development were faster at 25°C, with surviving fish larger and more developed at 1, 11 and 21 dph. Elevated pCO₂ affected size at 1 dph, but not at 11 or 21 dph, and did not affect survival or morphological development. Elevated temperature and pCO₂ had opposing effects on swimming performance at 21 dph. Critical swimming speed (U_crit) was increased by elevated temperature but reduced by elevated pCO₂. Additionally, elevated temperature increased the proportion of individuals that responded to a startle stimulus, reduced latency to respond and increased maximum escape speed, potentially due to the more advanced developmental stage of juveniles at 25°C. By contrast, elevated pCO₂ reduced the distance moved and average speed in response to a startle stimulus. Our results show that higher temperature is likely to be the primary driver of global change impacts on kingfish early life history; however, elevated pCO₂ could affect critical aspects of swimming performance in this pelagic species. Our findings will help parameterise and structure fisheries population dynamics models and improve projections of impacts to large pelagic fishes under climate change scenarios to better inform adaptation and mitigation responses.     

Comparison of the effects of symmetric and asymmetric temperature elevation and CO2 enrichment on yield and evapotranspiration of winter wheat (Triticum aestivum L.)

Under the changing climate, asymmetric warming pattern would be more likely during day and night time, instead of symmetric one. Concurrently, the growth responses and water use of plants may be different compared with those estimated based on symmetric warming. In this work, it was compared with the effects of symmetric (ETs) and asymmetric (ETa) elevation of temperature alone, and in interaction with elevated carbon dioxide concentration (EC), on the grain yield (GY) and evapotranspiration in winter wheat (Triticum aestivum L.) based on pot experiment in the North China Plain (NCP). The experiment was carried out in six enclosed‐top chambers with following climate treatments: (1) ambient temperature and ambient CO₂ (CON), (2) ambient temperature and elevated CO₂ (EC), (3) elevated temperature and ambient CO₂ (ETs; ETa), and (4) elevated temperature and elevated CO₂ (ECETs, ECETa). In symmetric warming, temperature was increased by 3°C and in asymmetric one by 3.5°C during night and 2.5°C during daytime, respectively. As a result, GY was in ETa and ETs 15.6 (P < 0.05) and 10.3% (P < 0.05) lower than that in CON. In ECETs and ECETa treatments, GY was 14.9 (P < 0.05) and 9.1% (P < 0.05) higher than that in CON. Opposite to GY, evapotranspiration was 7.8 (P < 0.05) and 17.9% (P < 0.05) higher in ETa and ETs treatments and 7.2 (P < 0.05) and 2.1% (P > 0.05) lower in ECETs and ECETa treatments compared with CON. Thus, GY of wheat could be expected to increase under the changing climate with concurrent elevation of CO₂ and temperature as a result of increased WUE under the elevated CO₂. However, the gain would be lower under ETa than that estimated based on ETs due to higher evapotranspiration.     

Response of the floating aquatic fern <Emphasis Type="Italic">Azolla filiculoides</Emphasis> to elevated CO<Subscript>2</Subscript>, temperature,and phosphorus levels

Azolla filiculoides is a floating aquatic fern growing in tropical and temperate freshwater ecosystems. As A. filiculoides has symbiotic nitrogen-fixing cyanobacteria (Anabaena azollae) within its leaf cavities, it is cultivated in rice paddies to improve N availability and suppress other wetland weeds. To understand how C assimilation and N accumulation in A. filiculoides respond to elevated atmospheric carbon dioxide concentration (CO₂) in combination with P addition and higher temperatures, we conducted pot experiments during the summer of 2007 and 2008. In 2007, we grew A. filiculoides in pots at two treatment levels of added P fertilizer and at two levels of [CO₂] (380 ppm for ambient and 680 ppm for elevated [CO₂]) in controlled-environment chambers. In 2008, we grew A. filiculoides in four controlled-environment chambers at two [CO₂] levels and two temperature levels (34/26°C (day/night) and 29/21°C). We found that biomass and C assimilation by A. filiculoides were significantly increased by elevated [CO₂], temperature, and P level (all P < 0.01), with a significant interaction between elevated [CO₂] and added P (P < 0.01). Tissue N content was decreased by elevated [CO₂] and increased by higher temperature and P level (all P < 0.01). The acetylene reduction assay showed that the N-fixation activity of A. filiculoides was not significantly different under ambient and elevated [CO₂] but was significantly stimulated by P addition. N-fixation activity decreased at higher temperatures (34/26°C), indicating that 29/21°C was more suitable for A. azollae growth. Therefore, we conclude that the N accumulation potential of A. filiculoides under future climate warming depends primarily on the temperature change and P availability, and C assimilation should be increased by elevated [CO₂].     

Adaptive evolution in the coccolithophore Gephyrocapsa oceanica following 1,000 generations of selection under elevated CO2

Coccolithophores are important oceanic primary producers not only in terms of photosynthesis but also because they produce calcite plates called coccoliths. Ongoing ocean acidification associated with changing seawater carbonate chemistry may impair calcification and other metabolic functions in coccolithophores. While short‐term ocean acidification effects on calcification and other properties have been examined in a variety of coccolithophore species, long‐term adaptive responses have scarcely been documented, other than for the single species Emiliania huxleyi. Here, we investigated the effects of ocean acidification on another ecologically important coccolithophore species, Gephyrocapsa oceanica, following 1,000 generations of growth under elevated CO₂ conditions (1,000 μatm). High CO₂‐selected populations exhibited reduced growth rates and enhanced particulate organic carbon (POC) and nitrogen (PON) production, relative to populations selected under ambient CO₂ (400 μatm). Particulate inorganic carbon (PIC) and PIC/POC ratios decreased progressively throughout the selection period in high CO₂‐selected cell lines. All of these trait changes persisted when high CO₂‐grown populations were moved back to ambient CO₂ conditions for about 10 generations. The results suggest that the calcification of some coccolithophores may be more heavily impaired by ocean acidification than previously predicted based on short‐term studies, with potentially large implications for the ocean's carbon cycle under accelerating anthropogenic influences.     

Effects of elevated CO2 on growth,calcification, and spectral dependence of photoinhibition in the coccolithophore Emiliania huxleyi (Prymnesiophyceae)1

We studied the effects of elevated CO₂ concentrations on cell growth, calcification, and spectral variation in the sensitivity of photosynthesis to inhibition by solar radiation in the globally important coccolithophore Emiliania huxleyi. Growth rates and chlorophyll a content per cell showed no significant differences between elevated (800 ppmv) and ambient (400 ppmv) CO₂ conditions. However, the production of organic carbon and the cell quotas for both carbon and nitrogen, increased under elevated CO₂ conditions, whilst particulate inorganic carbon production rates decreased under the same conditions. Biometric analyses of cells showed that coccoliths only presented significant differences due to treatments in the central area width. Most importantly, the size of the coccosphere decreased under elevated CO₂ conditions. The susceptibility of photosynthesis to inhibition by ultraviolet radiation (UVR) was estimated using biological weighting functions (BWFs) and a model that predicts photosynthesis under photosynthetically active radiation and UVR exposures. BWF results demonstrated that the sensitivity of photosynthesis to UVR was not significantly different between E. huxleyi cells grown under elevated and present CO₂ concentrations. We propose that the acclimation to elevated CO₂ conditions involves a physiological mechanism of regulation and allocation of energy and metabolites in the cell, which is also responsible for altering the sensitivity to UVR. In coccolithophores, this mechanism might be affected by the decrease in the calcification rates.     

Food availability outweighs ocean acidification effects in juvenile Mytilus edulis: laboratory and field experiments

Ocean acidification is expected to decrease calcification rates of bivalves. Nevertheless, in many coastal areas high pCO₂ variability is encountered already today. Kiel Fjord (Western Baltic Sea) is a brackish (12–20 g kg^?1) and CO₂ enriched habitat, but the blue mussel Mytilus edulis dominates the benthic community. In a coupled field and laboratory study we examined the annual pCO₂ variability in this habitat and the combined effects of elevated pCO₂ and food availability on juvenile M. edulis growth and calcification. In the laboratory experiment, mussel growth and calcification were found to chiefly depend on food supply, with only minor impacts of pCO₂ up to 3350 μatm. Kiel Fjord was characterized by strong seasonal pCO₂ variability. During summer, maximal pCO₂ values of 2500 μatm were observed at the surface and >3000 μatm at the bottom. However, the field growth experiment revealed seven times higher growth and calcification rates of M. edulis at a high pCO₂ inner fjord field station (mean pCO₂ ca. 1000 μatm) in comparison to a low pCO₂ outer fjord station (ca. 600 μatm). In addition, mussels were able to out‐compete the barnacle Amphibalanus improvisus at the high pCO₂ site. High mussel productivity at the inner fjord site was enabled by higher particulate organic carbon concentrations. Kiel Fjord is highly impacted by eutrophication, which causes bottom water hypoxia and consequently high seawater pCO₂. At the same time, elevated nutrient concentrations increase the energy availability for filter feeding organisms such as mussels. Thus, M. edulis can dominate over a seemingly more acidification resistant species such as A. improvisus. We conclude that benthic stages of M. edulis tolerate high ambient pCO₂ when food supply is abundant and that important habitat characteristics such as species interactions and energy availability need to be considered to predict species vulnerability to ocean acidification.     

Photoacclimatory and photoprotective responses to cold versus heat stress in high latitude reef corals

Corals at the world's southernmost coral reef of Lord Howe Island (LHI) experience large temperature and light fluctuations and need to deal with periods of cold temperature (<18°C), but few studies have investigated how corals are able to cope with these conditions. Our study characterized the response of key photophysiological parameters, as well as photoacclimatory and photoprotective pigments (chlorophylls, xanthophylls, and β‐carotene), to short‐term (5‐d) cold stress (~15°C; 7°C below control) in three LHI coral species hosting distinct Symbiodinium ITS2 types, and compared the coral–symbiont response to that under elevated temperature (~29°C; 7°C above control). Under cold stress, Stylophora sp. hosting Symbiodinium C118 showed the strongest effects with regard to losses of photochemical performance and symbionts. Pocillopora damicornis hosting Symbiodinium C100/C118 showed less severe bleaching responses to reduced temperature than to elevated temperature, while Porites heronensis hosting Symbiodinium C111* withstood both reduced and elevated temperature. Under cold stress, photoprotection in the form of xanthophyll de‐epoxidation increased in unbleached P. heronensis (by 178%) and bleached Stylophora sp. (by 225%), while under heat stress this parameter increased in unbleached P. heronensis (by 182%) and in bleached P. damicornis (by 286%). The xanthophyll pool size was stable in all species at all temperatures. Our comparative study demonstrates high variability in the bleaching vulnerability of these coral species to low and high thermal extremes and shows that this variability is not solely determined by the ability to activate xanthophyll de‐epoxidation.     

Effects of elevated CO2 and temperature on development and consumption rates of Octotoma championi and O. scabripennis feeding on Lantana camara

We carried out a factorial experiment to explore the effect of doubled CO₂ concentration and a 3 °C temperature increase on the development of a complete generation of the beetles Octotoma championi Baly and O. scabripennis Guérin‐Méneville (Coleoptera: Chrysomelidae). These species are biological control agents of Lantana camara L. (Verbenaceae), with a leaf‐mining larval phase and free‐living, leaf‐chewing adults. Plants grown at elevated CO₂ had enhanced above‐ground biomass, thicker leaves, reduced nitrogen concentration, and increased C:N ratios. Under the high temperature treatment, plants grown at ambient CO₂ suffered wilting and premature leaf loss, despite daily watering; this effect was ameliorated at elevated CO₂. The wilting of plants in the ambient CO₂/high temperature treatment reduced the emergence success of the beetles, particularly O. championi. Development time was accelerated by approximately 10–13 days at the higher temperature, but was not affected by CO₂. Neither CO₂ nor temperature affected adult beetle weight. Consumption rates of free‐living beetles were not affected by either CO₂ or temperature. By contrast, in the short‐term trials using excised foliage, beetles given no choice between ambient and elevated CO₂‐grown foliage, consumed more from ambient plants. When beetles were offered a choice between foliage grown at the two CO₂ levels, O. championi did not display a significant preference but O. scabripennis consumed more ambient CO₂‐grown foliage when feeding at the lower temperature. This study indicates that under future conditions of higher temperatures, amelioration of water stress in host plants growing in elevated CO₂ may benefit some endophagous insects by reducing premature leaf loss. Under some circumstances, this benefit may outweigh the deleterious effects of lower leaf nitrogen. Our results also indicate that foliage consumption under elevated CO₂ by mobile, adult insects on whole plants may not be significantly increased, as was previously indicated by short‐term experiments using excised foliage.     

Gas exchange and chlorophyll fluorescence responses of three south-western Yucca species to elevated CO2 and high temperature

The ability of seedlings to tolerate temperature extremes is important in determining the distribution of perennial plants in the arid south-western USA, and the manner in which elevated CO₂ impacts the ability of plants to tolerate high temperatures is relatively unknown. Whereas the effects of chronic high temperature (30–38°C) and elevated CO₂ are comparatively well understood, little research has assessed plant performance in elevated CO₂ during extreme (> 45 °C) temperature events. We exposed three species of Yucca to 360 and 700 μmol CO₂ mol^–1 for 8 months, then 9 d of high temperature (up to 53 °C) to evaluate the impacts of elevated CO₂ on the potential for photosynthetic function during external high temperature. Seedlings of a coastal C₃ species (Yucca whipplei), a desert C₃ species (Yucca brevifolia), and a desert CAM species (Yucca schidigera), were used to test for differences among functional groups. In general, Yuccas exposed to elevated CO₂ showed decreases in carboxylation efficiency as compared with plants grown at ambient before the initiation of high temperature. The coastal species (Y. whipplei) showed significant reductions (33%) in CO₂ saturated maximum assimilation rate (A_max), but the desert species (Y. brevifolia and Y. schidigera) showed no such reductions in A_max. Stomatal conductance was lower in elevated CO₂ as compared with ambient throughout the temperature event; however, there were species-specific differences over time. Elevated CO₂ enhanced photosynthesis in Y. whipplei at high temperatures for a period of 4 d, but not for Y. brevifolia or Y. schidigera. Elevated CO₂ offset photoinhibition (measured as F_v/F_m) in Y. whipplei as compared with ambient CO₂, depending on exposure time to high temperature. Stable F_v/F_m in Y. whipplei occurred in parallel with increases in the quantum yield of photosystem II (ΦPSII) at high temperatures in elevated CO₂. The value of ΦPSII remained constant or decreased with increasing temperature in all other treatment and species combinations. This suggests that the reductions in F_v/F_m resulted from thermal energy dissipation in the pigment bed for Y. brevifolia and Y. schidigera. The greater efficiency of photosystem II in Y. whipplei helped to maintain photosynthetic function at high temperatures in elevated CO₂. These patterns are in contrast to the hypothesis that high temperatures in elevated CO₂ would increase the potential for photoinhibition. Our results suggest that elevated CO₂ may offset high-temperature stress in coastal Yucca, but not in those species native to drier systems. Therefore, in the case of Y. whipplei, elevated CO₂ may allow plants to survive extreme temperature events, potentially relaxing the effects of high temperature on the establishment in novel habitats.     

No stimulation of nitrogen fixation by non‐filamentous diazotrophs under elevated CO2 in the South Pacific

Nitrogen fixation by diazotrophic cyanobacteria is a critical source of new nitrogen to the oligotrophic surface ocean. Research to date indicates that some diazotroph groups may increase nitrogen fixation under elevated pCO₂. To test this in natural plankton communities, four manipulation experiments were carried out during two voyages in the South Pacific (30–35^oS). High CO₂ treatments, produced using 750 ppmv CO₂ to adjust pH to 0.2 below ambient, and ‘Greenhouse’ treatments (0.2 below ambient pH and ambient temperature +3 °C), were compared with Controls in trace metal clean deckboard incubations in triplicate. No significant change was observed in nitrogen fixation in either the High CO₂ or Greenhouse treatments over 5 day incubations. qPCR measurements and optical microscopy determined that the diazotroph community was dominated by Group A unicellular cyanobacteria (UCYN‐A), which may account for the difference in response of nitrogen fixation under elevated CO₂ to that reported previously for Trichodesmium. This may reflect physiological differences, in that the greater cell surface area:volume of UCYN‐A and its lack of metabolic pathways involved in carbon fixation may confer no benefit under elevated CO₂. However, multiple environmental controls may also be a factor, with the low dissolved iron concentrations in oligotrophic surface waters limiting the response to elevated CO₂. If nitrogen fixation by UCYN‐A is not stimulated by elevated pCO₂, then future increases in CO₂ and warming may alter the regional distribution and dominance of different diazotroph groups, with implications for dissolved iron availability and new nitrogen supply in oligotrophic regions.     

Responses of Photosynthetic and Defence Systems to High Temperature Stress in Quercus suber L Seedlings Grown under Elevated CO2

Abstract: Growth in elevated CO₂ led to an increase in biomass production per plant as a result of enhanced carbon uptake and lower rates of respiration, compared to ambient CO₂-grown plants. No down-regulation of photosynthesis was found after six months of growth under elevated CO₂. Photosynthetic rates at 15°C or 35 °C were also higher in elevated than in ambient CO₂-grown plants, when measured at their respective CO₂ growth condition. Stomata of elevated CO₂-grown plants were less responsive to temperature as compared to ambient CO₂ plants. The after effect of a heat-shock treatment (4 h at 45 °C in a chamber with 80% of relative humidity and 800–1000 tmol m^-2 s^-1 photon flux density) on A_max was less in elevated than in ambient CO₂-grown plants. At the photochemical level, the negative effect of the heat-shock treatment was slightly more pronounced in ambient than in elevated CO₂-grown plants. A greater tolerance to oxidative stress caused by high temperatures in elevated CO₂-grown plants, in comparison to ambient CO₂ plants, is suggested by the increase in superoxide dismutase activity, after 1 h at 45 °C, as well as its relatively high activity after 2 and 4 h of the heat shock in the elevated CO₂-grown plants in contrast with the decrease to residual levels of superoxide dismutase activity in ambient CO₂-grown plants immediately after 1 h at 45 °C. The observed increase in catalase after 1 h at 45 °C in both ambient and elevated CO₂-grown plants, can be ascribed to the higher rates of photorespiration and respiration under this high temperature.     

Calcification is not the Achilles’ heel of cold‐water corals in an acidifying ocean

Ocean acidification is thought to be a major threat to coral reefs: laboratory evidence and CO₂ seep research has shown adverse effects on many coral species, although a few are resilient. There are concerns that cold‐water corals are even more vulnerable as they live in areas where aragonite saturation (Ω_ara) is lower than in the tropics and is falling rapidly due to CO₂ emissions. Here, we provide laboratory evidence that net (gross calcification minus dissolution) and gross calcification rates of three common cold‐water corals, Caryophyllia smithii, Dendrophyllia cornigera, and Desmophyllum dianthus, are not affected by pCO₂ levels expected for 2100 (pCO₂ 1058 μatm, Ω_ara 1.29), and nor are the rates of skeletal dissolution in D. dianthus. We transplanted D. dianthus to 350 m depth (pH_T 8.02; pCO₂ 448 μatm, Ω_ara 2.58) and to a 3 m depth CO₂ seep in oligotrophic waters (pH_T 7.35; pCO₂ 2879 μatm, Ω_ara 0.76) and found that the transplants calcified at the same rates regardless of the pCO₂ confirming their resilience to acidification, but at significantly lower rates than corals that were fed in aquaria. Our combination of field and laboratory evidence suggests that ocean acidification will not disrupt cold‐water coral calcification although falling aragonite levels may affect other organismal physiological and/or reef community processes.     

Effects of ocean acidification increase embryonic sensitivity to thermal extremes in Atlantic cod,Gadus morhua

Thermal tolerance windows serve as a powerful tool for estimating the vulnerability of marine species and their life stages to increasing temperature means and extremes. However, it remains uncertain to which extent additional drivers, such as ocean acidification, modify organismal responses to temperature. This study investigated the effects of CO₂‐driven ocean acidification on embryonic thermal sensitivity and performance in Atlantic cod, Gadus morhua, from the Kattegat. Fertilized eggs were exposed to factorial combinations of two PCO₂ conditions (400 μatm vs. 1100 μatm) and five temperature treatments (0, 3, 6, 9 and 12 °C), which allow identifying both lower and upper thermal tolerance thresholds. We quantified hatching success, oxygen consumption (MO₂) and mitochondrial functioning of embryos as well as larval morphometrics at hatch and the abundance of acid–base‐relevant ionocytes on the yolk sac epithelium of newly hatched larvae. Hatching success was high under ambient spawning conditions (3–6 °C), but decreased towards both cold and warm temperature extremes. Elevated PCO₂ caused a significant decrease in hatching success, particularly at cold (3 and 0 °C) and warm (12 °C) temperatures. Warming imposed limitations to MO₂ and mitochondrial capacities. Elevated PCO₂ stimulated MO₂ at cold and intermediate temperatures, but exacerbated warming‐induced constraints on MO₂, indicating a synergistic interaction with temperature. Mitochondrial functioning was not affected by PCO₂. Increased MO₂ in response to elevated PCO₂ was paralleled by reduced larval size at hatch. Finally, ionocyte abundance decreased with increasing temperature, but did not differ between PCO₂ treatments. Our results demonstrate increased thermal sensitivity of cod embryos under future PCO₂ conditions and suggest that acclimation to elevated PCO₂ requires reallocation of limited resources at the expense of embryonic growth. We conclude that ocean acidification constrains the thermal performance window of embryos, which has important implication for the susceptibility of cod to projected climate change.