The physiological response of marine diatoms to ocean acidification: differential roles of seawater pCO2 and pH

Although increasing the pCO₂ for diatoms will presumably down‐regulate the CO₂‐concentrating mechanism (CCM) to save energy for growth, different species have been reported to respond differently to ocean acidification (OA). To better understand their growth responses to OA, we acclimated the diatoms Thalassiosira pseudonana, Phaeodactylum tricornutum, and Chaetoceros muelleri to ambient (pCO₂ 400 μatm, pH 8.1), carbonated (pCO₂ 800 μatm, pH 8.1), acidified (pCO₂ 400 μatm, pH 7.8), and OA (pCO₂ 800 μatm, pH 7.8) conditions and investigated how seawater pCO₂ and pH affect their CCMs, photosynthesis, and respiration both individually and jointly. In all three diatoms, carbonation down‐regulated the CCMs, while acidification increased both the photosynthetic carbon fixation rate and the fraction of CO₂ as the inorganic carbon source. The positive OA effect on photosynthetic carbon fixation was more pronounced in C. muelleri, which had a relatively lower photosynthetic affinity for CO₂, than in either T. pseudonana or P. tricornutum. In response to OA, T. pseudonana increased respiration for active disposal of H⁺ to maintain its intracellular pH, whereas P. tricornutum and C. muelleri retained their respiration rate but lowered the intracellular pH to maintain the cross‐membrane electrochemical gradient for H⁺ efflux. As the net result of changes in photosynthesis and respiration, growth enhancement to OA of the three diatoms followed the order of C. muelleri > P. tricornutum > T. pseudonana. This study demonstrates that elucidating the separate and joint impacts of increased pCO₂ and decreased pH aids the mechanistic understanding of OA effects on diatoms in the future, acidified oceans.     

Temperature amplifies the effect of high CO2 on the photosynthesis,respiration, and calcification of the coralline algae Phymatolithon lusitanicum

The combination of ocean acidification (OA) and global warming is expected to have a significant effect on the diversity and functioning of marine ecosystems, particularly on calcifying algae such as rhodoliths (maërl) that form extensive beds worldwide, from polar to tropical regions. In addition, the increasing frequency of extreme events, such as heat waves, threatens coastal ecosystems and may affect their capacity to fix blue carbon. The few studies where the simultaneous effects of both temperature and CO₂ were investigated have revealed contradictory results. To assess the effect that high temperature spells can have on the maërl beds under OA, we tested the short‐time effects of temperature and CO₂ on the net photosynthesis, respiration, and calcification of the recently described species Phymatolithon lusitanicum, the most common maërl species of southern Portugal. Photosynthesis, calcification, and respiration increased with temperature, and the differences among treatments were enhanced under high CO₂. We found that in the short term, the metabolic rates of Phymatolithon lusitanicum will increase with CO₂ and temperature as will the coupling between calcification and photosynthesis. However, under high CO₂, this coupling will favor photosynthesis over calcification, which, in the long term, can have a negative effect on the blue carbon fixing capacity of the maërl beds from southern Portugal.     

CO2‐concentrating mechanisms in three southern hemisphere strains of Emiliania huxleyi

Rising global CO₂ is changing the carbonate chemistry of seawater, which is expected to influence the way phytoplankton acquire inorganic carbon. All phytoplankton rely on ribulose‐bisphosphate carboxylase oxygenase (RUBISCO) for assimilation of inorganic carbon in photosynthesis, but this enzyme is inefficient at present day CO₂ levels. Many algae have developed a range of energy demanding mechanisms, referred to as carbon concentrating mechanisms (CCMs), which increase the efficiency of carbon acquisition. We investigated CCM activity in three southern hemisphere strains of the coccolithophorid Emiliania huxleyi W. W. Hay & H. P. Mohler. Both calcifying and non‐calcifying strains showed strong CCM activity, with HCO₃^? as a preferred source of photosynthetic carbon in the non‐calcifying strain, but a higher preference for CO₂ in the calcifying strains. All three strains were characterized by the presence of pyrenoids, external carbonic anhydrase (CA) and high affinity for CO₂ in photosynthesis, indicative of active CCMs. We postulate that under higher CO₂ levels cocco‐lithophorids will be able to down‐regulate their CCMs, and re‐direct some of the metabolic energy to processes such as calcification. Due to the expected rise in CO₂ levels, photosynthesis in calcifying strains is expected to benefit most, due to their use of CO₂ for carbon uptake. The non‐calcifying strain, on the other hand, will experience only a 10% increase in HCO₃^?, thus making it less responsive to changes in carbonate chemistry of water.     

The role of external carbonic anhydrase in photosynthesis during growth of the marine diatom Chaetoceros muelleri

Carbon dioxide concentrating mechanisms (CCMs) act to improve the supply of CO₂ at the active site of ribulose‐1,5‐bisphosphate carboxylase/oxygenase. There is substantial evidence that in some microalgal species CCMs involve an external carbonic anhydrase (CA_ext) and that CA_ext activity is induced by low CO₂ concentrations in the growth medium. However, much of this work has been conducted on cells adapted to air‐equilibrium concentrations of CO₂, rather than to changing CO₂ conditions caused by growing microalgal populations. We investigated the role of CA_ext in inorganic carbon (C_i) acquisition and photosynthesis at three sampling points during the growth cycle of the cosmopolitan marine diatom Chaetoceros muelleri. We observed that CA_ext activity increased with decreasing C_i, particularly CO₂, concentration, supporting the idea that CA_ext is modulated by external CO₂ concentration. Additionally, we found that the contribution of CA_ext activity to carbon acquisition for photosynthesis varies over time, increasing between the first and second sampling points before decreasing at the last sampling point, where external pH was high. Lastly, decreases in maximum quantum yield of photosystem II (F_v/F_m), chlorophyll, maximum relative electron transport rate, light harvesting efficiency (α) and maximum rates of C_i‐ saturated photosynthesis (V_max) were observed over time. Despite this decrease in photosynthetic capacity an up‐regulation of CCM activity, indicated by a decreasing half‐saturation constant for CO₂ (K_0.5CO₂), occurred over time. The flexibility of the CCM during the course of growth in C. muelleri may contribute to the reported dominance and persistence of this species in phytoplankton blooms.     

Interactive effects of temperature and pCO2 on sponges: from the cradle to the grave

As atmospheric CO₂ concentrations rise, associated ocean warming (OW) and ocean acidification (OA) are predicted to cause declines in reef‐building corals globally, shifting reefs from coral‐dominated systems to those dominated by less sensitive species. Sponges are important structural and functional components of coral reef ecosystems, but despite increasing field‐based evidence that sponges may be ‘winners’ in response to environmental degradation, our understanding of how they respond to the combined effects of OW and OA is limited. To determine the tolerance of adult sponges to climate change, four abundant Great Barrier Reef species were experimentally exposed to OW and OA levels predicted for 2100, under two CO₂ Representative Concentration Pathways (RCPs). The impact of OW and OA on early life‐history stages was also assessed for one of these species to provide a more holistic view of species impacts. All species were generally unaffected by conditions predicted under RCP6.0, although environmental conditions projected under RCP8.5 caused significant adverse effects: with elevated temperature decreasing the survival of all species, increasing levels of tissue necrosis and bleaching, elevating respiration rates and decreasing photosynthetic rates. OA alone had little adverse effect, even under RCP8.5 concentrations. Importantly, the interactive effect of OW and OA varied between species with different nutritional modes, with elevated pCO₂ exacerbating temperature stress in heterotrophic species but mitigating temperature stress in phototrophic species. This antagonistic interaction was reflected by reduced mortality, necrosis and bleaching of phototrophic species in the highest OW/OA treatment. Survival and settlement success of Carteriospongia foliascens larvae were unaffected by experimental treatments, and juvenile sponges exhibited greater tolerance to OW than their adult counterparts. With elevated pCO₂ providing phototrophic species with protection from elevated temperature, across different life stages, climate change may ultimately drive a shift in the composition of sponge assemblages towards a dominance of phototrophic species.     

Decreased photosynthesis and growth with reduced respiration in the model diatom Phaeodactylum tricornutum grown under elevated CO2 over 1800 generations

Studies on the long‐term responses of marine phytoplankton to ongoing ocean acidification (OA) are appearing rapidly in the literature. However, only a few of these have investigated diatoms, which is disproportionate to their contribution to global primary production. Here we show that a population of the model diatom Phaeodactylum tricornutum, after growing under elevated CO₂ (1000 μatm, HCL, pH_T: 7.70) for 1860 generations, showed significant differences in photosynthesis and growth from a population maintained in ambient CO₂ and then transferred to elevated CO₂ for 20 generations (HC). The HCL population had lower mitochondrial respiration, than did the control population maintained in ambient CO₂ (400 μatm, LCL, pH_T: 8.02) for 1860 generations. Although the cells had higher respiratory carbon loss within 20 generations under the elevated CO₂, being consistent to previous findings, they downregulated their respiration to sustain their growth in longer duration under the OA condition. Responses of phytoplankton to OA may depend on the timescale for which they are exposed due to fluctuations in physiological traits over time. This study provides the first evidence that populations of the model species, P. tricornutum, differ phenotypically from each other after having been grown for differing spans of time under OA conditions, suggesting that long‐term changes should be measured to understand responses of primary producers to OA, especially in waters with diatom‐dominated phytoplankton assemblages.     

Atmospheric CO2 mole fraction affects stand‐scale carbon use efficiency of sunflower by stimulating respiration in light

Plant carbon‐use‐efficiency (CUE), a key parameter in carbon cycle and plant growth models, quantifies the fraction of fixed carbon that is converted into net primary production rather than respired. CUE has not been directly measured, partly because of the difficulty of measuring respiration in light. Here, we explore if CUE is affected by atmospheric CO₂. Sunflower stands were grown at low (200 μmol mol^?1) or high CO₂ (1000 μmol mol^?1) in controlled environment mesocosms. CUE of stands was measured by dynamic stand‐scale ¹³C labelling and partitioning of photosynthesis and respiration. At the same plant age, growth at high CO₂ (compared with low CO₂) led to 91% higher rates of apparent photosynthesis, 97% higher respiration in the dark, yet 143% higher respiration in light. Thus, CUE was significantly lower at high (0.65) than at low CO₂ (0.71). Compartmental analysis of isotopic tracer kinetics demonstrated a greater commitment of carbon reserves in stand‐scale respiratory metabolism at high CO₂. Two main processes contributed to the reduction of CUE at high CO₂: a reduced inhibition of leaf respiration by light and a diminished leaf mass ratio. This work highlights the relevance of measuring respiration in light and assessment of the CUE response to environment conditions.     

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.     

Effects of CO2 enrichment on whole-plant carbon budget of seedlings of Fagus grandifolia and Acer saccharum in low irradiance

Carbon exchange rates (CER) and whole-plant carbon balances of beech (Fagus grandifolia) and sugar maple (Acer saccharum) were compared for seedlings grown under low irradiance to determine the effects of atmospheric CO₂ enrichment on shade-tolerant seedlings of co-dominant species. Under contemporary atmospheric CO₂, photosynthetic rate per unit mass of beech was lower than for sugar maple, and atmospheric CO₂ enrich ment enhanced photosynthesis for beech only. Aboveground respiration per unit mass decreased with CO₂ enrichment for both species while root respiration per unitmass decreased for sugar maple only. Under contemporary atmoapheric CO₂, beech had lower C uptake per plant than sugar maple, while C losses per plant to nocturnal aboveground and root respiration were similar for both species. Under elevated CO₂, C uptake per plant was similar for both species, indicating a significant relative increase in whole-seedling CER with CO₂ enrich ment for beech but not for sugar maple. Total C loss per plant to aboveground respiration was decreased for beech only because increase in sugar maple leaf mass counterbalanced a reduction in respiration rates. Carbon loss to root respiration per plant was not changed by CO₂ enrichment for either species. However, changes in maintenance respiration cost and nitrogen level suggest changes in tissue composition with elevated CO₂. Beech had a greater net daily C gain with CO₂ enrichment than did sugar maple in contrast to a lower one under contemporary CO₂. Elevated CO₂ preferentially enhances the net C balance of beech by increasing photosynthesis and reducing respiration cost. In all cases, the greatest C lost was by roots, indicating the importance of belowground biomass in net C gain. Relative growth rate estimated from biomass accumulation was not affected by CO₂ enrichment for either species possibly because of slow growth under low light. This study indicates the importance of direct effects of CO₂ enrichment when predicting potential change in species distribution with global climate change.     

Energy costs of carbon dioxide concentrating mechanisms in aquatic organisms

Minimum energy (as photon) costs are predicted for core reactions of photosynthesis, for photorespiratory metabolism in algae lacking CO₂ concentrating mechanisms (CCMs) and for various types of CCMs; in algae, with CCMs; allowance was made for leakage of CO₂ from the internal pool. These predicted values are just compatible with the minimum measured photon costs of photosynthesis in microalgae and macroalgae lacking or expressing CCMs. More energy-expensive photorespiration, for example for organisms using Rubiscos with lower CO₂–O₂ selectivity coefficients, would be less readily accommodated within the lowest measured photon costs of photosynthesis by algae lacking CCMs. The same applies to the cases of CCMs with higher energy costs of active transport of protons or inorganic carbon species, or greater allowance for significant leakage from the accumulated intracellular pool of CO₂. High energetic efficiency can involve a higher concentration of catalyst to achieve a given rate of reaction, adding to the resource costs of growth. There are no obvious mechanistic interpretations of the occurrence of CCMs algae adapted to low light and low temperatures using the rationales adopted for the occurrence of C₄ photosynthesis in terrestrial flowering plants. There is an exception for cyanobacteria with low-selectivity Form IA or IB Rubiscos, and those dinoflagellates with low-selectivity Form II Rubiscos, for which very few natural environments have high enough CO₂:O₂ ratios to allow photosynthesis in the absence of CCMs.     

Acclimation of bloom‐forming and perennial seaweeds to elevated pCO2 conserved across levels of environmental complexity

Macroalgae contribute approximately 15% of the primary productivity in coastal marine ecosystems, fix up to 27.4 Tg of carbon per year, and provide important structural components for life in coastal waters. Despite this ecological and commercial importance, direct measurements and comparisons of the short‐term responses to elevated pCO₂ in seaweeds with different life‐history strategies are scarce. Here, we cultured several seaweed species (bloom forming/nonbloom forming/perennial/annual) in the laboratory, in tanks in an indoor mesocosm facility, and in coastal mesocosms under pCO₂ levels ranging from 400 to 2,000 μatm. We find that, across all scales of the experimental setup, ephemeral species of the genus Ulva increase their photosynthesis and growth rates in response to elevated pCO₂ the most, whereas longer‐lived perennial species show a smaller increase or a decrease. These differences in short‐term growth and photosynthesis rates are likely to give bloom‐forming green seaweeds a competitive advantage in mixed communities, and our results thus suggest that coastal seaweed assemblages in eutrophic waters may undergo an initial shift toward communities dominated by bloom‐forming, short‐lived seaweeds.     

STEM AND LEAF GAS EXCHANGE OF TWO ARID LAND SHRUBS

Gas exchange studies were conducted on two shrub species found in cool shrub-steppe communities of the American West, big sagebrush (Artemisia tridentata subsp. tridentata Nutt.) and broom snakeweed (Gutierrezia sarothrae [Pursh] Britt. and Rusby), with a goal of evaluating characteristics and relative contributions of green stem and leaf material to total shoot CO₂ exchange at different temperatures. Variations in tissue temperature exerted a pronounced effect on CO₂ exchange—net photosynthesis and dark respiration—of green stems and leaves of both species. Definite temperature optima of net photosynthesis were noted, and dark respiration rates consistently increased with increases in temperature. Green stems of both species exhibited sizable dark respiration rates, although stem rates at all temperatures were lower than corresponding leaf rates. Artemisia tridentata did not exhibit sizeable green stem net photosynthesis even under conditions of optimal temperature and water availability, and leaf net photosynthesis rates were much lower than those of G. sarothrae. However, A. tridentata in general possessed a greater leaf biomass than G. sarothrae. Green stems of G. sarothrae exhibited considerable rates of net photosynthesis under both optimal and sub-optimal temperature and water availability conditions. A higher optimum temperature of net photosynthesis was noted for stems than for leaves of G. sarothrae. The adaptive significance of these interspecific differences in CO₂ exchange characteristics is discussed.     

The dependence of respiration on photosynthetic substrate supply and temperature: integrating leaf, soil and ecosystem measurements

Interactions between photosynthetic substrate supply and temperature in determining the rate of three respiration components (leaf, belowground and ecosystem respiration) were investigated within three environmentally controlled, Populus deltoides forest bays at Biosphere 2, Arizona. Over 2 months, the atmospheric CO₂ concentration and air temperature were manipulated to test the following hypotheses: (1) the responses of the three respiration components to changes in the rate of photosynthesis would differ both in speed and magnitude; (2) the temperature sensitivity of leaf and belowground respiration would increase in response to a rise in substrate availability; and, (3) at the ecosystem level, the ratio of respiration to photosynthesis would be conserved despite week‐to‐week changes in temperature. All three respiration rates responded to the CO₂ concentration‐induced changes in photosynthesis. However, the proportional change in the rate of leaf respiration was more than twice that of belowground respiration and, when photosynthesis was reduced, was also more rapid. The results suggest that aboveground respiration plays a key role in the overall response of ecosystem respiration to short‐term changes in canopy photosynthesis. The short‐term temperature sensitivity of leaf respiration, measured within a single night, was found to be affected more by developmental conditions than photosynthetic substrate availability, as the Q₁₀ was lower in leaves that developed at high CO₂, irrespective of substrate availability. However, the temperature sensitivity of belowground respiration, calculated between periods of differing air temperature, appeared to be positively correlated with photosynthetic substrate availability. At the ecosystem level, respiration and photosynthesis were positively correlated but the relationship was affected by temperature; for a given rate of daytime photosynthesis, the rate of respiration the following night was greater at 25 than 20°C. This result suggests that net ecosystem exchange did not acclimate to temperature changes lasting up to 3 weeks. Overall, the results of this study demonstrate that the three respiration terms differ in their dependence on photosynthesis and that, short‐ and medium‐term changes in temperature may affect net carbon storage in terrestrial ecosystems.     

Direct and indirect effects of elevated CO2 on leaf respiration in a forest ecosystem

We measured the short‐term direct and long‐term indirect effects of elevated CO₂ on leaf dark respiration of loblolly pine (Pinus taeda) and sweetgum (Liquidambar styraciflua) in an intact forest ecosystem. Trees were exposed to ambient or ambient + 200 µmol mol^?1 atmospheric CO₂ using free‐air carbon dioxide enrichment (FACE) technology. After correcting for measurement artefacts, a short‐term 200 µmol mol^?1 increase in CO₂ reduced leaf respiration by 7–14% for sweetgum and had essentially no effect on loblolly pine. This direct suppression of respiration was independent of the CO₂ concentration under which the trees were grown. Growth under elevated CO₂ did not appear to have any long‐term indirect effects on leaf maintenance respiration rates or the response of respiration to changes in temperature (Q₁₀, R₀). Also, we found no relationship between mass‐based respiration rates and leaf total nitrogen concentrations. Leaf construction costs were unaffected by growth CO₂ concentration, although leaf construction respiration decreased at elevated CO₂ in both species for leaves at the top of the canopy. We conclude that elevated CO₂ has little effect on leaf tissue respiration, and that the influence of elevated CO₂ on plant respiratory carbon flux is primarily through increased biomass.     

Carbon dioxide assimilation in the flowerhead of Arctium

Summary The gas exchange of flowerheads was determined in Arctium tomentosum and A. lappa during their development. The light, temperature and CO₂ responses were used to estimate flowerhead photosynthesis and the in situ contribution of carbon assimilation to the carbon requirement of the plant for supporting a flowerhead. Changes in vapour pressure deficit had no effect on flowerhead photosynthesis rates and were not included in the model.In both species assimilatory capacity correlated with total bract chlorophyll content. Light, temperature and CO₂ response curves were very similar in form between species, differing only in absolute rates. During all stages of development, flowerheads always exhibited a net carbon loss, which was mainly determined by temperature. The respiration rate decreased in the light, the difference of CO₂ exchange in the dark and in the light was interpreted as photosynthesis. This rate was larger in A. lappa than in A. tomentosum. 30% of the total C requirement of A. lappa flowerheads was photosynthesized by its bracts, the total contribution offlowerhead photosynthesis in A. tomentosum was only 15%. The potential competitive advantages of variation in flowerhead photosynthesis are discussed.     

The influence of water content and leaf anatomy on carbon isotope discrimination and photosynthesis in Sphagnum

The relative effect of diffusional resistance due to water films (r_wf) and leaf anatomy (r_p) on rates of net photosynthesis and on-line measures of carbon isotope discrimination (Δ=Δδ¹³C) was investigated in Sphagnum. Sphagnum species differ in the exposure of photosynthetic cells at the leaf surface. In S. affine, photosynthetic cells are widely exposed at the surface, whereas in S. magellanicum, photo-synthetic cells are enclosed within water-filled hyaline cells. This difference is expected to lead to variation in diffusive resistance within leaves (r_p). Net photosynthesis and on-line Δ were measured at two water contents: greenhouse water content (wet) and blotted dry (dry). Without correcting for respiration, on-line Δ values differed significantly between wet (23.7%_o) and dry (30.9%_o) plants. However, there was no significant difference between species means and no species × water content interaction. Corrections for respiration lowered Δ values by approximately 8.1%_o and reduced the mean difference to 3.1%_o, but did not alter the rank order of treatments. Net photosynthesis also decreased by 16% in wet plants, but there was no significant difference between the two species. In addition, five populations of S. affine and S. magellanicum grown in a common garden were analysed for their organic matter carbon isotope composition (δ¹³C). These values varied more within each species (0.9–1.2%_o) than between the two species (0.6%_o). Therefore, we conclude that variation in surface water films leads to a greater difference in resistance to CO₂ uptake and carbon isotope discrimination than that due to variation in leaf anatomical properties in Sphagnum.     

SHORT‐ AND LONG‐TERM EFFECTS OF ELEVATED CO2 ON PHOTOSYNTHESIS AND RESPIRATION IN THE MARINE MACROALGA HIZIKIA FUSIFORMIS (SARGASSACEAE,PHAEOPHYTA) GROWN AT LOW AND HIGH N SUPPLIES1

The short‐term and long‐term effects of elevated CO₂ on photosynthesis and respiration were examined in cultures of the marine brown macroalga Hizikia fusiformis (Harv.) Okamura grown under ambient (375 μL · L^?1) and elevated (700 μL · L^?1) CO₂ concentrations and at low and high N availability. Short‐term exposure to CO₂ enrichment stimulated photosynthesis, and this stimulation was maintained with prolonged growth at elevated CO₂, regardless of the N levels in culture, indicating no down‐regulation of photosynthesis with prolonged growth at elevated CO₂. However, the photosynthetic rate of low‐N‐grown H. fusiformis was more responsive to CO₂ enrichment than that of high‐N‐grown algae. Elevation of CO₂ concentration increased the value of K_1/2(Ci) (the half‐saturation constant) for photosynthesis, whereas high N supply lowered it. Neither short‐term nor long‐term CO₂ enrichment had inhibitory effects on respiration rate, irrespective of the N supply, under which the algae were grown. Under high‐N growth, the Q₁₀ value of respiration was higher in the elevated‐CO₂‐grown algae than the ambient‐CO₂‐grown algae. Either short‐ or long‐term exposure to CO₂ enrichment decreased respiration as a proportion of gross photosynthesis (Pg) in low‐N‐grown H. fusiformis. It was proposed that in a future world of higher atmospheric CO₂ concentration and simultaneous coastal eutrophication, the respiratory carbon flux would be more sensitive to changing temperature.     

Sea anemones may thrive in a high CO2 world

Increased seawater pCO₂, and in turn ‘ocean acidification’ (OA), is predicted to profoundly impact marine ecosystem diversity and function this century. Much research has already focussed on calcifying reef‐forming corals (Class: Anthozoa) that appear particularly susceptible to OA via reduced net calcification. However, here we show that OA‐like conditions can simultaneously enhance the ecological success of non‐calcifying anthozoans, which not only play key ecological and biogeochemical roles in present day benthic ecosystems but also represent a model organism should calcifying anthozoans exist as less calcified (soft‐bodied) forms in future oceans. Increased growth (abundance and size) of the sea anemone (Anemonia viridis) population was observed along a natural CO₂ gradient at Vulcano, Italy. Both gross photosynthesis (P_G) and respiration (R) increased with pCO₂ indicating that the increased growth was, at least in part, fuelled by bottom up (CO₂ stimulation) of metabolism. The increase of P_G outweighed that of R and the genetic identity of the symbiotic microalgae (Symbiodinium spp.) remained unchanged (type A19) suggesting proximity to the vent site relieved CO₂ limitation of the anemones' symbiotic microalgal population. Our observations of enhanced productivity with pCO₂, which are consistent with previous reports for some calcifying corals, convey an increase in fitness that may enable non‐calcifying anthozoans to thrive in future environments, i.e. higher seawater pCO₂. Understanding how CO₂‐enhanced productivity of non‐ (and less‐) calcifying anthozoans applies more widely to tropical ecosystems is a priority where such organisms can dominate benthic ecosystems, in particular following localized anthropogenic stress.     

Sensitivity of C Sequestration in Reintroduced Sphagnum to Water-Level Variation in a Cutaway Peatland

The reintroduction of Sphagnum fragments has been found to be a promising method for restoring mire vegetation in a cutaway peatland. Although it is known that moisture controls Sphagnum photosynthesis, information concerning the sensitivity of carbon dynamics on water‐level variation is still scarce. In a 4‐year field experiment, we studied the carbon dynamics of reintroduced Sphagnum angustifolium material in a restored (rewetted) cutaway peatland. Cutaway peatland restored by Sphagnum reintroduction showed high sensitivity to variation in water level. Water level controlled both photosynthesis and respiration. Gross photosynthesis (P_G) had a unimodal response to water‐level variation with optimum level at ?12 cm. The range of water level for high P_G (above 60% of the maximum light‐saturated P_G) was between 22 and 1 cm below soil surface. Water level had a dual effect on total respiration. When the water level was below soil surface, peat respiration increased rapidly along the lowering water level until the respiration rate started to slow down at approximately ?30 cm. Contrary to peat respiration, the response of Sphagnum respiration to water‐level variation resembled that of photosynthesis with an optimum at ?12 cm. In optimal conditions, Sphagnum reintroduction turned the cutaway site from carbon source to a sink of 23 g C/m² per season (mid‐May to the end of September). In dry conditions, lowered photosynthesis together with the higher peat respiration led to a net loss of 56 g C/m². Although the water level above the optimum amplitude restricted CO₂ fixation, a decrease in peat respiration led to a positive CO₂ balance of 9 g C/m².     

Can Gas-Exchange Characteristics help Explain the Invasive Success of Lythrum salicaria?

Since its introduction to North America, Lythrum salicaria (L.) (purple loosestrife) has become invasive in marshy and riparian habitats. We compared gas-exchange responses to external CO₂ partial pressure and light, as well as related leaf structural and biochemical characteristics, of L. salicaria with those of co-occurring native Asclepias syriaca (common milkweed) and Solidago graminifolia (lance-leaved goldenrod) along a pond bank in the Black Rock Forest, Cornwall, New York, USA to examine if the invasive success of L. salicaria may be influenced by robust leaf gas-exchange characteristics, including relatively high rates of photosynthesis and low rates of respiration, compared with those of less successful co-occurring native plant species. Neither the mean rate of net photosynthesis measured at ambient CO₂ and saturating photon flux density (A) nor the mean dark respiration rate (R_D) differed significantly between L. salicaria and either of the native species, while both the mean maximum rate of photosynthesis at saturating CO₂ concentration and photon flux density (A _max) and the mean rate of respiration measured in light (R_L) were significantly higher in L. salicaria than A. syriaca, but no different between L. salicaria and S. graminifolia. Likewise, photosynthetic nitrogen-use efficiency was greater in L. salicaria than A. syriaca only, while photosynthetic water-use efficiency was significantly less in both L. salicaria and S. graminifolia than in A. syriaca. Despite limited interspecific differences in leaf photosynthesis, respiration, and resource-use efficiency, particularly between L. salicaria and S. graminifolia, we found that L. salicaria assimilated 208% more carbon per unit of energy invested in leaf biomass than either of the co-occurring native species, suggesting that increased photosynthetic energy-use efficiency may influence its observed invasive success. This revised version was published online in July 2006 with corrections to the Cover Date.