Regulation of carbon metabolic fluxes in response to CO<Subscript>2</Subscript> supplementation in phototrophic <Emphasis Type="Italic">Chlorella vulgaris</Emphasis>: a cytomic and biochemical study

As one of the promising species of microalgae for biofuel production, Chlorella vulgaris CS-42 was cultivated phototrophically in two cylindrical photobioreactors with aeration of 5 % (v/v) CO₂ or air for 13 days to evaluate the effects of CO₂ supplementation on biomass, CO₂ fixation performance, and biochemical content. Significant increases of specific growth rate and total carbon content in biomass resulting in a higher CO₂ fixation rate were found with 5 % CO₂. The maximum biomass concentration, carbohydrate and fatty acid contents with 5 % CO₂ were significantly higher than those with air, while carbohydrate biosynthesis was most affected as compared to other biochemical components. Cytomic analysis revealed a rapid accumulation of neutral lipid in the late growth phase with more lipid bodies visualized by confocal laser scanning microscopy (CLSM), when nitrate consumption was accelerated with CO₂ supplementation. Gas chromatography mass spectrometry (GC-MS) analysis indicated that 5 % CO₂ favored the formation of C18:2, which led to a decrease in the degree of lipid unsaturation (DLU). These results proved that CO₂ supplementation was one of the most efficient methods to significantly prompt the growth of microalgae and increase the C/N ratio in the medium, which in turn regulated the carbon metabolic flux to enhance neutral lipid and fatty acid production in C. vulgaris.     

Effects of NaCl or Na<Subscript>2</Subscript>SO<Subscript>4</Subscript> salinity on plant growth,ion content and photosynthetic activity in <Emphasis Type="Italic">Ocimum basilicum</Emphasis> L.

Basil (Ocimum basilicum L., cultivar Genovese) plants were grown in Hoagland solution with or without 50 mM NaCl or 25 mM Na₂SO₄. After 15 days of treatment, Na₂SO₄ slowed growth of plants as indicated by root, stem and leaf dry weight, root length, shoot height and leaf area, and the effects were major of those induced by NaCl. Photosynthetic response was decreased more by chloride salinity than by sulphate. No effects in both treatments on leaf chlorophyll content, maximal efficiency of PSII photochemistry (F _v/F _m) and electron transport rate (ETR) were recorded. Therefore, an excess of energy following the limitation to CO₂ photoassimilation and a down regulation of PSII photochemistry was monitored under NaCl, which displays mechanisms that play a role in avoiding PSII photodamage able to dissipate this excess energy. Ionic composition (Na⁺, K⁺, Ca²⁺, and Mg²⁺) was affected to the same extent under both types of salinity, thus together with an increase in leaves Cl⁻, and roots SO₄ ²⁻ in NaCl and Na₂SO₄-treated plants, respectively, may have resulted in the observed growth retardation (for Na₂SO₄ treatment) and photosynthesis activity inhibition (for NaCl treatment), suggesting that those effects seem to have been due to the anionic component of the salts.     

DFT study of CO<Subscript>2</Subscript> and H<Subscript>2</Subscript>O co-adsorption on carbon models of coal surface

The moisture content of coal affects the adsorption capacity of CO₂ on the coal surface. Since the hydrogen bonds are formed between H₂O and oxygen functional group, the H₂O cluster more easily adsorbs on the coal micropore than CO₂ molecule. The coal micropores are occupied by H₂O molecules that cannot provide extra space for CO₂ adsorption, which may leads to the reduction of CO₂ adsorption capacity. However, without considering factors of micropore and oxygen functional groups, the co-adsorption mechanisms of CO₂ and adsorbed H₂O molecule are not clear. Density functional theory (DFT) calculations were performed to elucidate the effect of adsorbed H₂O to CO₂ adsorption. This study reports some typical coal-H₂O···CO₂ complexes, along with a detailed analysis of the geometry, energy, electrostatic potential (ESP), atoms in molecules (AIM), reduced density gradient (RDG), and energy decomposition analysis (EDA). The results show that H₂O molecule can more stably adsorb on the aromatic ring surface than CO₂ molecule, and the absolute values of local ESP maximum and minimum of H₂O cluster are greater than CO₂. AIM analysis shows a detailed interaction path and strength between atoms in CO₂ and H₂O, and RDG analysis shows that the interactions among CO₂, H₂O, and coal model belong to weak van der Waals force. EDA indicates that electrostatic and long-range dispersion terms play a primary role in the co-adsorption of CO₂ and H₂O. According to the DFT calculated results without considering micropore structure and functional group, it is shown that the adsorbed H₂O can promote CO₂ adsorption on the coal surface. These results demonstrate that the micropore factor plays a dominant role in affecting CO₂ adsorption capacity, the attractive interaction of adsorbed H₂O to CO₂ makes little contribution.     

All-atom molecular dynamics simulations of an artificial sodium channel in a lipid bilayer: the effect of water solvation/desolvation of the sodium ion

All-atom molecular dynamics is used to investigate the transport of Na⁺ across a 1,2-dioleoyl-sn-glycero-3-phosphocholine lipid bilayer facilitated by a diazacrown hydraphile. Specifically, the free energy of Na⁺ passing through the bilayer is calculated using the adaptive biasing force method to study the free energy associated with the increase in Na⁺ transport in the presence of the hydraphile molecule. The results show that water interaction greatly influences Na⁺ transport through the lipid bilayer as water is pulled through the bilayer with Na⁺ forming a water channel. The hydraphile causes a reduction in the free energy barrier for the transport of Na⁺ through the head group part of the lipid bilayer since it complexes the Na⁺ reducing the necessity for water to be complexed and, therefore, dragged through with Na⁺, an energetically unfavorable process. The free energy associated with Na⁺ being desolvated within the bilayer is significantly decreased in the presence of the hydraphile molecule; the hydraphile increases the number of solvation states of Na⁺ that can be adopted, and this increase in the number of available configurations provides an entropic explanation for the success of the hydraphile.     

The effects of salinity on photosynthesis and growth of the single-cell C<Subscript>4</Subscript> species <Emphasis Type="Italic">Bienertia sinuspersici</Emphasis> (Chenopodiaceae)

Recent research on the photosynthetic mechanisms of plant species in the Chenopodiaceae family revealed that three species, including Bienertia sinuspersici, can carry out C₄ photosynthesis within individual photosynthetic cells, through the development of two cytoplasmic domains having dimorphic chloroplasts. These unusual single-cell C₄ species grow in semi-arid saline conditions and have semi-terete succulent leaves. The effects of salinity on growth and photosynthesis of B. sinuspersici were studied. The results show that NaCl is not required for development of the single-cell C₄ system. There is a large enhancement of growth in culture with 50–200 mM NaCl, while there is severe inhibition at 400 mM NaCl. With increasing salinity, the carbon isotope values (δ¹³C) of leaves increased from −17.3^o/_oo (C₄-like) without NaCl to −14.6^o/_oo (C₄) with 200 mM NaCl, possibly due to increased capture of CO₂ from the C₄ cycle by Rubisco and reduced leakiness. Compared to growth without NaCl, leaves of plants grown under saline conditions were much larger (~2 fold) and more succulent, and the leaf solute levels increased up to ~2000 mmol kg solvent⁻¹. Photosynthesis on an incident leaf area basis (CO₂ saturated rates, and carboxylation efficiency under limiting CO₂) and stomatal conductance declined with increasing salinity. On a leaf area basis, there was some decline in Rubisco content with increasing salinity up to 200 mM NaCl, but there was a marked increase in the levels of pyruvate, Pi dikinase, and phosphoenolpyruvate carboxylase (possibly in response to sensitivity of these enzymes and C₄ cycle function to increasing salinity). The decline in photosynthesis on a leaf area basis was compensated for on a per leaf basis, up to 200 mM NaCl, by the increase in leaf size. The influence of salinity on plant development and the C₄ system in Bienertia is discussed.     

Effects of salinity on chlorophyll fluorescence and CO<Subscript>2</Subscript> fixation in C<Subscript>4</Subscript> estuarine grasses

The effects of salinity (sea water at 0 ‰ versus 30 ‰) on gross rates of O₂ evolution (J _O2) and net rates of CO₂ uptake (P _N) were measured in the halotolerant estuarine C₄ grasses Spartina patens, S. alterniflora, S. densiflora, and Distichlis spicata in controlled growth environments. Under high irradiance, salinity had no significant effect on the intercellular to ambient CO₂ concentration ratio (C _i/C _a). However, during photosynthesis under limiting irradiance, the maximum quantum efficiency of CO₂ fixation decreased under salinity across species, suggesting there is increased leakage of the CO₂ delivered to the bundle sheath cells by the C₄ pump. Growth under salinity did not affect the maximum intrinsic efficiency of photosystem 2, PS2 (F_V/F_M) in these species, suggesting salinity had no effect on photosynthesis by inactivation of PS2 reaction centers. Under saline conditions and high irradiance, P _N was reduced by 75 % in Spartina patens and S. alterniflora, whereas salinity had no effect on P _N in S. densiflora or D. spicata. This inhibition of P _N in S. patens and S. alterniflora was not due to an effect on stomatal conductance since the ratio of C _i/C _a did not decrease under saline conditions. In growth with and without salt, P _N was saturated at ∼500 μmol(quantum) m⁻² s⁻¹ while J _O2 continued to increase up to full sunlight, indicating that carbon assimilation was not tightly coupled to photochemistry in these halophytic species. This increase in alternative electron flow under high irradiance might be an inherent function in these halophytes for dissipating excess energy.     

Interaction of a Ti-doped semi-fullerene (TiC<Subscript>30</Subscript>) with molecules of CO and CO<Subscript>2</Subscript>

Using density functional theory (DFT) and molecular dynamics (MD), we studied the interaction of a titanium atom with a half of a C₆₀ fullerene (i.e., C₃₀), formed from the corannulene structure with a pentagonal base. We considered atmospheric pressure and 300 K. We found that the most stable adsorption of the titanium atom on C₃₀ occurs in the concave surface of the molecule. Afterward, we investigated the interaction of the system C₃₀-titanium with carbon monoxide and carbon dioxide molecules, respectively. We found that each of these molecules is chemisorbed, with no dissociation. The value of the adsorption energy for the carbon monoxide molecule varies from ?0.897 to ?1.673 eV, and for the carbon dioxide molecule, it is between ?1.065 and ?1.274 eV. These values depend on the initial orientation of these molecules with respect to TiC₃₀.

Open image in new window

Graphical Abstract The TiC₃₀ system chemisorbs CO or CO₂?with no dissociation at atmospheric pressure and 300K

     

Molecular properties of metal difluorides and their interactions with CO<Subscript>2</Subscript> and H<Subscript>2</Subscript>O molecules: a DFT investigation

A computational study of metal difluorides (MF₂; M = Ca to Zn) and their interactions with carbon dioxide and water molecules was performed. The structural parameter values obtained and the results of AIM analysis and energy decomposition analysis indicated that the Ca–F bond is weaker and less ionic than the bonds in the transition metal difluorides. A deformation density plot revealed the stablizing influence of the Jahn–Teller effect in nonlinear MF₂ molecules (e.g., where M= Sc, Ti, Cr). An anaysis of the metal K-edge peaks of the difluorides showed that shifts in the edge energy were due to the combined effects of the ionicity, effective nuclear charge, and the spin state of the metal. The interactions of CO₂ with ScF₂ (Scc3 geometry) and TiF₂ (Tic2 geometry) caused CO₂ to shift from its usual linear geometry to a bent geometry (η²(C=O) binding mode), while it retained its linear geometry (η¹(O) binding mode) when it interacted with the other metal difluorides. Energy decomposition analysis showed that, among the various geometries considered, the Scc3 and Tic2 geometries possessed the highest interaction energies and orbital interaction energies. Heavier transition metal difluorides showed stronger affinities for H₂O, whereas the lighter transition metal (Sc and Ti) difluorides preferred CO₂. Overall, the results of this study suggest that fluorides of lighter transition metals with partially filled d orbitals (e.g., Sc and Ti) could be used for CO₂ capture under moist conditions.

Open image in new window

Graphical abstract Interaction of metal difluorides with carbon dioxide and water

     

Geographical Assessment of Microalgae Biofuels Potential Incorporating Resource Availability

Previous assessments of the economic feasibility and large-scale productivity of microalgae biofuels have not considered the impacts of land and carbon dioxide (CO₂) availability on the scalability of microalgae-based biofuels production. To accurately assess the near-term productivity potential of large-scale microalgae biofuel in the USA, a geographically realized growth model was used to simulate microalgae lipid yields based on meteorological data. The resulting lipid productivity potential of Nannochloropsis under large-scale cultivation is combined with land and CO₂ resource availability illustrating current geographically feasible production sites and corresponding productivity in the USA. Baseline results show that CO₂ transport constraints will limit US microalgae-based bio-oil production to 4 % of the 2030 Department of Energy (DOE) alternative fuel goal. The discussion focuses on synthesis of this large-scale productivity potential results including a sensitivity analysis to land and CO₂ resource assumptions, an evaluation of previous modeling efforts, and their assumptions regarding the transportation of CO₂, the feasibility of microalgae to meet DOE 2030 alternative fuel goals, and a comparison of the productivity potential in several key regions of the USA.     

Energy crosstalk between photosynthesis and the algal CO2-concentrating mechanisms

Microalgal photosynthesis is responsible for nearly half of the CO₂ annually captured by Earth’s ecosystems. In aquatic environments where the CO₂ availability is low, the CO₂-fixing efficiency of microalgae greatly relies on mechanisms – called CO_2-concentrating mechanisms (CCMs) – for concentrating CO₂ at the catalytic site of the CO₂-fixing enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco). While the transport of inorganic carbon (C_i) across membrane bilayers against a concentration gradient consumes part of the chemical energy generated by photosynthesis, the bioenergetics and cellular mechanisms involved are only beginning to be elucidated. Here, we review the current knowledge relating to the energy requirement of CCMs in the light of recent advances in photosynthesis regulatory mechanisms and the spatial organization of CCM components.     

Structural Effects and Translocation of Doxorubicin in a DPPC/Chol Bilayer: The Role of Cholesterol

We use molecular dynamics simulations to characterize the influence of cholesterol (Chol) on the interaction between the anticancer drug doxorubicin (DOX) and a dipalmitoyl phosphatidylcholine/Chol lipid bilayer. We calculate the potential of mean force, which gives us an estimate of the free energy barrier for DOX translocation across the membrane. We find free energy barriers of 23.1 ± 3.1 k_BT, 36.8 ± 5.1 k_BT, and 54.5 ± 4.7 k_BT for systems composed of 0%, 15%, and 30% Chol, respectively. Our predictions agree with Arrhenius activation energies from experiments using phospholipid membranes, including 20 k_BT for 0% Chol and 37.2 k_BT for 20% Chol. The location of the free energy barrier for translocation across the bilayer is dependent on composition. As Chol concentration increases, this barrier changes from the release of DOX into the water to flip-flop over the membrane center. The drug greatly affects local membrane structure by attracting dipalmitoyl phosphatidylcholine headgroups, curving the membrane, and allowing water penetration. Despite its hydrophobicity, DOX facilitates water transport via its polar groups.     

Effects of the interaction between vapor-pressure deficit and salinity on growth and photosynthesis of <Emphasis Type="Italic">Cucumis sativus</Emphasis> seedlings under different CO<Subscript>2</Subscript> concentrations

We studied growth and photosynthesis of cucumber (Cucumis sativus) seedlings under two vapor-pressure deficit levels (VPD; 0.4 and 3.0 kPa), two salinity levels (0 mM and 34 mM NaCl), and two CO₂ concentrations ([CO₂]; 400 and 1,000 μmol mol^–1). Relative growth rate (RGR) decreased with increasing VPD, but the causal factor differed between salinity levels and CO₂ concentrations. Under ambient [CO₂], RGR decreased with increasing VPD at low salinity mainly due to decreased leaf area ratio (LAR), and decreased net assimilation rate (NAR) at high salinity. The decrease in intercellular [CO₂] (C_i) with decreasing stomatal conductance caused by high VPD did not significantly limit net photosynthetic rate (P_N) at low salinity, but P_N was potentially limited by C_i at high salinity. At high [CO₂], high VPD reduced LAR, but did not affect NAR. This is because the decrease in C_i occurred where slope of P_N–C_i curve was almost flat.     

Immersion in CO<Subscript>2</Subscript>-rich water containing NaCl diminishes blood pressure fluctuation in anesthetized rats

Remarkably, bathing in hot springs containing high concentrations of carbon dioxide (CO₂) influences cardiovascular function more than bathing in fresh water. The CO₂-enriched water in hot springs generally contains many salts, whose interactions remain unknown. We separately evaluated the actions of individual factors in CO₂-enriched water and confirmed that CO₂ and NaCl have combined effects on blood pressure fluctuations in anesthetized rats. Animals equipped with sensor probes to monitor body temperature, skin blood flow and arterial blood pressure were immersed in bathwater (35°C) containing CO₂ with NaCl, KCl or sucrose. The effects of these factors on cardiovascular function were evaluated using power-spectral analysis of fluctuations in blood pressure and heart rate. Compared with immersion in tap water, heart rate and skin vascular resistance were reduced during immersion in CO₂-enriched water irrespective of the presence of other components. In terms of the very low frequency range (0.02–0.195 Hz), the power of blood pressure fluctuation during immersion was significantly reduced when the CO₂-enriched water contained more than 1.5% NaCl but was not influenced by other components of similar osmotic pressure and the same specific gravity. The results indicated that the coexistence of CO₂ and sodium ions in bathwater reduce blood pressure fluctuations, and suggested that this combination effect of CO₂ and salt contributes to the sedative effect on human cardiovascular functions while bathing in CO₂-hot springs.     

Localized density pulses in lipid membranes on the piсosecond time scale

Physical mechanisms for the modulation of lipid nanodomain dynamics and transport of small molecules across the lipid bilayer of biological membranes can have considerable functional significance. The longitudinal propagating compression–expansion mode in a single-component lipid bilayer is considered on a spatial scale comparable to the bilayer thickness. The expression used for free energy per lipid molecule includes a term with the gradient of the area per lipid molecule. The finite character of the stress relaxation time in the lipid bilayer, which causes significant attenuation of viscous damping at sufficiently high frequencies, is taken into account. A hydrodynamic ad hoc model that describes soliton-like excitations in the bilayer is proposed. The possibility of formation and propagation of nanoscale pulses of lipid density with velocities determined by surface compressibility of the bilayer (similarly to the propagation of sound) is demonstrated for the picosecond time scale.     

The effect of CO2 and salinity on the cultivation of Scenedesmus obliquus for biodiesel production

Biodiesel is a renewable and environmental friendly energy source that can be produced via tranesterification from various oil crops such as soy bean, sunflower, palm, and algae. In this work, the microalgae Scenedesmus obliquus, S. armatus and S. bernadii, isolated from natural water basins, were enriched in modified Chu 13 medium. Only S. obliquus showed significant oil accumulation and was thus further cultivated in 3 L tubular photo-reactors under mixotrophic conditions (16:8 h light-dark cycle) at room temperature and varying CO₂ (5, 10, and 15%) supply. The results indicated that S. obliquus can be grown under various CO₂ concentrations. A maximum biomass of 2.3 g/L was achieved when 15% CO₂ was used. The effect of salinity on oil storage was also considered, using sodium chloride (NaCl) solutions of varying concentrations (0.05, 0.2, and 0.3 M). Higher lipid contents were found in cells that were subjected to salt stress compared to those in conditions without salt stress. A maximum oil accumulation of 36% was observed within 15 days at 0.3 M NaCl. A biodiesel yield of up to 97.4% was obtained.     

Transitional slopes act as hotspots of both soil CO<Subscript>2</Subscript> emission and CH<Subscript>4</Subscript> uptake in a temperate forest landscape

Forest soils are an important component of CO₂ and CH₄ fluxes at the global scale, but the magnitude of these fluxes varies greatly in space and time within a landscape. Understanding the spatial and temporal distributions of these fluxes across complex landscapes remains a major challenge for researchers and land managers alike. We investigated the spatiotemporal variability of soil-atmosphere CO₂ and CH₄ fluxes and the relationships of these fluxes to chemical and physical soil properties distributed across a topographically-heterogeneous landscape. Soil CO₂ and CH₄ fluxes were measured along with soil temperature, moisture, bulk density, texture, carbon, sorption capacity, and dissolved organic matter quality over 2 years along hillslope transects spanning valley bottom, transition zone, and upland landscape positions in a temperate forest watershed. Transition zone soil CO₂ efflux was 54–160% higher than low-lying valley bottoms, and 15–54% higher than uplands. Net seasonal CH₄ uptake was 58–150% higher in transition zone soils than in uplands, while valley bottoms were occasionally large net sources (up to 19 nmol CH₄ m^?2 s^?1). Soil CO₂ efflux and net CH₄ uptake were both positively associated with seasonal temperature, and were highest in soils with relatively high carbon and clay content, and relatively low bulk density, moisture, and sorption capacity. We concluded that: (1) transition zone soils act as landscape hotspots for net CH₄ uptake in addition to CO₂ efflux, and (2) that this spatial distribution is more consistent across seasons for net CH₄ uptake than for CO₂ efflux.     

On-line stable isotope gas exchange reveals an inducible but leaky carbon concentrating mechanism in Nannochloropsis salina

Carbon concentrating mechanisms (CCMs) are common among microalgae, but their regulation and even existence in some of the most promising biofuel production strains is poorly understood. This is partly because screening for new strains does not commonly include assessment of CCM function or regulation despite its fundamental role in primary carbon metabolism. In addition, the inducible nature of many microalgal CCMs means that environmental conditions should be considered when assessing CCM function and its potential impact on biofuels. In this study, we address the effect of environmental conditions by combining novel, high frequency, on-line ¹³CO₂ gas exchange screen with microscope-based lipid characterization to assess CCM function in Nannochloropsis salina and its interaction with lipid production. Regulation of CCM function was explored by changing the concentration of CO₂ provided to continuous cultures in airlift bioreactors where cell density was kept constant across conditions by controlling the rate of media supply. Our isotopic gas exchange results were consistent with N. salina having an inducible “pump-leak” style CCM similar to that of Nannochloropsis gaditana. Though cells grew faster at high CO₂ and had higher rates of net CO₂ uptake, we did not observe significant differences in lipid content between conditions. Since the rate of CO₂ supply was much higher for the high CO₂ conditions, we calculated that growing cells bubbled with low CO₂ is about 40 % more efficient for carbon capture than bubbling with high CO₂. We attribute this higher efficiency to the activity of a CCM under low CO₂ conditions.     

Review of advances in biological CO<Subscript>2</Subscript> mitigation technology

CO₂ fixation by microalgae has emerged as a promising option for CO₂ mitigation. Intensive research work has been carried out to develop a feasible system for removing CO₂ from industrial exhaust gases. However, there are still several challenging points to overcome in order to make the process more practical. In this paper, recent research activities on three key technologies of biological CO₂ fixation, an identification of a suitable algal strain, development of high efficient photobioreactor and utilization of algal cells produced, are described. Finally the barriers, progress, and prospects of commercially developing a biological CO₂ fixation process are summarized.     

Climatic influences on net ecosystem CO<Subscript>2</Subscript> exchange during the transition from wintertime carbon source to springtime carbon sink in a high-elevation,subalpine forest

The transition between wintertime net carbon loss and springtime net carbon assimilation has an important role in controlling the annual rate of carbon uptake in coniferous forest ecosystems. We studied the contributions of springtime carbon assimilation to the total annual rate of carbon uptake and the processes involved in the winter-to-spring transition across a range of scales from ecosystem CO₂ fluxes to chloroplast photochemistry in a coniferous, subalpine forest. We observed numerous initiations and reversals in the recovery of photosynthetic CO₂ uptake during the initial phase of springtime recovery in response to the passage of alternating warm- and cold-weather systems. Full recovery of ecosystem carbon uptake, whereby the 24-h cumulative sum of NEE (NEE_daily) was consistently negative, did not occur until 3–4 weeks after the first signs of photosynthetic recovery. A key event that preceded full recovery was the occurrence of isothermality in the vertical profile of snow temperature across the snow pack; thus, providing consistent daytime percolation of melted snow water through the snow pack. Interannual variation in the cumulative annual NEE (NEE_annual) was mostly explained by variation in NEE during the snow-melt period (NEE_snow-melt), not variation in NEE during the snow-free part of the growing season (NEE_snow-free). NEE_snow-melt was highest in those years when the snow melt occurred later in the spring, leading us to conclude that in this ecosystem, years with earlier springs are characterized by lower rates of NEE_annual, a conclusion that contrasts with those from past studies in deciduous forest ecosystems. Using studies on isolated branches we showed that the recovery of photosynthesis occurred through a series of coordinated physiological and biochemical events. Increasing air temperatures initiated recovery through the upregulation of PSII electron transport caused in part by disengagement of thermal energy dissipation by the carotenoid, zeaxanthin. The availability of liquid water permitted a slightly slower recovery phase involving increased stomatal conductance. The most rate-limiting step in the recovery process was an increase in the capacity for the needles to use intercellular CO₂, presumably due to slow recovery of Rubisco activity. Interspecific differences were observed in the timing of photosynthetic recovery for the dominant tree species. The results of our study provide (1) a context for springtime CO₂ uptake within the broader perspective of the annual carbon budget in this subalpine forest, and (2) a mechanistic explanation across a range of scales for the coupling between springtime climate and the carbon cycle of high-elevation coniferous forest ecosystems.