Photosynthetic performance of a cyanobacterium in a vertical flat-plate photobioreactor for outdoor microalgal production and fixation of CO2

A vertical flat-plate photobioreactor was developed for the outdoor culture of microalgae using sunlight as the light source. The ability for biomass production and CO₂ fixation was evaluated by using a cyanobacterium, Synechocystis aquatilis SI-2. The average areal productivity was 31 g biomass m^–2 d^–1, which corresponded to a CO₂ fixation rate of 51 g CO₂ m^–2 d^–1, sustainable in the northern region of Japan during the winter time (January and February). The relationships between the efficiency of solar energy utilization of the reactor and its effect factors (cell concentration and irradiation) were investigated.     

Effects of elevated CO2 and phosphorus addition on productivity and community composition of intact monoliths from calcareous grassland

We investigated the effects of elevated CO₂ (600 μl l⁻¹ vs 350 μl l⁻¹) and phosphorus supply (1 g P m⁻² year⁻¹ vs unfertilized) on intact monoliths from species-rich calcareous grassland in a greenhouse. Aboveground community dry mass remained almost unaffected by elevated CO₂ in the first year (+6%, n.s.), but was significantly stimulated by CO₂ enrichment in year two (+26%, P<0.01). Among functional groups, only graminoids contributed significantly to this increase. The effect of phosphorus alone on community biomass was small in both years and marginally significant only when analyzed with MANOVA (+6% in year one, +9% in year two, 0.1 ≥P > 0.05). Belowground biomass and stubble after two seasons were not different in elevated CO₂ and when P was added. The small initial increase in aboveground community biomass under elevated CO₂ is explained by the fact that some species, in particular Carex flacca, responded very positively right from the beginning, while others, especially the dominant Bromus erectus, responded negatively to CO₂ enrichment. Shifts in community composition towards more responsive species explain the much larger CO₂ response in the second year. These shifts, i.e., a decline in xerophytic elements (B. erectus) and an increase in mesophytic grasses and legumes occurred independently of treatments in all monoliths but were accelerated significantly by elevated CO₂. The difference in average biomass production at elevated compared to ambient CO₂ was higher when P was supplied (at the community level the CO₂ response was enhanced from 20% to 33% when P was added, in graminoids from 17% to 27%, in legumes from 4% to 60%, and in C. flacca from 120% to 298% by year two). Based on observations in this and similar studies, we suggest that interactions between CO₂ concentration, species presence, and nutrient availability will govern community responses to elevated CO₂. Received: 12 July 1997 / Accepted: 28 March 1998     

Utilization of flue gas for cultivation of microalgae <Emphasis Type="Italic">Chlorella</Emphasis> sp.) in an outdoor open thin-layer photobioreactor

Flue gas generated by combustion of natural gas in a boiler was used for outdoor cultivation of Chlorella sp. in a 55 m² culture area photobioreactor. A 6 mm thick layer of algal suspension continuously running down the inclined lanes of the bioreactor at 50 cm s⁻¹ was exposed to sunlight. Flue gas containing 6–8% by volume of CO₂ substituted for more costly pure CO₂ as a source of carbon for autotrophic growth of algae. The degree of CO₂ mitigation (flue gas decarbonization) in the algal suspension was 10–50% and decreased with increasing flue gas injection rate into the culture. A dissolved CO₂ partial pressure (pCO₂) higher than 0.1 kPa was maintained in the suspension at the end of the 50 m long culture area in order to prevent limitation of algal growth by CO₂. NO_X and CO gases (up to 45 mg m⁻³ NO_X and 3 mg m⁻³ CO in flue gas) had no negative influence on the growth of the alga. On summer days the following daily net productivities of algae [g (dry weight) m⁻²] were attained in comparative parallel cultures: flue gas = 19.4–22.8; pure CO₂ = 19.1–22.6. Net utilization (η) of the photosynthetically active radiant (PAR) energy was: flue gas = 5.58–6.94%; pure CO₂ = 5.49–6.88%. The mass balance of CO₂ obtained for the flue gas stream and for the algal suspension was included in a mathematical model, which permitted the calculation of optimum flue gas injection rate into the photobioreactor, dependent on the time course of irradiance and culture temperature. It was estimated that about 50% of flue gas decarbonization can be attained in the photobioreactor and 4.4 kg of CO₂ is needed for production of 1 kg (dry weight) algal biomass. A scheme of a combined process of farm unit size is proposed; this includes anaerobic digestion of organic agricultural wastes, production and combustion of biogas, and utilization of flue gas for production of microalgal biomass, which could be used in animal feeds. A preliminary quantitative assessment of the microalgae production is presented.     

CO2 and O2 gas exchange in outdoor thin-layer high density microalgal cultures

Two thin layer culture units operated as batch cultures with the algaChlorella kessleri were used in gas exchange experiments. The mass transfer coefficient K_g [g m^–2 h^–1 kPa^–1] of O₂ and CO₂ desorption from culture surface decreased with increasing culture temperature. Between 60–70% of supplied CO₂ was used for algal growth. It was estimated that the length of growth surface may be extended to about 50 m, without additional saturation by CO₂. On average 1.35 g CO₂ was consumed by the alga per 1 g of produced O₂. Net CO₂ consumption (R_CO2) and O₂ production (R_O2) were not inhibited by irradiance. R_O2 did not decrease (in some cases it even increased) along the culture surface, despite increased accumulation of O₂. Measurement of pO₂ where the culture leaves the reactor before being pumped back onto the illuminated surface, correlated with O₂ production and CO₂ consumption and may be used to monitor the reactors growth performance.     

The profiles of nitrate reductase and carbonic anhydrase activity in batch cultivation of the marine microalgae Tetraselmis gracilis growing under different aeration conditions

Tetraselmis gracilis, a Prasinophycean alga found in estuaries and in the open ocean, was cultivated under different conditions of aeration, which resulted in variations of inorganic carbon in the medium. Relative growth rates, nitrate reductase and carbonic anhydrase activities were daily determined and correlated to the concentration of nitrate, nitrite, phosphate, inorganic and organic carbon in the media. Nitrate reductase catalyzes the reversible carbon dioxide hydration reaction. The activity profiles of both enzymes during 10 days of cultivation under aeration with air showed an inverse relationship: the maximum in the activity of nitrate reductase coincided with the minimum of carbonic anhydrase activity. An ionizable organic carbon species with pKa in the range of metabolites of the photorespiratory path was found parallel with the increase of carbonic anhydrase activity and the decrease of nitrate reductase activity. The onset of photorespiration is probably one of the factors involved in the simultaneous regulation of these enzymatic processes. Cultures aerated with air containing 5% CO2 showed different profiles for nitrate reductase activity and nitrate uptake.     

Effect of light intensity on the proximate biochemical and fatty acid composition of Isochrysis sp. and Nannochloropsis oculata for use in tropical aquaculture

The total protein, carbohydrate, lipid and ash compositions, and fatty acid contents of two species of marine microalgae, the eustigmatophyte Nannochloropsis oculata (formerly ‘Chlorella sp., Japan’) and the chrysophyte Isochrysis sp. (Tahitian) used in tropical Australian mariculture, were studied. The microalgae were grown under a range of culture conditions (41 and 601 laboratory culture, 3001 bag culture, and 80001 outdoor culture) and four light regimes (100 to 107 μ E m⁻² s⁻¹, 240 to 390 μ E m⁻² s⁻¹, 340 to 620 μ E m⁻² s⁻¹, and 1100 to 1200 μE m⁻² s⁻¹ respectively) to determine the effect of light intensity on the chemical composition of large scale outdoor cultures. Laboratory and bag cultures were axenic and cultured in Walne medium while outdoor cultures were grown in a commercial medium designed for optimum nutrition in tropical outdoor aquaculture operations. Change in growth medium and photon flux density produced only small changes in the proximate biochemical composition of both algae. N. oculata and Isochrysis sp. both showed a trend towards slightly lower carbohydrate and higher chlorophyll a in shaded outdoor culture. Isochrysis sp. showed significant concentrations of the essential polyunsaturated fatty acid 22:6(n−3) (docosahexaenoic acid) from 5.3 to 10.3% of total fatty acid, and 20:5(n−3) (eicosapentaenoic acid) ranged from 0.6 to 4.1%. In contrast, N. oculata had high concentrations of 20:5(n−3) (17.8 to 39.9%) and only traces of 22:6(n−3). The fatty acid composition of Isochrysis sp. grown at high photon flux density (1100–1200 μE m⁻² s⁻¹) under outdoor culture showed a decrease in the percentage of several highly unsaturated fatty acids, including 20:5(n−3), and an increase in 22:6(n−3). N. oculata showed a similar decrease in the percentage of 20:5(n−3). High light intensity caused a decrease in the ratio of total C₁₆ unsaturated fatty acids to saturated 16:0 in N. oculata, and a decrease in the ratio of total C₁₈ unsaturated fatty acids to saturated 18:0 together with a decrease in the ratio of total unsaturated fatty acids to total saturated fatty acids in both microalgae.     

Effect of solar near ultraviolet radiation on growth of Chlorella cells

Cells of Chlorella regularis (Artari) Oltmanns (S-50) were grown under solar radiation in Tokyo, using a newly constructed outdoor culture system. The maximum specific growth rate (log₂ unit h⁻¹) was about 0.3, which was a little lower than the highest value reported using artificial light in the laboratory. The near ultraviolet light, consisting mostly of UV-A, inhibited the growth from 10 to 40% in summer mornings; the inhibition was not significant in the afternoon. The percentage inhibition was correlated with the total dose of the ultraviolet radiation.     

Environmental productivity indices and productivity for Opuntia ficus-indica under current and elevated atmospheric CO2 levels*

Abstract. The productivity of the prickly-pear cactus Opuntia ficus-indica, which is cultivated worldwide for its fruits and stem segments, was predicted based on the responses of its net CO₂ uptake to soil water status, air temperature and photosynthetic photon flux density (PPFD). Each of these environmental factors was represented by an index with a maximum value of unity when that factor was not limiting net CO₂ uptake over a 24-h period. The water index, the temperature index, and the PPFD index were determined for 87 sites in the contiguous United States using data from 189 weather stations and for 148 sites worldwide using data from 1464 weather stations. The product of these three indices, the environmental productivity index (EPI), was used to predict the productivity of O. ficus-indica under current climatic conditions and under those accompanying a possible increase in the atmospheric CO₂ level to 650μumol mol^?1. Sites with temperatures always above -10°C and hence suitable for prickly-pear cultivation numbered 37 in the United States and 110 worldwide; such sites increased by 43 and 5%, respectively, for the global warming accompanying the elevated CO₂. Productivity of O. ficus-indica was at least 15 tonnes dry weight hectare^?1 year^?1, comparable to that of many agronomic crops, for 20 sites with temperatures always above -10°C in the contiguous United States and for 12 such sites worldwide under current climatic conditions; such sites increased by 85 and 117%, respectively, under the elevated CO₂ condition, mainly because of direct effects of the atmospheric CO₂ level on net CO₂ uptake. In summary, simulations based on EPI indicate that O. ficus-indica may presently be advantageously cultivated over a substantial fraction of the earth's surface, such regions increasing markedly with a future doubling in atmospheric CO₂ levels.     

Effect of CO2 enhancement in an outdoor algal production system usingTetraselmis

One of the objectives of microalgal culture is to provide reliable production technology for important live aquaculture feed organisms. Presented here are the results of experiments designed to provide a better understanding of the relationship between inorganic carbon availability and algal production.Our results suggest that through additions of CO₂ gas we were able to maintain sufficient dissolved carbon to stabilize outdoor algal cultures. Increases in the rate of addition of CO₂ increased levels of dissolved CO₂, total dissolved inorganic carbon (CO₂), and decreased pH in the growth medium. This translated into improved buffering capacity of the culture medium and higher growth rate. A minimum of 2.4 mM CO₂ was found necessary to maintain a maximal growth rate of 0.7 doublings/day. We also found that the increased productivity more than offsets the cost of adding the CO₂.     

Toxicity of inorganic cadmium salts to the microalga Scenedesmus armatus (Chlorophyta) with respect to medium composition, pH and CO2 concentration

Batch cultures of algae grown at low (0.1 %) and elevated (2.0 %) concentrations of CO₂, as well as in original BBM (Bold Basal Medium) and BBM modified with phosphate, EDTA and a combination of both, were exposed to cadmium (Cd(NO₃)₂·4H₂O, 3CdSO₄·8H₂O and CdCl₂·H₂O) for 24 h. Regardless of the salt applied, the concentration-dependent relationships of Cd toxicity were found to be biphasic, suggesting the different affinity of target sites to cadmium. Nominal values of EC₅₀ obtained for algae grown in original BBM and at low CO₂ were 18.0, 16.44 and 15.37 mg·dm⁻³ for cadmium nitrate, sulphate and chloride, respectively. However, it was estimated that 97 % of the free cadmium in the added salts were bound by components of original BBM such as EDTA, phosphates, chloride and sulphate. The effect of Cd-salts at concentrations corresponding to EC₅₀ values on algae were tested in media with 10-fold reduced phosphates (BBM-P), BBM depleted of EDTA (BBM-EDTA) and of both phosphates and EDTA (BBM-P-EDTA). For algae grown at low CO₂ and BBM-P, cadmium was about 25 % less toxic than those applied in original BBM. Cadmium greatly inhibited (about 85 % of the control) the growth of algae cultured in BBM-EDTA; this effect was only slightly dependent on the CO₂ concentration. Deficits of both EDTA and P led to effects similar to those brought about by the absence of EDTA only. The toxicity of cadmium depends on CO₂ concentration only when algae are grown in original BBM. The growth of algae under high CO₂ conditions was reduced considerably less (about 80% of control) compared with low CO₂ concentrations (about 50 % of control). A relationship was found between the toxicity of cadmium salts and final pH values only in variants of low-CO₂ grown algae; with an increase of medium pH the toxicity decreased. The results suggest that both growth conditions and the binding ability of the medium markedly affect the toxicity of cadmium towards microalgae.     

Carbon isotope composition of biochemical fractions isolated from leaves of Bryophyllum daigremontianum berger,a plant with crassulacean acid metabolism: Some physiological aspects related to CO2 dark fixation

Isotope analysis of the biochemical fractions isolated quantitatively from young and mature leaves of Bryophyllum daigremontianum Berger have been carried out before and after a dark period of accumulation of organic acids. The mature leaf is enriched in ¹³C compared to the young leaf. The ¹³C values of the different leaf constituents vary between the ¹³C values of C₄ plants (-11) and those of C₃ plants (-27). During the dark period, the two types of leaves store organic acids with ¹³C values of -15 and lose insoluble sugars, including starch with a ¹³C value of -12. Furthermore, young leaves store phosphorylated compounds with ¹³C values of -11 and lose weakly polymerised sugars with ¹³C values of -18. These results led to the formulation of a hypothesis of the origin of the two substrates of -carboxylation: phosphoenolpyruvate arises from the glycolytic breakdown of the insoluble sugars rich in ¹³C, and the major portion of the CO₂ is the result of the complete breakdown (respiration) of the soluble sugars rich in ¹²C. The existence of two independent sugar pools leads to the assumption that there are two separate glycolytic pathways. The ¹³C enrichment of the stored products of the young leaves in the day seems to be the result of a weak discrimination for ¹³C by ribulose diphosphate carboxylase, which reassimilates to a great extent the CO₂ released from malate accumulated in the night.Abbreviations CAM crassulacean acid metabolism - C₃ metabolism metabolism with primary carbon fixed by the Calvin and Benson pathway - C₄ metabolism metabolism with primary carbon fixed by the Hatch and Slack pathway - ¹³C() (R_sample-R_PDB) 10³/R_PDB where PDB=Pee Dee belemnite (belemite from the Pee Dee formation South Carolina) and R=¹³C/¹²C - NAD-MDH(EC1.1.1.37) NAD-malate dehydrogenase - NADP-ME (EC1.1.1.40) NADP-malic enzyme - PEP phosphoenolpyruvate - PEPC (EC4.1.1.31) PEP carboxylase - PGA phosphoglyceric acid - Py.di-PK(EC2.7.9.1) pyruvate, Pi-dikinase - RuDP ribulose diphosphate - RuDPC (EC4.1.1.39) RuDP carboxylase     

Root biomass and nutrient dynamics in a scrub-oak ecosystem under the influence of elevated atmospheric CO2

Elevated CO₂ can increase fine root biomass but responses of fine roots to exposure to increased CO₂ over many years are infrequently reported. We investigated the effect of elevated CO₂ on root biomass and N and P pools of a scrub-oak ecosystem on Merritt Island in Florida, USA, after 7 years of CO₂ treatment. Roots were removed from 1-m deep soil cores in 10-cm increments, sorted into different categories (<0.25 mm, 0.25–1 mm, 1–2 mm, 2 mm to 1 cm, >1 cm, dead roots, and organic matter), weighed, and analyzed for N, P and C concentrations. With the exception of surface roots <0.25 mm diameter, there was no effect of elevated CO₂ on root biomass. There was little effect on C, N, or P concentration or content with the exception of dead roots, and <0.25 mm and 1–2 mm diameter live roots at the surface. Thus, fine root mass and element content appear to be relatively insensitive to elevated CO₂. In the top 10 cm of soil, biomass of roots with a diameter of <0.25 mm was depressed by elevated CO₂. Elevated CO₂ tended to decrease the mass and N content of dead roots compared to ambient CO₂. A decreased N concentration of roots <0.25 mm and 1–2 mm in diameter under elevated CO₂ may indicate reduced N supply in the elevated CO₂ treatment. Our study indicated that elevated CO₂ does not increase fine root biomass or the pool of C in fine roots. In fact, elevated CO₂ tends to reduce biomass and C content of the most responsive root fraction (<0.25 mm roots), a finding that may have more general implications for understanding C input into the soil at higher atmospheric CO₂ concentrations.     

Mid-season gas exchange of an alpine grassland under elevated CO2

Ecosystem net CO₂ uptake, evapotranspiration (ET) and night-time CO₂ efflux were measured in an alpine grassland dominated by Carex curvula, treated with doubled ambient partial pressure of CO₂ via open-top chambers. One quarter of the plots were treated with mineral nutrients to simulate the effect of lowland nitrogen deposition rates. Depending upon fertilizer supply, ecosystem net CO₂ uptake per ground area in full sunlight (NCE_max) was 41–81% higher in open-top chambers supplied with doubled ambient partial pressure (p _a) of CO₂ than in plots receiving ambient CO₂. Short-term reversals of the CO₂ level suggest that the extent of downward adjustment of canopy photosynthesis under elevated CO₂ was 30–40%. ET tended to decline, while water use efficiency (WUE), expressed as the NCE_max:ET ratio, increased more than twofold under elevated CO₂. Night-time ecosystem CO₂ efflux did not respond to changes in CO₂ p _a. NCE_max and night-time CO₂ efflux were more responsive to mineral fertilizer than the doubling of CO₂. This suggests that in these alpine plant communities, atmospheric nutrient input may induce equal or greater effects on gas exchange than increased CO₂.     

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.     

Carbon isotope composition,water-use efficiency and biomass productivity of Eucalyptus microtheca populations under different water supplies

Variation in carbon isotope composition (δ¹³C), water-use efficiency (WUE) and biomass productivity were compared among three populations of Eucalyptus microtheca F. Muell. in a greenhouse. Seedlings were maintained under one well-watered (Control, keeping the soil at field capacity) and two different water deficit conditions (Drought stress I, keeping the same soil water content; Drought stress II, keeping the same soil water supply). In each treatment, significant population differences in δ¹³C, WUE, and dry matter accumulation and allocation were detected. A negative correlation between WUE and biomass productivity was detected under control and drought stress I, but a positive correlation under drought stress II. The results suggested that there were different water-use strategies among the populations, the southeastern population with lower WUE may employ a prodigal water-use strategy, whereas the northwestern and central populations with higher WUE may employ a conservative water-use strategy. This knowledge may be useful as criteria for genotype selection within a breeding program for this species. This revised version was published online in June 2006 with corrections to the Cover Date.     

CO2浓度升高对斜生栅藻生长和光合作用的影响

升高大气中CO2 浓度可提高斜生栅藻的生物量和光合作用速率 ,对光合效率、暗呼吸速率、光饱和点和光系统Ⅱ的光化学效率 (Fv Fm)没有明显影响 ,但藻细胞光合作用对无机碳的亲和力降低     

Relations between soil microflora and CO2 evolution upon decomposition of cellulose

Summary The investigation was carried out to study the relation between CO₂ evolution and the changes in microflora in the case of cellulose-decomposition in soil with special attention to the heterogeneity of soil crumbs.The rates of CO₂ evolution correlated with the number of Gram-negative bacteria, while the number of cellulose-decomposing microorganisms did not. The Gram-negative bacteria probably contribute directly to CO₂ evolution by decomposing simple sugars produced from cellulose by cellulose-decomposing microorganisms. Both the Gram-negative bacteria and cellulose-decomposing microorganisms seemed to grow luxuriantly on the surface area of soil crumbs with added cellulose powder. Therefore, it is speculated that there is a cooperation between the Gram-negative bacteria and the cellulose-decomposing microorganisms with respect to cellulose decomposition in soil. The main locus where this reaction takes place may be the surface area of soil crumbs.This work was carried out at the Laboratory of Microbiology, Agricultural University, Wageningen, The Netherlands during author's stay as guest researcher.     

Soil and biomass carbon pools in model communities of tropical plants under elevated CO2

The experimental data presented here relate to the question of whether terrestrial ecosystems will sequester more C in their soils, litter and biomass as atmospheric CO₂ concentrations rise. Similar to our previous study with relatively fertile growth conditions (Körner and Arnone 1992), we constructed four rather nutrient-limited model communities of moist tropical plant species in greenhouses (approximately 7 m² each). Plant communities were composed of seven species (77 individuals per community) representing major taxonomic groups and various life forms found in the moist tropics. Two ecosystems were exposed to 340 l CO₂ l^–1 and two to 610 l l^–1 for 530 days of humid tropical growth conditions. In order to permit precise determination of C deposition in the soil, plant communities were initially established in C-free unwashed quartz sand. Soils were then amended with known amounts of organic matter (containing C and nutrients). Mineral nutrients were also supplied over the course of the experiment as timed-release full-balance fertilizer pellets. Soils represented by far the largest repositories for fixed C in all ecosystems. Almost 5 times more C (ca. 80% of net C fixation) was sequestered in the soil than in the biomass, but this did not differ between CO₂ treatments. In addition, at the whole-ecosystem level we found a remarkably small and statistically non-significant increase in C sequestration (+4%; the sum of C accretion in the soil, biomass, litter and necromass). Total community biomass more than quadrupled during the experiment, but at harvest was, on average, only 8% greater (i.e. 6% per year; n.s.) under elevated CO₂, mainly due to increased root biomass (+15%, P=0.12). Time courses of leaf area index of all ecosystems suggested that canopy expansion was approaching steady state by the time systems were harvested. Net primary productivity (NPP) of all ecosystems-i.e. annual accumulation of biomass, necromass, and leaf litter (but not plant-derived soil organic matter)-averaged 815 and 910 g m^–2 year^–1 at ambient and elevated CO₂, respectively. These NPPs are remarkably similar to those of many natural moist tropical forested ecosystems. At the same time net productivity of soil organic matter reached 7000 g dry matter equivalent per m² and year (i.e. 3500 g C m^–2 year^–1). Very slight yet statistically significant CO₂-induced shifts in the abundance of groups of species occurred by the end of the experiment, with one group of species (Elettaria cardamomum, Ficus benjamina, F. pumila, Epipremnum pinnatum) gaining slightly, and another group (Ctenanthe lubbersiana, Heliconia humilis, Cecropia peltata) losing. Our results show that: (1) enormous amounts of C can be deposited in the ground which are normally not accounted for in estimates of NPP and net ecosystem productivity; (2) any enhancement of C sequestration under elevated atmospheric CO₂ may be substantially smaller than is believed will occur (yet still very important), especially under growth conditions which permit close to natural NPP; and (3) species dominance in plant communities is likely to change under elevated CO₂, but that changes may occur rather slowly.