Carbon isotope ratios of photolithotrophs from allt meall nan damh,a burn at ardeonaig,Perthshire, and their ecophysiological significance

Summary

δ¹³C measurements were made of dissolved inorganic C, and of submerged benthic cyanobacteria, algae and bryophytes, from Allt Meall nan Damh, a burn at Ardeonaig, Perthshire. The δ¹³C of the CO₂, HCO3/- and CO3/2- components of the inorganic C were computed, and the Δ values of the organic C in the photolithotrophs were then calculated relative to dissolved CO₂. The decreasing order of A values in the Ardeonaig Burn is Lemanea and bryophytes ≥ green macroalgae and Audouinella > diatom mats, which is the same as in the Dighty Burn. However, the Δ values of Lemanea and the bryophytes, which depend on diffusive CO₂ entry, are lower at Ardeonaig than in the Dighty Burn, suggesting greater diffusive limitation to photosynthesis in the Ardeonaig Burn. It is not easy to relate this difference in Δ values in Lemanea to the higher C:N atomic ratio in the Ardeonaig Burn (21.2 ± 0.64) than in the Dighty Burn (9.5–11.0). The Δ values relative to HCO3/- for the HCO3/--using diatom mat in the Ardeonaig Burn is also lower than that in the Dighty Burn; this is consistent with a greater diffusion limitation of photosynthesis in the thicker mats in the Ardeonaig Burn. The δ¹³C of a Lyngbya mat overlying a Lemanea population stranded by low summer water levels indicates that some of the C fixed by the HCO3/--using Lyngbya comes from respiration of low-δ¹³C inorganic C by the Lemanea which is shaded by the Lyngbya. The δ¹³C values of Mesotaenium in its mucilage sheath on a thinly vegetated bank is suggestive of predominant use of the higher CO₂ concentrations with lower δ¹³C from groundwater rather than of atmospheric CO₂ yielding lower dissolved CO₂ concentrations with a higher δ¹³C value.     

Carbon fixation and carbon availability in marine phytoplankton

It is widely believed that inorganic C does not limit the rate of short-term photosynthesis, the net productivity, or the maximum biomass, of marine phytoplankton. This lack of inorganic C restriction is less widely believed to hold for phytoplankton in many low alkalinity freshwaters or for seaweed in nutrient-enriched rock pools. These views are examined in the context of the physical chemistry of the inorganic C system in natural waters and of the ways in which various taxa of phytoplankton deal with inorganic C and discriminate between ¹²C and ¹³C. Using this information to interpret data obtained in the ocean or in freshwater suggests that short-term photosynthesis, production rate, and achieved biomass, of phytoplankton are rarely limited by inorganic C supply but, rather, that the widely suggested factors of limited light, nitrogen or phosphorus supply are the resource inputs which restrict productivity. Global change, by increasing atmospheric CO₂ partial pressure and global mean temperatures, is likely to increase the mean CO₂ concentration in the atmosphere, but the corresponding change in the oceans will be much less. There are, however, genotypic differences in the handling of inorganic C among the diversity of marine phytoplankton, and in impact on use of limiting nutrients, so increases in the mean CO₂ and HCO₃ ^- concentrations in surface ocean waters could cause changes in species composition. However, the rarity of inorganic C limitation of marine phytoplankton short-term photosynthesis, net productivity, or the maximum biomass, in today's ocean means that global change is unlikely to increase these three values in the ocean.     

Detection of a variable intracellular acid‐labile carbon pool in Thalassiosira weissflogii (Heterokontophyta) and Emiliania huxleyi (Haptophyta) in response to changes in the seawater carbon system

Accumulation of an intracellular pool of carbon (C_i pool) is one strategy by which marine algae overcome the low abundance of dissolved CO₂ (CO_2(aq)) in modern seawater. To identify the environmental conditions under which algae accumulate an acid‐labile C_i pool, we applied a ¹⁴C pulse‐chase method, used originally in dinoflagellates, to two new classes of algae, coccolithophorids and diatoms. This method measures the carbon accumulation inside the cells without altering the medium carbon chemistry or culture cell density. We found that the diatom Thalassiosira weissflogii [(Grunow) G. Fryxell & Hasle] and a calcifying strain of the coccolithophorid Emiliania huxleyi [(Lohmann) W. W. Hay & H. P. Mohler] develop significant acid‐labile C_i pools. Ci pools are measureable in cells cultured in media with 2–30 µmol l^?1 CO_2(aq), corresponding to a medium pH of 8.6–7.9. The absolute C_i pool was greater for the larger celled diatoms. For both algal classes, the C_i pool became a negligible contributor to photosynthesis once CO_2(aq) exceeded 30 µmol l^?1. Combining the ¹⁴C pulse‐chase method and ¹⁴C disequilibrium method enabled us to assess whether E. huxleyi and T. weissflogii exhibited thresholds for foregoing accumulation of DIC or reduced the reliance on bicarbonate uptake with increasing CO_2(aq). We showed that the C_i pool decreases with higher CO₂:HCO₃^? uptake rates.     

Photosynthetic carbon assimilation by Crassula helmsii

Photosynthesis of Crassula helmsii, an amphibious aquatic macrophyte weed species, has been measured with respect to pH and irradiance. C. helmsii shows a marked diel fluctuation in titratable acidity, which can be accounted for by changing levels of malic acid. C. helmsii is unable to use HCO _inf3 ^sup- for photosynthesis and exhibits generally low photosynthetic rates when CO₂ is not limiting. The photon flux density at which the onset of light saturation of photosynthesis is reached (E _K ) is low for aquatic macrophytes. Some advantages conferred on C. helmsii by the possession of crassulacean acid metabolism are an extension of the period of assimilation of dissolved inorganic carbon, resulting in a reduction in the limitation imposed on photosynthesis in aquatic environments by a very high CO₂ diffusion resistance.     

Effect of photon flux density on inorganic carbon accumulation and net CO2 exchange in a high-CO2-requiring mutant of Chlamydomonas reinhardtii

The effect of photon flux density on inorganic carbon accumulation and photosynthetic CO₂ assimilation was determined by CO₂ exchange studies at three, limiting CO₂ concentrations with a ca-1 mutant of Chlamydomonas reinhardiii. This mutant accumulates a large internal inorganic carbon pool in the light which apparently is unavailable for photosynthetic assimilation. Although steady-state photosynthetic CO₂ assimilation did not respond to the varying photon flux densities because of CO₂ limitation, components of inorganic-carbon accumulation were not clearly light saturated even at 1100 mol photons m^-2 s^-1, indicating a substantial energy requirement for inorganic carbon transport and accumulation. Steady-state photosynthetic CO₂ assimilation responded to external CO₂ concentrations but not to changing internal inorganic carbon concentrations, confirming that diffusion of CO₂ into the cells supplies most of the CO₂ for photosynthetic assimilation and that the internal inorganic carbon pool is essentially unavailable for photosynthetic assimilation. The estimated concentration of the internal inorganic carbon pool was found to be relatively insensitive to the external CO₂ concentration over the small range tested, as would be expected if the concentration of this pool is limited by the internal to external inorganic carbon gradient. An attempt to use this CO₂ exchange method to determine whether inorganic carbon accumulation and photosynthetic CO₂ assimilation compete for energy at low photon flux densities proved inconclusive.     

Interactions between carbon dioxide and oxygen in the photosynthesis of three species of marine red macroalgae

Summary

Red algae have the highest known selectivity factor (S_rel) for CO₂ over O₂ of ribulose bisphosphate carboxylase-oxygenase (RUBISCO). This allows the prediction that a red alga relying on diffusive supply of CO₂ to RUBISCO from air-equilibrated solution should have less O₂ inhibition of photosynthesis than would an otherwise similar non-red alga with a lower S_rel of RUBISCO. Furthermore, RUBISCO shows an increased S_rel values at low temperatures. The prediction that O ₂inhibition of photosynthesis should be small for marine red algae relying on diffusive CO₂ entry growing in the North Sea with an annual temperature range of 4–16°C was tested in O₂ electrode experiments at 12°C. Phycodrys rubens and Plocamium cartilagineum, which rely on diffusive CO₂ entry showed, as predicted, only a small inhibition at lower inorganic C concentrations. Palmaria palmata, which has a CO₂ concentrating mechanism, had the expected negligible O ₂ inhibition of photosynthesis at any inorganic C concentration except (non-significantly) for saturating inorganic C.     

Water status related photosynthesis and carbon isotope discrimination in species of the lichen genusPseudocyphellaria with green or blue-green photobionts and in photosymbiodemes

Summary Green lichens have been shown to attain positive net photosynthesis in the presence of water vapour while blue-green lichens require liquid water (Lange et al. 1986). This behaviour is confirmed not only for species with differing photobionts in the genusPseudocyphellaria but for green and blue-green photobionts in a single joined thallus (photosymbiodeme), with a single mycobiont, and also when adjacent as co-primary photobionts. The different response is therefore a property of the photobiont. The results are consistent with published photosynthesis/water content response curves. The minimum thallus water content for positive net photosynthesis appears to be much lower in green lichens (15% to 30%, related to dry weight) compared to blue-greens (85% to 100%). Since both types of lichen rehydrate to about 50% water content by water vapour uptake only green lichens will show positive net photosynthesis. It is proposed that the presence of sugar alcohols in green algae allow them to retain a liquid pool (concentrated solution) in their chloroplasts at low water potentials and even to reform it by water vapour uptake after being dried. The previously shown difference in δ¹³C values between blue-green and green lichens is also retained in a photosymbiodeme and must be photobiont determined. The wide range of δ¹³C values in lichens can be explained by a C₃ carboxylation system and the various effects of different limiting processes for photosynthetic CO₂ fixation. If carboxylation is rate limiting, there will be a strong discrimination of¹³CO₂, at high internal CO₂ partial pressure. The resulting very low δ¹³C values (-31 to-35‰) have been found only in green lichens which are able to photosynthesize at low thallus water content by equilibraiton with water vapour. When the liquid phase diffusion of CO₂ becomes more and more rate limiting and the internal CO₂ pressure decreases, the¹³C content of the photosynthates increases and less negative δ¹³C values results, as are found for blue-green lichens.     

Mechanism of c(4) photosynthesis: a model describing the inorganic carbon pool in bundle sheath cells

A theoretical model of the composition of the inorganic carbon pool generated in C₄ leaves during steady-state photosynthesis was derived. This model gives the concentrations of CO₂ and O₂ in the bundle sheath cells for any given net photosynthesis rate and inorganic carbon pool size. The model predicts a bundle sheath CO₂ concentration of 70 micromolar during steady state photosynthesis in a typical C₄ plant, and that about 13% of the inorganic carbon generated in bundle sheath cells would leak back to the mesophyll cells, predominantly as CO₂. Under these circumstances the flux of carbon through the C₄ acid cycle would have to exceed the net rate of CO₂ assimilation by 15.5%. With the calculated O₂ concentration of 0.44 millimolar, the potential photorespiratory CO₂ loss in bundle sheath cells would be about 3% of CO₂ assimilation. Among the factors having a critical influence on the above values are the permeability of bundle sheath chloroplasts to HCO₃⁻, the activity of carbonic anhydrase within these chloroplasts, the assumed stromal volume, and the permeability coefficients for CO₂ and O₂ diffusion across the interface between bundle sheath and mesophyll cells. The model suggests that as the net photosynthesis rate changes in C₄ plants, the level and distribution of the components of the inorganic carbon pool change in such a way that C₄ acid overcycling is maintained in an approximately constant ratio with respect to the net photosynthesis rate.     

Inorganic C-sources for Lemanea,Cladophora and Ranunculus in a fast-flowing stream: Measurements of gas exchange and of carbon isotope ratio and their ecological implications

Summary CO₂-and O₂-exchange characteristics and ¹³C values have been measured in a rhodophycean haptophyte (Lemanea mamillosa), a chlorophycean haptophyte (Cladophora glomerata) and a magnoliophyte rhizophyte (Ranunculus sp.) from a 5 m stretch of the Dichty Burn near Dundee. Light-and CO₂-saturated rates of photosynthesis are greatest on a dry weight basis for Cladophora and lowest for Lemanea; the order is reversed on a surface area basis. The CO₂ concentration at pH 6.5 at which photosynthesis is half-saturated is 25–40 M, with Lemanea rather lower than Cladophora or Ranunculus; these half-saturation values are similar to the free CO₂ concentration in the Burn water. Lemanea cannot use HCO ₃ ^- in photosynthesis, while Cladophora and Ranunculus can. Despite being within a factor or two of saturation with free CO₂ in terms of the bulk water concentration, the growth habit of Cladophora and, particularly, Ranunculus means that the high water velocity in the Burn does not necessarily prevent C depletion effects around the plants, thus providing a possible role for HCO ₃ ^- use by these plants. Lemanea lives in the fastest-growing parts of the Burn, and its growth habit insures that it is exposed to this high water velocity, thus minimising CO₂ depletion during photosynthesis despite the low surface/volume ratio for this plant. ¹³C measurements on the inorganic C in the Burn water are consistent with at least part of its excess (above air-equilibrium) inorganic C levels coming from heterotrophic activity. Lemanea has the most negative ¹³C value of the three plants, consistent with CO₂ use and small diffusion resistances. Ranunculus has the least negative ¹³C value, consistent with some CO₂ depletion and/or HCO ₃ ^- use in situ related to a high diffusion resistance in a rhizophyte which does not have to obtain all of its N and P from the bulk water but can obtain some from the sediments. Cladophora is intermediate, suggesting some CO₂ depletion and/or HCO ₃ ^- use in this densely growing haptophyte.Abbreviations RuBPc-o Ribulose bisphosphate carboxylase-oxygenese (E. C. 4.1.1.39) - PEPc Phosphenolpyruvate carboxylase (E.C 4.1.1.31) - PEPck Phosphoenolpyruvate carboxykinase (ATP) (E.C. 4.1.1.48)     

Photosynthetic gas exchange under emersed conditions in eulittoral and normally submersed members of the Fucales and the Laminariales: interpretation in relation to C isotope ratio and N and water use efficiency

Summary Ten species of brown macroalgae (five eulittoral and one submersed species of the Fucales; four submersed species of the Laminariales) from a rocky shore at Arbroath, Scotland, were examined for characteristics of emersed photosynthesis in relation to the partial pressure of CO₂ and O₂. The five eulittoral species of the Fucaceae were approaching CO₂ saturation for light-saturated photosynthesis at normal air levels of CO₂ (35 Pa) in 21 kPa O₂. The normally submersed algae are further from CO₂ saturation under these conditions, especially in the case of the four members of the Laminariales. The rate of net photosynthesis in the Fucaceae is O₂-independent in the range 2–21 kPa O₂ over the entire range of CO₂ partial pressure tested (compensation up to 95 Pa). For the other five algae tested, net photosynthesis is slightly inhibited by O₂ at 21 kPa relative to 2 kPa over the entire range of CO₂ partial pressures tested (compensation up to 95 Pa). CO₂ compensation partial pressures are low (<0.5 Pa) for the Fucaceae and independent of O₂ in the range 2–42 kPa. For the other five algae, the CO₂ compensation partial pressure are higher, and increased with O₂ partial pressure in the range 2–42 kPa. These gas exchange data show that the Fucaceae exhibit more C₄-like characteristics of their photosynthetic physiology than do the other five species tested, although even the Laminariales and Halidrys siliquosa are not classic C₃ plants in their photosynthetic physiology. These data suggest that, in emersed conditions as well as in the previously reported work on submersed photosynthesis, a CO₂ concentrating mechanism is operating which, by energized transmembrane transport of inorganic C, accumulates CO₂ at the site of RUBISCO and, at least in part, suppresses the oxygenase activity. Work with added extracellular carbonic anhydrase (CA), and with a relatively membrane-impermeant inhibitor of the native extracellular CA activity (acetazolamide), suggests that, in emersed conditions as well as in the previously reported work on algae submersed in seawater at pH 8, HCO _inf3 ^sup– is the major inorganic C species entering the cell. At optimal hydration, the rate of emersed photosynthesis in air is not less than the rate of photosynthesis when submersed in seawater, at least for the Fucaceae. ¹³C ratios of organic C for the Fucaceae are slightly more negative than is the case for the other five algae; these data are consitent with substantial (half or more of the entering inorganic C) leakage of CO₂ from the accumulated pool, and with some contribution of atmospheric CO₂ to the organic C gain by the eulittoral algae. The predicted increase in N use efficiency of photosynthesis in the Fucaceae, with their more strongly developed CO₂ concentrating mechanism, is consistent with data on emersed, but not submersed, photosynthesis for the algae collected from the wild and thus at a poorly defined N status. The more C₄-like gas exchange charateristics of photosynthesis in the eulittoral Fucaceae may be important in increasing the water use efficiency of emersed photosynthesis from the limited capital of water available for transpiration by a haptophyte.     

Discrimination between12C and13C by marine plants

Summary The natural abundance¹³C/¹²C ratios (as δ¹³C) of organic matter of marine macroalgae from Fife and Angus (East Scotland) were measured for comparison with the species' ability to use CO₂ and HCO ₃ ^- for photosynthesis, as deduced from previously published pH-drift measurements. There was a clear difference in δ¹³C values for species able or unable to use HCO ₃ ^- . Six species of Chlorophyta, 12 species of Phaeophyta and 8 species of Rhodophyta that the pH-drift data suggested could use HCO ₃ ^- had δ¹³C values in the range -8.81‰ to -22.55‰. A further 6 species of Rhodophyta which the pH-drift data suggested could only use CO₂ had δ¹³C values in the range -29.90‰ to-34.51‰. One of these six species (Lomentaria articulata) is intertidal; the other five are subtidal and so have no access to atmospheric CO₂ to complicate the analysis. For these species, calculations based on the measured δ¹³C of the algae, the δ¹³C of CO₂ in seawater, and the known¹³C/¹²C discrimination of CO₂ diffusion and RUBISCO carboxylation suggest that only 15–21% of the limitation to photosynthesisin situ results from CO₂ diffusion from the bulk medium to the plastids; the remaining 79–85% is associated with carboxylation reactions (and, via feedback effects, down-stream processes). This analysis has been extended for one of these five species,Delesseria sanguinea, by incorporating data onin situ specific growth rates, respiratory rates measured in the laboratory, and applying Fick's law of diffusion to calculate a boundary layer thickness of 17–24 μm. This value is reasonable for aDelesseria sanguinea frondin situ. For HCO ₃ ^- -using marine macroalgae the range of δ¹³C values measured can be accommodated by a CO₂ efflux from algal cells which range from 0.306 of the gross HCO ₃ ^- influx forEnteromorpha intestinalis (δ¹³C=-8.81‰) in a rockpool to 0.787 forChondrus crispus (δ¹³C=-22.55‰). The relatively high computed CO₂ efflux for those HCO ₃ ^- -users with the more negative δ¹³C values implies a relatively high photon cost of C assimilation; the observed photon costs can be accommodated by assuming coupled, energy-independent inorganic carbon influx and efflux. The observed δ¹³C values are also interpreted in terms of water movement regimes and obtaining CO₂ from the atmosphere. Published δ¹³C values for freshwater macrophytes were compared with the ability of the species to use CO₂ and HCO ₃ ^- and again there was an apparent separation in δ¹³C values for these two groups. δ¹³C values obtained for marine macroalgae for which no pH-drift data are available permit predictions, as yet untested, as to whether they use predominantly CO₂ or HCO ₃ ^-     

Diurnal variation in light and carbon limitation of photosynthesis by two species of submerged freshwater macrophyte with a differential ability to use bicarbonate

SUMMARY.

• 1 Rates of photosynthetic oxygen evolution by Callitriche cophocarpa and Ranunculus peltatus in stream were measured on live occasions during the light period on 2 days at ambient light and ambient inorganic carbon, ambient light and saturating inorganic carbon, saturating light and ambient inorganic carbon, saturating light and saturating inorganic carbon and air-equilibrium inorganic carbon and ambient light.
• 2 Despite an ambient CO₂ concentration of about 220 μm , which is about ten times air-equilibrium, the concentration of inorganic carbon was more limiting than light on all the occasions that rates were measured. On average, rates of photosynthesis at ambient concentrations of CO₂ were about 130 and 425 μmol O₂ g^?1 DW h^?1 for C. cophocarpa and R. peltatus, respectively. These rates as a percentage of carbon saturated rates were only about 35% for C. cophocarpa and about 60% for R. peltatus. Ambient rates as a percentage of light saturated rates were about 80% for C. cophocarpa and about 95% for R. peltatus. Only in early morning and late evening where the photon irradiance was below 160 μmol m^?2 s^?1 was there evidence for slight light limitation.
• 3 Based on results from pH-drift experiments and from rates of photosynthesis as a function of CO₂ concentration in the presence and absence of HCO₃^?, C. cophocarpa was unable, but R. peltatus able to use HCO₃^? at an ambient HCO₃^? concentration of about 0·84 mm . The greater rates of photosynthesis at ambient CO₂ concentration and the lesser limitation by inorganic carbon shown by R. peltatus compared to C. cophocarpa was the result of HCO₃^?-use as laboratory experiments showed that R. peltatus performed similarly to C. cophocarpa if the HCO₃^? concentration was reduced to 60 μm .

     

Stable carbon isotope ratio variations in marine macrophytes along intertidal gradients

Summary The hypothesis that relative water motion and boundary layer diffusion processes affect carbon isotope ratios of aquatic plants was tested in tidal pool and surge zone comparisons of the surfgrass Phyllospadix spp. No evidence was found that submerged plants growing in still upper tidal pools were isotopically different from those growing submerged in lower tidal surge zones. Significant decreases in ¹³C/¹²C ratios for plants growing emersed in the intertidal may have been caused by uptake of atmospheric carbon dioxide. Marine algae (Egregia menziesii and Halosaccion americanum) growing at the same location and tidal elevations as the seagrasses showed somewhat different isotopic fractionation patterns, suggesting that causes of isotopic variability in the seagrasses were not necessarily the same as those in the two marine algae.     

Exogenous inorganic carbon sources for photosynthesis in seawater by members of the Fucales and the Laminariales (Phaeophyta): ecological and taxonomic implications

Summary Characteristics of inorganic carbon assimilation by photosynthesis in seawater were investigated in six species of the Fucales (five Fucaceae, one Cystoseiraceae) and four species of the Laminariales (three Laminariaceae, one Alariaceae) from Arbroath, Scotland. All of the algae tested could photosynthesise faster at high external pH values than the uncatalysed conversion of HCO ₃ ^- to CO₂ can occur, i.e. can use external HCO ₃ ^- . They all had detectable extracellular carbonic anhydrase activity, suggesting that HCO ₃ ^- use could involve catalysis of external CO₂ production, a view supported to some extent by experiments with an inhibitor of carbonic anhydrase. All of the algae tested had CO₂ compensation concentrations at pH 8 which were lower than would be expected from diffusive entry of CO₂ supplying RUBISCO as the initial carboxylase, consistent with the operation of energized entry of HCO ₃ ^- and / or CO₂ acting as a CO₂ concentrating mechanism. Quantitative differences among the algae examined were noted with respect to characteristics of inorganic C assimilation. The most obvious distinction was between the eulittoral Fucaceae, which are emersed for part of, or most of, the tidal cycle, and the other three families (Cystoseiraceae, Laminariaceae, Alariaceae) whose representatives are essentially continually submersed. The Fucaceae examined are able to photosynthesise at high pH values, and have lower CO₂ compensation concentrations, and lower K_1/2 values for inorganic C use in photosynthesis, at pH 8, than the other algae tested. Furthermore, the Fucaceae are essentially saturated with inorganic C for photosynthesis at the normal seawater concentration at pH 8 and 10°C. These characteristics are consistent with the dominant role of a CO₂ concentrating mechanism in CO₂ acquisition by these plants. Other species tested have characteristcs which suggest a less effective HCO ₃ ^- use and CO₂ concentrating mechanism, with the Laminariaceae being the least effective; unlike the Fucaceae, photosynthesis by these algae is not saturated with inorganic C in normal seawater. Taxonomic and ecological implications of these results are considered in relation to related data in the literature.     

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.     

Mechanism of c(4) photosynthesis: the size and composition of the inorganic carbon pool in bundle sheath cells

We sought to characterize the inorganic carbon pool (CO₂ plus HCO₃⁻) formed in the leaves of C₄ plants when C₄ acids derived from CO₂ assimilation in mesophyll cells are decarboxylated in bundle sheath cells. The size and kinetics of labeling of this pool was determined in six species representative of the three metabolic subgroups of C₄ plants. The kinetics of labeling of the inorganic carbon pool of leaves photosynthesizing under steady state conditions in ¹⁴CO₂ closely paralleled those for the C-4 carboxyl of C₄ acids for all species tested. The inorganic carbon pool size, determined from its ¹⁴C content at radioactivity saturation, ranged between 15 and 97 nanomoles per milligram of leaf chlorophyll, giving estimated concentrations in bundle sheath cells of between 160 and 990 micromolar. The size of the pool decreased, together with photosynthesis, as light was reduced from 900 to 95 microeinsteins per square meter per second or as external CO₂ was reduced from 400 to 98 microliters per liter. A model is developed which suggests that the inorganic carbon pool existing in the bundle sheath cells of C₄ plants during steady state photosynthesis will comprise largely of CO₂; that is, CO₂ will only partially equlibrate with bicarbonate. This predominance of CO₂ is believed to be vital for the proper functioning of the C₄ pathway.     

REVIEW: Time lag between photosynthesis and carbon dioxide efflux from soil: a review of mechanisms and controls

CO₂ efflux from soil depends on the availability of organic substances respired by roots and microorganisms. Therefore, photosynthetic activity supplying carbohydrates from leaves to roots and rhizosphere is a key driver of soil CO₂. This fact has been overlooked in most soil CO₂ studies because temperature variations are highly correlated with solar radiation and mask the direct effect of photosynthesis on substrate availability in soil. This review highlights the importance of photosynthesis for rhizosphere processes and evaluates the time lag between carbon (C) assimilation and CO₂ release from soil. Mechanisms and processes contributing to the lag were evaluated. We compared the advantages and shortcomings of four main approaches used to estimate this time lag: (1) interruption of assimilate flow from leaves into the roots and rhizosphere, and analysis of the decrease of CO₂ efflux from soil, (2) time series analysis (TSA) of CO₂ fluxes from soil and photosynthesis proxies, (3) analysis of natural δ¹³C variation in CO₂ with photosynthesis‐related parameters or δ¹³C in the phloem and leaves, and (4) pulse labeling of plants in artificial ¹⁴CO₂ or ¹³CO₂ atmosphere with subsequent tracing of ¹⁴C or ¹³C in CO₂ efflux from soil. We concluded that pulse labeling is the most advantageous approach. It allows clear evaluation not only of the time lag, but also of the label dynamics in soil CO₂, and helps estimate the mean residence time of recently assimilated C in various above‐ and belowground C pools. The impossibility of tracing the phloem pressure–concentration waves by labeling approach may be overcome by its combination with approaches based on TSA of CO₂ fluxes and its δ¹³C with photosynthesis proxies. Numerous studies showed that the time lag for grasses is about 12.5±7.5 (SD) h. The time lag for mature trees was much longer (～4–5 days). Tree height slightly affected the lag, with increasing delay of 0.1 day m^?1. By evaluating bottle‐neck processes responsible for the time lag, we conclude that, for trees, the transport of assimilates in phloem is the rate‐limiting step. However, it was not possible to predict the lag based on the phloem transport rates reported in the literature. We conclude that studies of CO₂ fluxes from soil, especially in ecosystems with a high contribution of root‐derived CO₂, should consider photosynthesis as one of the main drivers of C fluxes. This calls for incorporating photosynthesis in soil C turnover models.     

Assimilate partitioning affects 13C fractionation of recently assimilated carbon in maize

Coupling ¹³C natural abundance and ¹⁴C pulse labelling enabled us to investigate the dependence of ¹³C fractionation on assimilate partitioning between shoots, roots, exudates, and CO₂ respired by maize roots. The amount of recently assimilated C in these four pools was controlled by three levels of nutrient supply: full nutrient supply (NS), 10 times diluted nutrient supply (DNS), and deionised water (DW). After pulse labelling of maize shoots in a ¹⁴CO₂ atmosphere, ¹⁴C was traced to determine the amounts of recently assimilated C in the four pools and the δ¹³C values of the four pools were measured. Increasing amounts of recently assimilated C in the roots (from 8% to 10% of recovered ¹⁴C in NS and DNS treatments) led to a 0.3‰ ¹³C enrichment from NS to DNS treatments. A further increase of C allocation in the roots (from 10% to 13% of recovered ¹⁴C in DNS and DW treatments) resulted in an additional enrichment of the roots from DNS to DW treatments by 0.3‰. These findings support the hypothesis that ¹³C enrichment in a pool increases with an increasing amount of C transferred into that pool. δ¹³C of CO₂ evolved by root respiration was similar to that of the roots in DNS and DW treatments. However, if the amount of recently assimilated C in root respiration was reduced (NS treatment), the respired CO₂ became 0.7‰ ¹³C depleted compared to roots. Increasing amounts of recently assimilated C in the CO₂ from NS via DNS to DW treatments resulted in a 1.6‰ δ¹³C increase of root respired CO₂ from NS to DW treatments. Thus, for both pools, i.e. roots and root respiration, increasing amounts of recently assimilated C in the pool led to a δ¹³C increase. In DW and DNS plants there was no ¹³C fractionation between roots and exudates. However, high nutrient supply decreased the amount of recently assimilated C in exudates compared to the other two treatments and led to a 5.3‰ ¹³C enrichment in exudates compared to roots. We conclude that ¹³C discrimination between plant pools and within processes such as exudation and root respiration is not constant but strongly depends on the amount of C in the respective pool and on partitioning of recently assimilated C between plant pools. Section Editor: H. Lambers     

Effects of atmospheric CO2 enrichment on δ 13C, δ 15N values and turnover times of soil organic matter pools isolated by thermal techniques

CO₂ applied for Free-Air CO₂ Enrichment (FACE) experiments is strongly depleted in ¹³C and thus provides an opportunity to study C turnover in soil organic matter (SOM) based on its δ ¹³C value. Simultaneous use of ¹⁵N labeled fertilizers allows N turnover to be studied. Various SOM fractionation approaches (fractionation by density, particle size, chemical extractability etc.) have been applied to estimate C and N turnover rates in SOM pools. The thermal stability of SOM coupled with C and N isotopic analyses has never been studied in experiments with FACE. We tested the hypothesis that the mean residence time (MRT) of SOM pools is inversely proportional to its thermal stability. Soil samples from FACE plots under ambient (380 ppm) and elevated CO₂ (540 ppm; for 3 years) treatments were analyzed by thermogravimetry coupled with differential scanning calorimetry (TG-DSC). Based on differential weight losses (TG) and energy release or consumption (DSC), five SOM pools were distinguished. Soil samples were heated up to the respective temperature and the remaining soil was analyzed for δ ¹³C and δ ¹⁵N by IRMS. Energy consumption and mass losses in the temperature range 20–200°C were mainly connected with water volatilization. The maximum weight losses occurred from 200–310°C. This pool contained the largest amount of carbon: 61% of the total soil organic carbon in soil under ambient treatment and 63% in soil under elevated CO₂, respectively. δ ¹³C values of SOM pools under elevated CO₂ treatment showed an increase from −34.3‰ of the pool decomposed between 20–200°C to −18.1‰ above 480°C. The incorporation of new C and N into SOM pools was not inversely proportional to its thermal stability. SOM pools that decomposed between 20–200 and 200–310°C contained 2 and 3% of the new C, with a MRT of 149 and 92 years, respectively. The pool decomposed between 310–400°C contained the largest proportion of new C (22%), with a MRT of 12 years. The amount of fertilizer-derived N after 2 years of application in ambient and elevated CO₂ treatments was not significantly different in SOM pools decomposed up to 480°C having MRT of about 60 years. In contrast, the pool decomposed above 480°C contained only 0.5% of new N, with a MRT of more than 400 years in soils under both treatments. Thus, the separation of SOM based on its thermal stability was not sufficient to reveal pools with contrasting turnover rates of C and N. Responsible Editor: Bernard Nicolardot.     

C, N and P nutrition of Lemanea mamillosa Kütz. (Batrachospermales, Rhodophyta) in the Dighty Burn, Angus, U.K.

Abstract. Field measurements of the growth rate of the red freshwater macroalga Lemanca mamillosa Kutz, in the Dighty Burn, together with measurements of water velocity, [CO₂], [NO₃], [NH₃+ NH₄⁺] and [phosphate], have been made between February and July. This period covers the growth of the erect gametophyte and later of the carposporophyte inside the gametophyte. Hydrodynamic studies in the laboratory on benzoic acid models of the gametophyte suggest an average in situ unstirred layer some 12 μm thick. For growth of the gametophyte, this estimated boundary layer thickness, together with the measured inorganic C transport pathway within the plant, suggest that growth is not significantly restricted by CO₂ transport from the bulk phase to the plastids. δ¹³C measurements on source CO₂ and on plant organic C bear this out. Habitat choice (low temperatures: CO₂ enrichment from ground-water input: rapid water flow), plant morphology and anatomy (turbulence-generating ‘knobbles’ on the nodes; plastids close to the outside of the plant), and plant biochemistry (high CO₂ affinity of the RUBISCO carboxylase; quite high carbonic anhydrase activity) are responsible for this lack of limitation by inorganic C transport in the growing gametophyte which lacks HCO₃ transport and a CO₂ concentrating mechanism. Transport through the boundary layer does not significantly restrict acquisition by the plant of N (probably as NH₄⁺, despite the preponderance of NO₃ in the environment) or of P (as orthophosphate) in the field. The membrane transporters, which have high substrate affinities (K½'s about 2 mmol NH₄⁺ m^-3 and < 2 mmol inorganic phosphate m^?3), probably impose the major limitation. The development of the carposporophyte later in the season, and an increase in the thickness of the cortex of the gametophyte, result in an increased (less negative) δ¹³C, suggesting a significant diffusion limitation to CO₂ transport. This conclusion is reinforced by consideration of the opposing effect on Δδ¹³ C of the decreased demand for products of phosphoenolpyruvate carboxylase activity as the N/C ratio decreases late in the growing season.