The expression of a carbon concentrating mechanism in Chlamydomonas acidophila under variable phosphorus, iron, and CO2 concentrations

The CO(2) acquisition was analyzed in Chlamydomonas acidophila at pH 2.4 in a range of medium P and Fe concentrations and at high and low CO(2) condition. The inorganic carbon concentrating factor (CCF) was related to cellular P quota (Q(p)), maximum CO(2)-uptake rate by photosynthesis (V(max,O2)), half saturation constant for CO(2) uptake (K(0.5)), and medium Fe concentration. There was no effect of the medium Fe concentration on the CCF. The CCF increased with increasing Q(p) in both high and low CO(2) grown algae, but maximum Q(p) was 6-fold higher in the low CO(2) cells. In high CO(2) conditions, the CCF was low, ranging between 0.8 and 3.5. High CCF values up to 9.1 were only observed in CO(2)-limited cells, but P- and CO(2)-colimited cells had a low CCF. High CCF did not relate with a low K(0.5) as all CO(2)-limited cells had a low K(0.5) (<4 μM CO(2)). High C(i)-pools in cells with high Q(p) suggested the presence of an active CO(2)-uptake mechanism. The CCF also increased with increasing V(max,O2) which reflect an adaptation to the nutrient in highest demand (CO(2)) under balanced growth conditions. It is proposed that the size of the CCF in C. acidophila is more strongly related to porter density for CO(2) uptake (reflected in V(max,O2)) and less- to high-affinity CO(2) uptake (low K(0.5)) at balanced growth. In addition, high CCF can only be realized with high Q(p).     

Interactions between P-limitation and different C conditions on the fatty acid composition of an extremophile microalga

The extremophilic microalga Chlamydomonas acidophila inhabits very acidic waters (pH 2-3.5), where its growth is often limited by phosphorus (P) or colimited by P and inorganic carbon (CO(2)). Because this alga is a major food source for predators in acidic habitats, we studied its fatty acid content, which reflects their quality as food, grown under a combination of P-limited and different carbon conditions (either mixotrophically with light + glucose or at high or low CO(2), both without glucose). The fatty acid composition largely depended on the cellular P content: stringent P-limited cells had a higher total fatty acid concentration and had a lower percentage of polyunsaturated fatty acids. An additional limitation for CO(2) inhibited this decrease, especially reflected in enhanced concentrations of 18:3(9,12,15) and 16:4(3,7,10,13), resulting in cells relatively rich in polyunsaturated fatty acids under colimiting growth conditions. The percentage of polyunsaturated to total fatty acid content was positively related with maximum photosynthesis under all conditions applied. The two factors, P and CO(2), thus interact in their effect on the fatty acid composition in C. acidophila, and colimited cells P-limited algae can be considered a superior food source for herbivores because of the high total fatty acid content and relative richness in polyunsaturated fatty acids.     

Interactive effects of elevated CO2, warming, and drought on photosynthesis of Deschampsia flexuosa in a temperate heath ecosystem

Global change factors affect plant carbon uptake in concert. In order to investigate the response directions and potential interactive effects, and to understand the underlying mechanisms, multifactor experiments are needed. The focus of this study was on the photosynthetic response to elevated CO(2) [CO2; free air CO(2) enrichment (FACE)], drought (D; water-excluding curtains), and night-time warming (T; infrared-reflective curtains) in a temperate heath. A/C(i) curves were measured, allowing analysis of light-saturated net photosynthesis (P(n)), light- and CO(2)-saturated net photosynthesis (P(max)), stomatal conductance (g(s)), the maximal rate of Rubisco carboxylation (V(cmax)), and the maximal rate of ribulose bisphosphate (RuBP) regeneration (J(max)) along with leaf δ(13)C, and carbon and nitrogen concentration on a monthly basis in the grass Deschampsia flexuosa. Seasonal drought reduced P(n) via g(s), but severe (experimental) drought decreased P(n) via a reduction in photosynthetic capacity (P(max), J(max), and V(cmax)). The effects were completely reversed by rewetting and stimulated P(n) via photosynthetic capacity stimulation. Warming increased early and late season P(n) via higher P(max) and J(max). Elevated CO(2) did not decrease g(s), but stimulated P(n) via increased C(i). The T×CO2 synergistically increased plant carbon uptake via photosynthetic capacity up-regulation in early season and by better access to water after rewetting. The effects of the combination of drought and elevated CO(2) depended on soil water availability, with additive effects when the soil water content was low and D×CO2 synergistic stimulation of P(n) after rewetting. The photosynthetic responses appeared to be highly influenced by growth pattern. The grass has opportunistic water consumption, and a biphasic growth pattern allowing for leaf dieback at low soil water availability followed by rapid re-growth of active leaves when rewetted and possibly a large resource allocation capability mediated by the rhizome. This growth characteristic allowed for the photosynthetic capacity up-regulations that mediated the T×CO2 and D×CO2 synergistic effects on photosynthesis. These are clearly advantageous characteristics when exposed to climate changes. In conclusion, after 1 year of experimentation, the limitations by low soil water availability and stimulation in early and late season by warming clearly structure and interact with the photosynthetic response to elevated CO(2) in this grassland species.     

Acclimation of wild-type cells and CO2-insensitive mutants of the green alga Chlorella ellipsoidea to elevated [CO2

CO(2)-insensitive mutants of the green alga Chlorella ellipsoidea were previously shown to be unable to repress an inorganic carbon-concentrating mechanism (CCM) when grown under 5% CO(2). When air-grown, wild-type (WT) cells were transferred to 5% CO(2), an abrupt drop of P(max) to 43% the original level of air-grown cells was observed within the initial 12 h. Photosynthetic affinities of WT cells to dissolved inorganic carbon (DIC) were maintained at high levels for the initial 4 d of acclimation, and then decreased gradually to lower levels over the next 6 d. In contrast to WT cells, the CO(2)-insensitive mutant, ENU16, exhibited a constant P(max) at maximum levels and a low K(1/2)[DIC] throughout the acclimation period. The rapid P(max) drop within 12 h of acclimation in WT cells was significantly reduced by treatment with 0.5 mm of 6-ethoxybenzothiazole-2-sulphonamide (EZA), a specific membrane-permeable inhibitor of carbonic anhydrase (CA), suggesting the participation of internal CAs in the temporary drop in P(max) in WT cells. WT and ENU16 cells were grown in controlled equilibrium [CO(2)], and the photosynthetic rate of each acclimated cell type was measured under equilibrated growth [DIC] conditions. In WT cells acclimated to 0.14-0.4% [CO(2)], K(1/2)[DIC] values increased as [CO(2)] increased, and the photosynthetic rates at growth DIC conditions were shown to decrease to about 70% the P(max) level in this intermediate [CO(2)] range. Such decreases in the net photosynthetic rates were not observed in ENU16. These results suggest that algal primary production could be depressed significantly under moderately enriched CO(2) conditions as a result of acquiring intermediate affinities for DIC because of their sensitive responses to changes in the ambient [CO(2)].     

Phosphorus uptake and growth of blue-green alga, Microcystis aeruginosa

The specific uptake rate Q(p) of orthophosphate (expressed throughout as phosphorus) and the specific growth rate mu of Microcystis aeruginosa were measured using batch-precultured cells, whose growth phase, and intracellular and extracellular phosphorus concentrations f(p) and P, respectively, had been changed. When the cells from phosphorus-rich precultures were used, smaller values of Q(p) (0.1-0.3 mug P mg dry wt. (-1) h (-1)) were observed. However, if phosphorusstarved cells were used, the initial value of Q(p) was enhanced to more than ten times those smaller values referred to above, but declined rapidly with time after the transfer. Q(p) leveled off at around t = 4 h, when f(p) approached the maximum value, even if phosphorus was still available in the medium. A new correlation was presented here with respect to Q(p) as a function of P and f(p) as follows: \documentclass{article}\pagestyle{empty}\begin{document}$$ Q_p = Q_{p,\max } \frac{P}{{K_p + P}}\frac{{(f_{p,\max } - f_p )}}{{(f_{p,\max } - f_p )}} $$\end{document} Although numerical values of parameters involved in the equation depend on physiological state (or preculture history) of the cells, the above equation could account not only for phosphorus uptake, during which changes in phosphorus content in the cells were observed, but also for initial rates of uptake presented previously by other workers. mu Values were confirmed to be a hyperbolic function of f(p) as has been suggested by previous workers.     

Regulation of Nitrite Reductase Activity under CO2 Limitation in the Cyanobacterium Synechococcus sp. PCC7942

During photoautotrophic growth under CO2-limited conditions, cells of Synechococcus sp. PCC7942 excreted into the medium about 30% of the nitrite produced by reduction of nitrate. No nitrite was excreted under CO2-sufficient conditions. After transfer of high-CO2-grown cells to CO2-limited conditions, nitrite reductase activity started to decline within 0.5 h and decreased to 50% of the initial level in 3 h, whereas nitrate reductase activity was virtually unchanged. Nitrite started to accumulate in the medium about 3 h after the transfer of the cells to CO2-limited conditions and reached a concentration of >0.4 mM at 17 h. These findings suggested that the nitrite excretion was due to an imbalance of the activities of nitrite reductase and nitrate reductase. Since ammonium, the product of nitrite reduction, was not detected in the medium, it was concluded that the step of nitrite reduction limits the rate of nitrate assimilation under CO2-limited conditions. The extent of decrease in nitrite reductase activity under CO2-limited conditions was much larger than that caused by rifampicin (an inhibitor of RNA synthesis) treatment under high-CO2 conditions. Addition of CO2, in the form of sodium bicarbonate, to the CO2-limited culture increased the nitrite reductase activity, but rifampicin inhibited this increase. These findings suggested the presence of a mechanism that irreversibly inactivates nitrite reductase under CO2-limited conditions.     

Nutrient and genotypic effects on CO2-responsiveness: photosynthetic regulation in Leucadendron species of a nutrient-poor environment

Four South African Leucadendron congenerics with divergent soil N and P preferences were grown as juveniles at contrasting nutrient concentrations at ambient (350 mol mol^-1) and elevated (700 mol mol^-1) atmospheric CO2 levels. Photosynthetic parameters were related to leaf nutrient and carbohydrate status to reveal controls of carbon uptake rate. In all species, elevated CO2 depressed both the maximum Rubisco catalytic activity (Vc,max, by 19-44%) and maximum electron transport rate (Jmax, by 13-39%), indicating significant photosynthetic acclimation of both measures. Even so, all species had increased maximum light-saturated rate of net CO2 uptake (Amax)) at the elevated growth CO2 level, due to higher intercellular CO2 concentration (ci). Leaf nitrogen concentration was central to photosynthetic performance, correlating with Amax, Vc,max and Jmax, Vc,max and Jmax were linearly co-correlated, revealing a relatively invariable Jmax:Vc,max ratio, probably due to N resource optimization between light harvesting (RuBP regeneration) and carboxylation. Leaf total non-structural carbohydrate concentration (primarily starch) increased in high CO2, and was correlated with the reduction in Vc,max and Jmax. Apparent feedback control of Vc,max and Jmax was thus surprisingly consistent across all species, and may regulate carbon exchange in response to end-product fluctuation. If so, elevated CO2 may have emulated an excess end-product condition, triggering both Vc,max and Jmax down-regulation. In Leucadendron, a general physiological mechanism seems to control excess carbohydrate formation, and photosynthetic responsiveness to elevated CO2, independently of genotype and nutrient concentration. This mechanism may underlie photosynthetic acclimation to source:sink imbalances resulting from such diverse conditions as elevated CO2, low sink strength, low carbohydrate export, and nutrient limitation.     

Effect of CO2 on the fermentation capacities of the acetogen Peptostreptococcus productus U-1.

The fermentative capacities of the acetogenic bacterium Peptostreptococcus productus U-1 (ATCC 35244) were examined. Although acetate was formed from all the substrates tested, additional products were produced in response to CO2 limitation. Under CO2-limited conditions, fructose-dependent growth yielded high levels of lactate as a reduced end product; lactate was also produced under CO2-enriched conditions when fructose concentrations were elevated. In the absence of supplemental CO2, xylose-dependent growth yielded lactate and succinate as major reduced end products. Although supplemental CO2 and acetogenesis stimulated cell yields on fructose, xylose-dependent cell yields were decreased in response to CO2 and acetogenesis. In contrast, glycerol-dependent growth yielded high levels of ethanol in the absence of supplemental CO2, and pyruvate was subject to only acetogenic utilization independent of CO2. CO2 pulsing during the growth of CO2-limited fructose cultures stopped lactate synthesis immediately, indicating that CO2-limited cells were nonetheless metabolically poised to respond quickly to exogenous CO2. Resting cells that were cultivated at the expense of fructose without supplemental CO2 readily consumed fructose in the absence of exogenous CO2 and formed only lactate. Although the specific activity of lactate dehydrogenase was not appreciably influenced by supplemental C02 during cultivation, cells cultivated on fructose under CO2-enriched conditions displayed minimal capacities to consume fructose in the absence of exogenous CO2. These results demonstrate that the utilization of alternative fermentations for the conservation of energy and growth of P. productus U-1 is augmented by the relative availability of CO2 and growth substrate.     

Biosynthesis of Poly(3-Hydroxyalkanoic Acid) Copolymer from CO(inf2) in Pseudomonas acidophila through Introduction of the DNA Fragment Responsible for Chemolithoautotrophic Growth of Alcaligenes hydrogenophilus

Pseudomonas acidophila is a bacterial strain producing a poly(3-hydroxyalkanoic acid) (PHA) copolymer from low-molecular-weight organic compounds such as formate and acetate. The genes responsible for PHA production were cloned in cosmid pIK7 containing a 14.8-kb HindIII fragment of P. acidophila DNA. With the aim of developing a means of producing a PHA copolymer from CO(inf2), cosmid pIK7 was introduced into a polymer-negative mutant of the chemolithoautotrophic bacterium Alcaligenes eutrophus PHB(sup-)4. However, the recombinant strain produced a homopolymer of 3-hydroxybutyric acid (polyhydroxybutyric acid) from CO(inf2). Since it was thought that the composition of the accumulated polymer might depend not on the PHA biosynthetic genes but on the metabolism of the host strain, a recombinant plasmid, pFUS, containing the genes for chemolithoautotrophic growth of the hydrogen-oxidizing bacterium A. hydrogenophilus was introduced into P. acidophila by conjugation. The recombinant plasmid pFUS was stably maintained in P. acidophila in the absence of chemolithoautotrophic or antibiotic selection. This pFUS-harboring strain possessed the ability to grow under a gas mixture of H(inf2), O(inf2), and CO(inf2) in a mineral salts medium, and PHA copolymer accumulation was confirmed by nuclear magnetic resonance spectral analysis. A gas chromatogram obtained by gas chromatography-mass spectrometry showed the composition of the polymer to be 52.8% 3-hydroxybutyrate, 41.1% 3-hydroxyoctanoate, and 6.1% 3-hydroxydecanoate. This is the first report of the production of a PHA copolymer from CO(inf2) as sole carbon source.     

Reduced exercise arteriovenous O2 difference in Type 2 diabetes.

Maximal O(2) consumption (Vo(2 max)) is lower in individuals with Type 2 diabetes than in sedentary nondiabetic individuals. This study aimed to determine whether the lower Vo(2 max) in diabetic patients was due to a reduction in maximal cardiac output (Q(max)) and/or peripheral O(2) extraction. After 11 Type 2 diabetic patients and 12 nondiabetic subjects, matched for age and body composition, who had not exercised for 2 yr, performed a bicycle ergometer exercise test to determine Vo(2 max), submaximal cardiac output, Q(max), and arterial-mixed venous O(2) (a-v O(2)) difference were assessed. Maximal workload, Vo(2 max), and maximal a-v O(2) difference were lower in Type 2 diabetic patients (P < 0.05). Q(max) was low in both groups but not significantly different: 11.2 and 10.0 l/min for controls and diabetic patients, respectively (P > 0.05). Submaximal O(2) uptake and heart rate were lower at several workloads in diabetic patients; respiratory exchange ratio was similar between groups at all workloads. Vo(2 max) was linearly correlated with a-v O(2) difference, but not Q(max) in diabetic patients. These data suggest that a reduction in maximal a-v O(2) difference contributes to a decreased Vo(2 max) in Type 2 diabetic patients.     

Comparison of the continuous flotation performances of Saccharomyces cerevisiae LBG H620 and DSM 2155 strains

Saccharomyces cerevisiae LBG H620 and DSM 2155 strains were continuously cultivated under carbon (C)-limited, phosphorus (P)-limited and nitrogen (N)-limited growth conditions. Cell and protein concentrations in feed, foam, and residue as well as the degree of cell recovery and the rate of foaming were measured, and the concentration and enrichment factors were evaluated at different dilution rates (D). The LBG H620 cells were reduced, while the DSM 2155 cells were enriched in the foam. The highest concentration factors in DSM 2155 cells were attained if they were cultivated under strong P-limitation at a low D. Fairly high concentration factors were also found under C-limitation. Under N-limitation, low concentration factors were found with low Ds. At the beginning of the continuous cultivations, all of the cells were recovered, but with advancing time the degree of recovery and cell concentration and the enrichment factor ratio diminished. The cellular properties of the yeast were characterized by flow cytometry, and the surface properties by measurements of their hydrophobicity, electrophoretic mobility, and chemical composition (using X-ray photoelectron spectroscopy, XPS). These investigations indicated that the large difference in flotation between the two strains is due to different surface properties. Strain DSM 2155 has higher surface hydrophobicity and lower electrokinetic potential. Cell wall properties and the cell flotation depend on medium composition and age of the culture.Correspondence to: K. Schügerl     

Expression of an aromatic-dependent decarboxylase which provides growth-essential CO2 equivalents for the acetogenic (Wood) pathway of Clostridium thermoaceticum.

The acetogen Clostridium thermoaceticum generates growth-essential CO2 equivalents from carboxylated aromatic compounds (e.g., 4-hydroxybenzoate), and these CO2 equivalents are likely integrated into the acetogenic pathway (T. Hsu, S. L. Daniel, M. F. Lux, and H. L. Drake, J. Bacteriol. 172:212-217, 1990). By using 4-hydroxybenzoate as a model substrate, an assay was developed to study the expression and activity of the decarboxylase involved in the activation of aromatic carboxyl groups. The aromatic-dependent decarboxylase was induced by carboxylated aromatic compounds in the early stages of growth and was not repressed by glucose or other acetogenic substrates; nonutilizable carboxylated aromatic compounds did not induce the decarboxylase. The decarboxylase activity displayed saturation kinetics at both whole-cell and cell extract levels, was sensitive to oxidation, and was not affected by exogenous energy sources. However, at the whole-cell level, metabolic inhibitors decreased the decarboxylase activity. Supplemental biotin or avidin did not significantly affect decarboxylation. The aromatic-dependent decarboxylase was specific for benzoates with a hydroxyl group in the para position of the aromatic ring; the meta position could be occupied by various substituent groups (-H, -OH, -OCH3, -Cl, or -F). The carboxyl carbon from [carboxyl-14C] vanillate went primarily to 14CO2 in short-term decarboxylase assays. During growth, the aromatic carboxyl group went primarily to CO2 under CO2-enriched conditions. However, under CO2-limited conditions, the aromatic carboxyl carbon went nearly totally to acetate, with equal distribution between the carboxyl and methyl carbons, thus demonstrating that acetate could be totally synthesized from aromatic carboxyl groups. In contrast, when cocultivated (i.e., supplemented) with CO under CO2-limited conditions, the aromatic carboxyl group went primarily to the methyl carbon of acetate.     

Aquatic adventitious roots of the wetland plant Meionectes brownii can photosynthesize: implications for root function during flooding

? Many wetland plants produce aquatic adventitious roots from submerged stems. Aquatic roots can form chloroplasts, potentially producing endogenous carbon and oxygen. Here, aquatic root photosynthesis was evaluated in the wetland plant Meionectes brownii, which grows extensive stem-borne aquatic roots during submergence. ? Underwater photosynthetic light and CO(2) response curves were determined for aquatic-adapted leaves, stems and aquatic roots of M. brownii. Oxygen microelectrode and (14)CO(2)-uptake experiments determined shoot inputs of O(2) and photosynthate into aquatic roots. ? Aquatic adventitious roots contain a complete photosynthetic pathway. Underwater photosynthetic rates are similar to those of stems, with a maximum net photosynthetic rate (P(max)) of 0.38 μmol O(2) m(-2) s(-1); however, this is c. 30-fold lower than that of aquatic-adapted leaves. Under saturating light with 300 mmol m(-3) dissolved CO(2), aquatic roots fix carbon at 0.016 μmol CO(2) g(-1) DM s(-1). Illuminated aquatic roots do not rely on exogenous inputs of O(2). ? The photosynthetic ability of aquatic roots presumably offers an advantage to submerged M. brownii as aquatic roots, unlike sediment roots, need little O(2) and carbohydrate inputs from the shoot when illuminated.     

Growth yield coefficients of Sphingomonas sp. strain P5 on various chlorophenols in chemostat culture

A polychlorophenol-degrading bacterium, Sphingomonas sp. strain P5, was grown in 2,6-dichlo-rophenol(26-DCP)-limited, 2,3,6-trichlorophenol(236-TCP)-limited, 2,4,6-trichlorophenol(246-TCP)-limited, 2,3,4,6-tetrachlorophenol(2346-TeCP)-limited, and pentachlorophenol(PCP)-limited chemostat cultures at a dilution rate of 0.02 ± 0.002 h⁻¹. The cultures were analyzed for the yield coefficient for growth on chlorophenol during steady-state conditions. The average growth yields coefficients (as carbon conversion efficiencies) were 0.252, 0.230, 0.219, 0.157, and 0.121 mol C mol C⁻¹ for 26-DCP, 236-TCP, 246-TCP, 2346-TeCP, and PCP respectively. The differences in growth yield can be interpreted in terms of the energetics of chlorinated carbon metabolism; i.e. substitution of the phenol moiety reduces the available metabolic energy by one electron per chlorine. The growth yield coefficients on chlorinated phenols were lower than the yield coefficients of heterotrophic growth reported in the literature on non-chlorinated and aliphatic compounds. Metabolic origins for low growth yield coefficients on (chlorinated) aromatic compounds are postulated. Received: 7 April 1997 / Received revision: 7 July 1997 / Accepted: 12 July 1997     

Effect of mild carboxy-hemoglobin on exercising skeletal muscle: intravascular and intracellular evidence

We studied muscle blood flow, muscle oxygen uptake (VO(2)), net muscle CO uptake, Mb saturation, and intracellular bioenergetics during incremental single leg knee-extensor exercise in five healthy young subjects in conditions of normoxia, hypoxia (H; 11% O(2)), normoxia + CO (CO(norm)), and 100% O(2) + CO (CO(hyper)). Maximum work rates and maximal oxygen uptake (VO(2 max)) were equally reduced by approximately 14% in H, CO(norm), and CO(hyper). The reduction in arterial oxygen content (Ca(O(2))) (approximately 20%) resulted in an elevated blood flow (Q) in the CO and H trials. Net muscle CO uptake was attenuated in the CO trials. Suprasystolic cuff measurements of the deoxy-Mb signal were not different in terms of the rate of signal rise or maximum signal attained with and without CO. At maximal exercise, calculated mean capillary PO(2) was most reduced in H and resulted in the lowest Mb-associated PO(2). Reductions in ATP, PCr, and pH during H, CO(norm), and CO(hyper) occurred earlier during progressive exercise than in normoxia. Thus the effects of reduced Ca(O(2)) due to mild CO poisoning are similar to H.     

Carbohydrate and Amino Acid Fermentation in the Free-Living Primitive Protozoon Hexamita sp

Hexamita sp. is an amitochondriate free-living diplomonad which inhabits O(2)-limited environments, such as the deep waters and sediments of lakes and marine basins. C nuclear magnetic resonance spectroscopy reveals ethanol, lactate, acetate, and alanine as products of glucose fermentation under microaerobic conditions (23 to 34 muM O(2)). Propionic acid and butyric acid were also detected and are believed to be the result of fermentation of alternative substrates. Production of organic acids was greatest under microaerobic conditions (15 muM O(2)) and decreased under anaerobic (<0.25 muM O(2)) and aerobic (200 to 250 muM O(2)) conditions. Microaerobic incubation resulted in the production of high levels of oxidized end products (70% acetate) compared to that produced under anoxic conditions (20% acetate). In addition, data suggest that Hexamita cells contain the arginine dihydrolase pathway, generating energy from the catabolism of arginine to citrulline, ornithine, NH(4), and CO(2). The rate of arginine catabolism was higher under anoxic conditions than under microaerobic conditions. Hexamita cells were able to grow in the absence of a carbohydrate source, albeit with a lower growth rate and yield.     

Inorganic carbon limitation and mixotrophic growth in Chlamydomonas from an acidic mining lake

Plankton communities in acidic mining lakes (pH 2.5-3.3) are species-poor because they face extreme environmental conditions, e.g. 150mg l(-1) Fe2+ +Fe3+. We investigated the growth characteristics of the dominant pigmented species, the flagellate Chlamydomonas acidophila, in semi-continuous culture experiments under in situ conditions. The following hypotheses were tested: (1) Low inorganic carbon (IC) concentrations in the epilimnion (e.g. 0.3 mg l(-1)) arising from the low pH limit phototrophic growth (H-1); (2) the additional use of dissolved organic carbon (mixotrophy) leads to higher growth rates under IC-limitation (H-2), and (3) phagotrophy is not relevant (H-3). H-1 was supported as the culture experiments, in situ PAR and IC concentrations indicated that IC potentially limited phototrophic growth in the mixed surface layers. H-2 was also supported: mixotrophic growth always exceeded pure phototrophic growth even when photosynthesis was saturated. Dark growth in filtered lake water illuminated prior to inoculation provided evidence that Chlamydomonas was able to use the natural DOC. The alga did not grow on bacteria, thus confirming H-3. Chlamydomonas exhibited a remarkable resistance to starvation in the dark. The compensation light intensity (ca. 20 micromol photons m(-2) s(-1)) and the maximum phototrophic growth (1.50 d(-1)) fell within the range of algae from non-acidic waters. Overall, Chlamydomonas, a typical r-strategist in circum-neutral systems, showed characteristics of a K-strategist in the stable, acidic lake environment in achieving moderate growth rates and minimizing metabolic losses.     

Independent colimitation for carbon dioxide and inorganic phosphorus

Simultaneous limitation of plant growth by two or more nutrients is increasingly acknowledged as a common phenomenon in nature, but its cellular mechanisms are far from understood. We investigated the uptake kinetics of CO(2) and phosphorus of the algae Chlamydomonas acidophila in response to growth at limiting conditions of CO(2) and phosphorus. In addition, we fitted the data to four different Monod-type models: one assuming Liebigs Law of the minimum, one assuming that the affinity for the uptake of one nutrient is not influenced by the supply of the other (independent colimitation) and two where the uptake affinity for one nutrient depends on the supply of the other (dependent colimitation). In addition we asked whether the physiological response under colimitation differs from that under single nutrient limitation.We found no negative correlation between the affinities for uptake of the two nutrients, thereby rejecting a dependent colimitation. Kinetic data were supported by a better model fit assuming independent uptake of colimiting nutrients than when assuming Liebigs Law of the minimum or a dependent colimitation. Results show that cell nutrient homeostasis regulated nutrient acquisition which resulted in a trade-off in the maximum uptake rates of CO(2) and phosphorus, possibly driven by space limitation on the cell membrane for porters for the different nutrients. Hence, the response to colimitation deviated from that to a single nutrient limitation. In conclusion, responses to single nutrient limitation cannot be extrapolated to situations where multiple nutrients are limiting, which calls for colimitation experiments and models to properly predict growth responses to a changing natural environment. These deviations from single nutrient limitation response under colimiting conditions and independent colimitation may also hold for other nutrients in algae and in higher plants.     

Influence of elevated CO(2) on the sensitivity of two soybean cultivars to sulphur dioxide

Two cultivars of soybean (Glycine max cv. Bragg and PK 472) were subjected to elevated concentrations of CO(2) (600 &mgr;l l(-1)) and/or SO(2) (0.06 &mgr;l l(-1)), for 8 h from germination to grain maturity in open top chambers under field conditions to assess the modification in response to SO(2) exposure resulting form CO(2) enrichment. Exposure to SO(2) alone resulted in reductions in plant growth, biomass and yield, as well as declines in foliar starch and protein content in both the cultivars of soybean. Elevated CO(2) stimulated plant growth, yield and enhanced foliar starch content, photosynthesis and WUE in both the cultivars. In plants exposed to the combination of elevated CO(2)+SO(2), the adverse influence of SO(2) was mitigated by CO(2) enrichment. This effect was considered to result from the provision of extra carbon sources required for repair and detoxification processes and a reduction in SO(2) uptake through reduction in stomatal conductance. PK 472 exhibited greater sensitivity to SO(2) than Bragg. PK 472 also showed greater stimulation of yield under CO(2)+SO(2) treatment than Bragg.     

Importance of Viral Lysis and Dissolved DNA for Bacterioplankton Activity in a P-Limited Estuary,Northern Baltic Sea

Through lysis of bacterioplankton cells, viruses mediate an important, but poorly understood, pathway of carbon and nutrients from the particulate to the dissolved form. Via this activity, nutrient-rich cell lysates may become available to noninfected cells and support significant growth. However, the nutritional value of lysates for noninfected bacteria presumably depends on the prevailing nutrient limitation. In the present study, we examined dynamics of dissolved DNA (D-DNA) and viruses along a transect in the phosphorus (P)-limited Ore Estuary, northern Baltic Sea. We found that viruses were an important mortality factor for bacterioplankton and that their activity mediated a significant recycling of carbon and especially of P. Uptake of dissolved DNA accounted for up to 70% of the bacterioplankton P demand, and about a quarter of the D-DNA pool was supplied through viral lysis of bacterial cells. Generally, the importance of viral lysates and uptake of D-DNA was highest at the estuarine and offshore stations and was positively correlated with P limitation measured as alkaline phosphatase activity. Our results highlight the importance of viral activity for the internal recycling of principal nutrients and pinpoints D-DNA as a particularly relevant compound in microbial P dynamics.