Ammonia uptake and retention in some cyanobacteria

The internal pool of ammonia in strains of unicellular and filamentous cyanobacteria was found to be 6–12 nmol·mg^-1 protein. In nitrate grown Anacystis nidulans R-2 the pool size averaged 12 nmol·mg^-1 protein, which corresponds to 2.3 mM, and was little affected by N-source or medium pH during growth. Cells from NH ₄ ⁺ -limited continuous culture contained comparable pools, and cell yield was independent of medium pH (7.2–8.5). The internal pool was not bound to macromolecules. The pool fell transiently to about one-third within 2 h on shifting cells to N-free medium, but was slowly regenerated over 24 h.Added ammonia was removed from solution by illuminated cell suspensions at a linear rate, adequate to supply biosynthetic needs, to residual concentrations less than 5 M. An apparent K _m of less than 1 M can be inferred. Uptake rates were independent of N-source during growth, and of assay pH over the range 6.2–8.7. Bicarbonate was needed for uptake, but the rate of uptake was not influenced by the simultaneous presence of NaNO₃ (10 mM) or CH₃NH₃Cl (0.15 mM). Uptake was energydependent, and was eliminated in dark, anaerobic conditions or by the addition of protonophores. Uptake was also strongly inhibited by dicyclohexylearbodiimide, an ATPase inhibitor, by — SH reagents and methionine sulfoximine, suggesting that interference with energy supply or with ammonia metabolism prevented further entry into the cells.Non-standard abbreviations CCCP carbonyl cyanide m-chlorophenylhydrazone - DCCD dicyclohexylcarbodiimide - DCMU dichlorophenyl dimethylurea - NEM N-ethylmaleimide - pCMB p-chloromercuribenzoate - MSX L-methionine Dl-sulfoximine     

Replacement of potassium ions by ammonium ions in different micro-organisms grown in potassium-limited chemostat culture

The biomass concentration extant in potassiumlimited cultures of either Klebsiella pneumoniae or Bacillus stearothermophilus (when growing at a fixed temperature and dilution rate in a glucose/ammonium salts medium) increased progressively as the medium pH value was raised step-wise from 7.0 to 8.5. Because the macromolecular composition of the organisms did not vary significantly, this increase in biomass could not be attributed to an accumulation of storage-type polymers but appeared to reflect a pH-dependent decrease in the cells' minimum K⁺ requirement. Significantly, this effect of pH was not eviden with cultures in which no ammonium salts were present and in which either glutamate or nitrate was added as the sole nitrogen source; however, it was again manifest when various concentrations of NH₄Cl were added to the glutamate-containing medium. This suggested a functional replacement of K⁺ by NH ₄ ⁺ , a proposition consistent with the close similarity of the ionic radii of the potassium ion (1.33 Å) and the ammonium ion (1.43 Å). At pH 8.0, and with a medium containing both glutamate (30 mM) and NH₄Cl (100 mM), cultures of B. stearothermophilus would grow without added potassium at a maximum rate of 0.7 h^-1. Under these conditions the cells contained maximally 0.1% (w/w) potassium (derived from contaminating amounts of this element in the medium constituents), a value which should be compared with one of 1.4% (w/w) for cells growing in a potassiumlimited medium containing initially 0.5 mM K⁺. Qualitatively similar findings were made with cultures of K. pneumoniae; and whereas one may not conclude that NH ₄ ⁺ can totally replace K⁺ in the growth of these bacteria, it can clearly do so very extensively.     

The Synergistic Effects of High Nitrate Concentrations on Sediment Bioreduction

Groundwaters at nuclear sites can be characterized by low pH and high nitrate concentrations (10–100 mM). These conditions are challenging for bioremediation, often inhibiting microbial Fe(III)-reduction which can limit radionuclide migration. Here, sediment microcosms representative of the UK Sellafield site were used to study the influence of variable pH and nitrate concentrations on microbially-mediated TEAP (terminal electron accepting processes) progression. The rate of reduction through the terminal electron accepting cascade NO^? ₃ > NO^? ₂ > Mn(IV)/Fe(III) > SO^2? ₄ at low pH (～5.5) was slower than that in bicarbonate buffered systems (pH ～ 7.0), but in the low pH systems, denitrification and associated pH buffering resulted in conditioning of the sediments for subsequent Fe(III) and sulfate-reduction. Under very high nitrate conditions (100 mM), bicarbonate buffering (pH ～ 7.0) was necessary for TEAP progression beyond denitrification and the reduction of 100 mM nitrate created alkaline conditions (pH 9.5). 16S rRNA gene analysis showed that close relatives of known nitrate reducers Bacillus niacini and Ochrobactrum grignonense dominated the microbial communities in this reduced sediment. In Fe(III)-reducing enrichment cultures from the 100 mM nitrate system, close relatives of the Fe(III)-reducing species Alkaliphilus crotonatoxidans and Serratia liquifaciens were observed. These results highlight that under certain conditions and contrary to expectations, denitrification may support bioreduction via pH conditioning for optimal metal reduction and radionuclide immobilization.     

Isolation and characterization of a thermophilic acetotrophic strain of Methanothrix

An enrichment culture which converted acetate to methane at 60°C was obtained from a thermophilic anaerobic bioreactor. The predominant morphotype in the enrichment was a sheathed gas-vacuolated rod with marked resemblence to the mesophile Methanothrix soehngenii. This organism was isolated using vancomycin treatments and serial dilutions and was named Methanothrix sp. strain CALS-1. Strain CALS-1 grew as filaments typically 2–5 cells long, and cultures showed opalescent turbidity rather than macroscopic clumps. The cells were enclosed in a striated subunit-type sheath and there were distinct cross-walls between the cells, similar to M. soehngenii. The gas vesicles in cells were typically 70 nm in diameter and up to 0.5 m long, and were collapsed by pressures over 3 atm (ca. 300 kPa). Stationary-phase cells tended to have a higher vesicle content than did growing cells, and occasionally bands of cells were seen floating at the top of the liquid in stationary-phase cultures. Acetate was the only substrate of those tested which was used for methanogenesis by strain CALS-1, and acetate was decarboxylated by the aceticlastic reaction. The optimum temperature for growth of strain CALS-1 was near 60°C (doubling time=24–26 h), with no growth occurring at 70°C and 37°C. The optimum pH value for growth was near 6.5 in bicarbonate/CO₂ buffered medium and no growth occurred at pH 5.5 or pH 8.4. No growth was obtained below pH 7 when the medium was buffered with 20 mM phosphate. Strain CALS-1 grew in a chemically defined medium and required biotin. Sulfide concentrations over 1 mM were inhibitory to the culture, and growth was more rapid with 1 mM 2-mercaptoethane sulfonate (coenzyme M) or 1 mM titanium citrate as an accessory reductant than with 1 mM cysteine. It is likely that strain CALS-1 represents a new species in the genus Methanothrix.     

Aluminium effect on nitrate assimilation in cucumber (Cucumis sativus L.) roots

In vivo effect of aluminium on nitrate uptake and reduction by cucumber seedlings was investigated. The high-performance liquid chromatography was used to analyse the rate of nitrate uptake. Low (0.5 mM) concentration of AlCl₃ in the nutrient solution stimulated nitrate uptake during the first 3 h. On the other hand, 6 h exposure of the cucumber seedlings to 1 or 5 mM of AlCl₃ resulted in inhibition of nitrate uptake and at 5 mM concentration of AlCl₃ the efflux of nitrate was observed. Furthermore, the amount of nitrate accumulated in cucumber roots after aluminium treatment was decreased. The noteworthy fact was observed, that at all concentrations of aluminium tested on increase of the nitrate reductase activity. This stimulation was concentration depended, but independent of the source of the enzyme. The activity of both the cytosolic and the plasma membrane bound nitrate reductase activity was enhanced in vivo. On the other hand, AlCl₃ applied in vitro only slighty decreased nitrate reductase activity.     

Selection of Bradyrhizobium strains and provenances of Acacia mangium and Faidherbia albida: Relationship with their tolerance to acidity and aluminium

This work was designed to determine the role of the acidity and aluminium stress in the selection of partners in the Acacia symbioses with relevance to the persistence of the microsymbiont Bradyrhizobium in the soil and the growth and nodulation of the host plant respectively. Fifteen strains of Bradyrhizobium from Acacia mangium and Faidherbia albida formed a very homogenous acid tolerant group as indicated by their ability to grow better in a medium at pH 4.5 than in a medium at pH 6.8. By contrast, a growth experiment using an acid liquid media (pH 4.5), containing different concentrations of aluminium successfully identified strains sensitive to aluminium toxicity and those able to grow even in the presence of 100 M AlCl₃.Our results suggest that high amounts of aluminium in the soil rather than acidity (pH 4.5) were a major soil factor for selection of Bradyrhizobium strains capable of establishing a permanently high population under natural conditions.Unlike the behaviour of the microsymbiont, growth and nodulation of Acacia mangium and Faidherbia albida were not affected by aluminium, even at 100 M, but they might be significantly affected by medium acidity (pH 4.5) depending on plant provenances. It is therefore suggested that ability of the host plant to tolerate acidity stress should be taken into account first when screening effective Acacia-Bradyrhizobium combinations for use in afforestation trials.     

Transport kinetics of maltotriose in strains ofSaccharomyces

Summary Maltotriose transport was studied in two brewer's yeast strains, an ale strain 3001 and a lager strain 3021, using laboratory-synthesized¹⁴C-maltotriose. The maltotriose transport systems preferred a lower pH (pH 4.3) to a higher pH (pH 6.6). Two maltotriose transport affinity systems have been indentified. The high affinity system hasK _m values of 1.3 mM for strain 3021 and 1.4 mM for strain 3001. The low affinity competitively inhibited by maltose and glucose withK _i values of 58 mM and 177 mM. respectively, for strain 3021, and 55 mM and 147 mM, respectively, for strain 3001. Cells grown in maltotriose and maltose had higher maltotriose and maltose transport rates, and cells grown in glucose had lower maltortriose and maltose transport rates. Early-logarithmic phase cells transported glucose faster than either maltose or maltotriose. Cells harvested later in the growth phase had increased maltotriose and maltose transport activity. Neither strain exhibited significant differences with respect to maltose and maltotriose transport activity.     

Production and partial characterization of two types of phytase from Aspergillus niger NCIM 563 under submerged fermentation conditions

Novel extracellular phytase was produced by Aspergillus niger NCIM 563 under submerged fermentation conditions at 30 °C in medium containing dextrin and glucose as carbon sources along with sodium nitrate as nitrogen source. Maximum phytase activity (41.47 IU/mL at pH 2.5 and 10.71 IU/mL at pH 4.0) was obtained when dextrin was used as carbon source along with glucose and sodium nitrate as nitrogen source. Nearly 13 times increase in phytase activity was observed when phosphate in the form of KH₂PO₄ (0.004 g/100 mL) was added in the fermentation medium. Physic-chemical properties of partially purified enzyme indicate the possibility of two distinct forms of phytases, Phy I and Phy II. Optimum pH and temperature for Phy I was 2.5 and 60 °C while Phy II was 4.0 and 60 °C, respectively. Phy I was stable in the pH range 1.5–3.5 while Phy II was stable in the wider pH range, 2.0–7.0. Molecular weight of Phy I and Phy II on Sephacryl S-200 was approximately 304 kDa and 183 kDa, respectively. Phy I activity was moderately stimulated in the presence of 1 mM Mg²⁺, Mn²⁺, Ca²⁺ and Fe³⁺ ions and inhibited by Zn²⁺ and Cd²⁺ ions while Phy II activity was moderately stimulated by Fe³⁺ ions and was inhibited by Hg²⁺, Mn²⁺ and Zn²⁺ ions at 1 mM concentration in reaction mixture. The Km for Phy I and II was 3.18 and 0.514 mM while Vmax was 331.16 and 59.47 μmols/min/mg protein, respectively.     

Artemisinin production in Artemisia annua hairy root cultures with improved growth by altering the nitrogen source in the medium

Murashige & Skoog medium was modified for enhancing artemisinin production in Artemisia annua hairy root cultures by altering the ratio of NO ₃ ^– /NH ₄ ⁺ and the total amount of initial nitrogen. Increasing ammonium to 60 mM decreased both growth and artemisinin accumulation in hairy root cultures. With NO ₃ ^– /NH ₄ ⁺ at 5:1 (w/w), the optimum concentration of total initial nitrogen for artemisinin production was 20 mM. After 24 days of cultivation with 16.7 mM nitrate and 3.3 mM ammonium, the maximum artemisinin production of hairy roots was about 14 mg l^–1, a 57% increase over that in the standard MS medium.     

HYDROGEN ION BUFFERING OF CULTURE MEDIA FOR ALGAE FROM MODERATELY ACIDIC,OLIGOTROPHIC WATERS 1

Five hydrogen ion buffers were compared for their usefulness in regulating pH in a model oligotrophic, moderately acidic (pH 6.0) algal growth medium. These were 3,3-dimethylglutaric acid (DMGA), tricarbaliylic acid (TCA), trans-aconitic acid (tAA), N-2-hydroxyethylpiperazine-N′-2-ethanesulfonic acid (HEPES) and 2-(N-morpholino) ethanesulfonic acid (MES). All buffers (2.5 mM) except HEPES limited the reduction of pH in a NH₄⁺-based medium during growth of Chrysochromulina breviturrita Nich. to less than 0.12 units, compared with more than 2 units in an unbuffered medium. Long term growth of C. breviturrita in these media was significantly inhibited (P < 0.05) by TCA and tAA. MES was able to control pH with the minimum amount of NaOH (1.0 mM) added to the medium to adjust to pH 6.0. Four of five bacterial isolates were capable of utilizing tAA as a sole organic-C source, and no isolate could metabolize HEPES or MES. No significant differences (P > 0.05) were found in the maximum growth rates of six algal species (from five classes) in a medium with or without MES buffer, although significantly greater cell yields of Ochromonas danica Prings. were obtained in the buffered medium. MES (pK₄=6.15) was considered to be the most useful buffer in the pH range 5.0–6.5, due to its biological inertness, buffering capacity, the minimal requirement for excess base to adjust pH and its minimal metal complexing ability.     

Assessment of Al3+ availability in callus culture media for screening tolerant genotypes of Cynodon dactylon

Aluminium-tolerant genotypes of Cynodon dactylon are potential candidates for the vegetation of gold mine tailings in South Africa. As a prerequisite to in vitro selection of tolerant genotypes, this work aimed at assessing and adapting micropropagation media to ensure Al³⁺ activity and toxicity. This was investigated using MINTEQA2, a chemical equilibrium speciation model. The maximum AlAl³⁺ activity achieved in any medium was 7.5 M. Of the seven published media investigated, four never achieved an activity greater than 4 M at 3–4 mM aluminium. The most appropriate medium was that of Yamamoto et al. (1996) (modified MS without KH₂PO₄ and EDTA), as it showed an increasing range of AlAl³⁺ activities from 2 to 7.5 M at aluminium concentrations from 0.25–2.5 mM. An improved modified MS formulation retaining phosphate was investigated because phosphate is an important component of our medium for callus induction in C. dactylon. Using MINTEQA2, no reduction in AlAl³⁺ activity by phosphate was detected in standard MS medium at pH 4. Through further simulations a new modified MS medium was derived with 1 mM SO ₄ ^2– and no EDTA at pH 4, which gave the maximum AlAl³⁺ activity (7.5 M) at 2 mM aluminium. This medium gave the highest AlAl³⁺ activities for the 0.25–2 mM concentration range of all the tested formulations, including the seven published media. It also resulted in significantly higher callus growth rates than standard MS media and other tested media. This new medium is currently being used to screen C. dactylon for aluminium tolerance at pH 4.     

Toxicity of and mutagenesis by chlorate are independent of nitrate reductase activity in Chlamydomonas reinhardtii

Summary Spontaneous chlorate-resistant (CR) mutants have been isolated from Chlamydomonas reinhardtii wildtype strains. Most of them, 244, were able to grow on nitrate minimal medium, but 23 were not. Genetic and in vivo complementation analyses of this latter group of mutants indicated that they were defective either at the regulatory locus nit-2, or at the nitrate reductase (NR) locus nit-1, or at very closely linked loci. Some of these nit-1 or nit-2 mutants were also defective in pathways not directly related to nitrate assimilation, such as those of amino acids and purines. Chlorate treatment of wild-type cells resulted in both a decrease in cell survival and an increase in mutant cells resistant to a number of different chemicals (chlorate, methylammonium, sulphanilamide, arsenate, and streptomycin). The toxic and mutagenic effects of chlorate in minimal medium were not found when cells were grown either in darkness or in the presence of ammonium, conditions under which nitrate uptake is drastically inhibited. Chlorate was also able to induce reversion of nit ^– mutants of C. reinhardtii, but failed to produce His ⁺ revertants or Ara^r mutants in the BA-13 strain of Salmonella typhimurium. In contrast, chlorate treatment induced mutagenesis in strain E1F1 of the phototrophic bacterium Rhodobacter capsulatus. Genetic analyses of nitrate reductase-deficient CR mutants of C. reinhardtii revealed two types of CR, to low (1.5 mM) and high (15 mM) chlorate concentrations. These two traits were recessive in heterozygous diploids and segregated in genetic crosses independently of each other and of the nit-1 and nit-2 loci. Three her loci and four lcr loci mediating resistance to high (HC) and low (LC) concentrations of chlorate were identified. Mutations at the nit-2 locus, and deletions of a putative locus for nitrate transport were always epistatic to mutations responsible for resistance to either LC or HC. In both nit ⁺ and nit ^– chlorate-sensitive (CS) strains, nitrate and nitrite gave protection from the toxic effect of chlorate. Our data indicate that in C. reinhardtii chlorate toxicity is primarily dependent on the nitrate transport system and independent of the existence of an active NR enzyme. At least seven loci unrelated to the nitrate assimilation pathway and mediating CR are thought to control indirectly the efficiency of the nitrate transporter for chlorate transport. In addition, chlorate appears to be a mutagen capable of inducing a wide range of mutations unrelated to the nitrate assimilation pathway.     

The extremely high Al resistance of Penicillium janthineleum F-13 is not caused by internal or external sequestration of Al

Penicillium janthinellum F-13 has been isolated in previous work as a fungus tolerating the presence of high concentrations of Al (as high as 100 mM AlCl₃). Here its growth rate and yield in three acidic (pH 3.0) media of different composition with varying concentrations of Al are reported. The presence of Al did not affect these parameters, except that the growth yield was somewhat lower in GM (a glucose/peptone/yeast extract-containing medium) with the highest concentration tested (100 mM AlCl₃). The amount of Al found in the mycelium was so low that it cannot lead to a significant decrease in the medium for the higher Al concentrations applied. Although citric acid was excreted at growth on GM, and the presence of Al even promoted this, the concentration of this was far too low to diminish (by chelation) the high Al concentrations in the medium to a non-toxic level, i.e. the level (of approx. 1 mM) that is tolerated by low-resistance fungi. At growth on SLBM (a peptone/yeast extract/soil extract-containing medium), a rise in pH occurred. The same was found for SM (a glucose/mineral salts-containing medium), although in this case the picture was more complicated because the initial rise in pH was followed by a lowering due to the excretion of oxalic acid. Although both phenomena can diminish Al toxicity (by decreasing the external concentration of monomeric Al, regarded to be the toxic species), again the decrease is far too low to attain a non-toxic level when high Al concentrations are applied. Therefore, although in principal the metabolic phenomena observed for P. janthinellum F-13 at growth on different media can diminish Al toxicity, the tolerance of this organism for high external Al concentrations must be caused by another mechanism.     

Toxicity of Al to Desulfovibrio desulfuricans

The toxicity of Al to Desulfovibrio desulfuricans G20 was assessed over a period of 8 weeks in a modified lactate C medium buffered at four initial pHs (5.0, 6.5, 7.2, and 8.3) and treated with five levels of added Al (0, 0.01, 0.1, 1.0, and 10 mM). At pH 5, cell population densities decreased significantly and any effect of Al was negligible compared to that of the pH. At pHs 6.5 and 7.2, the cell population densities increased by 30-fold during the first few days and then remained stable for soluble-Al concentrations of <5 × 10⁻⁵ M. In treatments having total-Al concentrations of ≥1 mM, soluble-Al concentrations exceeded 5 × 10⁻⁵ M and limited cell population growth substantially and proportionally. At pH 8.3, soluble-Al concentrations were below the 5 × 10⁻⁵ M toxicity threshold and cell population density increases of 20- to 40-fold were observed. An apparent cell population response to added Al at pH 8.3 was attributed to the presence of large, spirilloidal bacteria (accounting for as much as 80% of the cells at the 10 mM added Al level). Calculations of soluble-Al speciation for the pH 6.5 and 7.2 treatments that showed Al toxicity suggested the possible presence of the Al₁₃O₄(OH)₂₄(H₂O)₁₂⁷⁺ “tridecamer” cation and an inverse correlation of the tridecamer concentration and the cell population density. Analysis by ²⁷Al nuclear magnetic resonance spectroscopy, however, yielded no evidence of this species in freshly prepared samples or those taken 800 days after inoculation. Exclusion of the tridecamer species from the aqueous speciation calculations at pHs 6.5 and 7.2 yielded inverse correlations of the neutral Al(OH)₃ and anionic Al(OH)₄⁻ monomeric species with cell population density, suggesting that one or both of these ions bear primary responsibility for the toxicity observed.     

Mediated uptake of folate by a high-affinity binding protein in sublines of L1210 cells adapted to nanomolar concentrations of folate

Summary An L1210 cell line (JT-1), which can grow in medium supplemented with 1nm folate, has been isolated. These cells exhibit a slower growth rate than folate-replete parental cells and have a lower ability to transport folate or methotrexate via the reduced folate transport system. Measurements at nanomolar concentrations of folate revealed that the adapted cells have acquired a high-affinity folate-binding protein. Binding to this component at 37°C was rapid and reached a maximum value after 30 min which corresponded in amount to 0.23±0.3 pmol/mg protein, and excess unlabeled folate added 30 min subsequent to the [³H]folate led to a rapid release of the bound substrate. Radioactivity bound to or released from the cells after 30 min at 37°C remained as unmetabolized folic acid. Binding was also rapid at 0°C but uptake at the plateau was only one-half the value obtained at 37°C. Half-maximal saturation of the binding component (K _D) occurred at a folate concentration of 0.065nm at pH 7.4, while the affinity for folate decreased 30-fold when the pH was reduced to 6.2 (K _D=2.0nm). 5-Methyltetrahydrofolate was also bound by this component (K _i=13nm at pH 7.4) but with a much lower affinity than for folate, while progressively weaker interactions were observed with 5-formyltetrahydrofolate (K _i=45nm) and methotrexate (K _i=325nm). When the same adaptation procedure was performed with limiting amounts of 5-formyltetrahydrofolate, two additional cell lines, JT-2 and JT-3, were isolated which expressed elevated levels of the folate-binding protein. The binding activity of the latter cells was 0.46 and 1.4 pmol/mg protein, respectively. When the level of binding protein was compared in cells grown at different concentrations of folate, an increase in medium folate from 1 to 500nm caused a sevenfold reduction in binding activity in the JT-3 cell line, while these same growth conditions had no effect on binding by the other cells. These results indicate that L1210 cells adapted to low concentrations of folate or 5-formyltetrahydrofolate contain elevated levels of a high-affinity binding protein and that this protein is able to mediate the intracellular accumulation of folate compounds. L1210 cells thus appear to have two potential uptake routes for folate compounds, the previously characterized anion-exchange system and a second route mediated by a high-affinity binding protein. An additional low-affinity, high-capacity transport system for folate that had been proposed previously was not observed under a variety of experimental conditions in either the adapted or parental cells.     

Axenic mass cultivation of the free-living soil amoeba, Acanthamoeba castellanii in a laboratory fermentor

Axenic mass cultivation of Acanthamoeba castellanii in laboratory fermentors (14 l) yielded after 20 days approximately 3 g cells (wet weight). After a short lag phase amoebal cell numbers increased exponentially to a maximum of 3.5×10⁵ cells per ml until cell death occurred after 20 days. Optical density and protein concentrations revealed identical patterns. During amoebal growth only 12–19% of the initially added glucose (100 mM) as sole carbon source was used. Large amounts of ammonia (1 g in 10.5 l culture volume) were excreted into the medium which subsequently raised the pH from 6.6 to 7.7, and from 6.6 to 6.8 in 2 and 20 mM buffered media, respectively. Growth inhibition and cell death could not be explained by a depletion of glucose or oxygen limitations during growth. The production of ammonia had a growth inhibitory effect, however, the sudden termination of the exponential growth phase and cell death could not be explained by the toxic influence of ammonia only.     

Substitution of inosine for yeastolate in the culture medium forDrosophila cells

Summary The nutritional requirement ofDrosophila cells (GM₁ and GM₂) was studied. TC Yeastolate contained in the medium forDrosophila cell culture was found to be replaceable with adenosine or inosine without appreciable changes in the generation time of cells. The optimal concentration of either adenosine or inosine was 0.01 mM. Whereas adenosine manifested cell toxicity at concentrations higher than 0.1 mM, in the case of inosine, such an inhibitory effect was not observed up to and at the concentration of 1.0 mM. Further-more, the plating efficiency at cell densities as low as 2×10³ cells per cm² was raised from 0 to 10% by supplementing inosine (0.1 mM) for the TC Yeastolate. Therefore inosine is in practice more useful than adenosine. Experiments using radioactive nucleosides suggested that both adenosine and inosine were exclusively incorporated into RNA as adenosine-monophosphate.     

Calcium/aluminium interactions in the cell wall and plasma membrane of Chara

The proposal that aluminium (Al) toxicity in plants is caused by either inhibition of Ca²⁺ influx or by displacement of Ca²⁺ from the cell wall, was examined. For this study the giant alga Chara corallina Klein ex Will. em. R.D. Wood was selected because it shows a similar sensitivity to Al as in roots of higher plants and, more importantly, it is possible to use the large single internodal cells to make accurate and unambiguous measurements of Ca²⁺ influx and Ca²⁺ binding in cell walls. Growth of Chara was inhibited by Al at concentrations comparable to those required to inhibit growth of roots, and with a similar speed of onset and pH dependence. At Al concentrations which inhibited growth, influx of calcium (Ca²⁺) was only slightly sensitive to Al. The maximum inhibition of Ca²⁺ influx at 0.1 mol·m^–3 Al at pH 4.4 was less than 50%. At the same concentration, lanthanum (La³⁺) inhibited influx of Ca²⁺ by 90% but inhibition of growth was similar for both La³⁺ and Al. Removal of Ca²⁺ from the external solution did not inhibit growth for more than 8 h whereas inhibition of growth by Al was apparent after only 2.5 h. Ca²⁺ influx was more sensitive to Al when stimulated by addition of high concentrations of potassium (K⁺) or by action potentials generated by electrical stimulation. Other membrane-related activities such as sodium influx, rubidium influx and membrane potential difference and conductance, were not strongly affected by Al even at high concentrations. In isolated cell walls equilibrated in 0.5 mol·m^–3 Ca²⁺ at pH 4.4, 0.1 mol·m^–3 Al displaced more than 80% of the bound Ca²⁺ with a half-time of 25 min. From the poor correlation between inhibition of growth and reduction in Ca²⁺ influx, it was concluded that Al toxicity was not caused by limitation of the Ca²⁺ supply. Short-term changes in other membrane-related activities induced by Al also appeared to be too small to explain the toxicity. However the strong displacement, and probable replacement, of cell wall ca²⁺ by Al may be sufficient to disrupt normal cell development.Abbreviations CPW artificial pond water - PD potential difference The technical assistance of Dawn Verlin is gratefully acknowledged. This work was supported by the Australian Research Council.     

Effect of high nitrate concentrations on growth and nitrate uptake by free-living and immobilizedChlorella vulgaris cells

Growth and nitrate uptake were studied on free-living and immobilizedChlorella vulgaris cells cultivated in medium containing different nitrate concentrations. First, the effect of nitrate concentrations on growth indicated that cells can live in the presence of high concentrations as high as 97 mM. Although no lethal effect on cells was observed such concentration a slow down in growth and a decrease in biomass produced was observed. The rate of nitrate uptake increased with the nitrate concentration in the medium. The maximum uptake rate was reached in first days of culture in both free-living and immobilized cells. The rate dropped more rapidly for cells growing in 2 mM nitrate than for cells growing in higher nitrate concentration. The maximum rate was very much the same for free-living and immobilized and was within the order of 0.45 to 0.57 g NO₃ h^–1 10^–6 cells. Immobilization modified the changes of nitrate uptake rate for concentration higher than 2 mM.     

Importance of seed Zn content for wheat growth on Zn-deficient soil

Eggplants (Solanum melongena L. cv. Bonica) were grown in a glasshouse during summer under natural light with one unbranched shoot or one shoot with 3 to 4 branches and with or without fruit in quartz sand buffered and not buffered with 0.5% CaCO₃ (w : v), respectively. Nutrient solutions supplied contained nitrate or ammonium as the sole nitrogen source. Compared with nutrient solutions containing nitrate (10 mM), solutions containing ammonium (10 mM) caused a decrease in net photosynthesis of eggplants during early stages of vegetative growth when grown in quartz sand not buffered with CaCO₃. The decrease was not observed before leaves showed interveinal chlorosis. In contrast, net photosynthesis after bloom at first increased more rapidly in eggplants supplied with ammonium than with nitrate nitrogen. However, even in this case, net photosynthesis decreased four weeks later when ammonium nutrition was continued. The decrease was accompanied by epinasty and interveinal chlorosis on the lower leaves and later by severe wilting, leaf drop, stem lesions, and hampered growth of stems, roots, and fruits. These symptoms appeared later on plants not bearing fruits than on plants bearing fruits. If nutrient solutions containing increasing concentrations of ammonium (0.5–30 mM) were supplied after the time of first fruit ripening, shoot growth and set of later flowers and fruits were promoted. In contrast, vegetative growth and reproduction was only slightly affected by increasing the concentration of nitrate in the nutrient solutions. In quartz sand buffered with CaCO₃ ammonium nutrition caused deleterious effects only under low light conditions (shade) and on young plants during rapid fruit growth. If eggplants were supplied with ammonium nitrogen before bloom, vegetative growth was promoted, and set of flowers and fruit occurred earlier than on plants supplied with nitrate. Furthermore, the number of flowers and fruit yield increased. These effects of ammonium nutrition were more pronounced when plants were grown with branched shoots than with unbranched shoots. The results indicate that vegetative and reproductive growth of eggplants may be manipulated without causing injury to the plants by supplying ammonium nitrogen as long as the age of the plants, carbohydrate reserves of the roots, quantity of ammonium nitrogen supplied, and pH of the growth medium are favourable. T W Rufty Section editor