Acid-tolerant Strain Research Articles

Interactions of an acid tolerant strain of phosphate solubilizing bacteria with a few acid tolerant crops

Phosphate solubilizing bacteria (PSB) were isolated from sixty soil samples of various soil classes and cropping histories in Himalayan regions of Uttar Pradesh, India by enrichment culture techniques. Phosphate solubilization and acid tolerance of each strain was estimated. A strain (PAS-2) isolated froma pasture and waste land of pH 4.8, organic matter 2.6% available N 265kg ha-1, available P2O5(Bray's II) 2.3kg ha-1 and available K2O 353 kg ha-1 had the highest P-solubilization (45 µg P per mL per day) and also highest acid tolerance rating 42. The strain was identified as Bacillus sp. Seed inoculation of this bacterial strain resulted in significant increases in grain and vegetative yield of fingermillet (Elosine coracana), maize (Zea mays), amaranth (Amaranthus hypochondriacus), buckwheat (Fagopyrium esculentum), frenchbean (Phaseolus vulgaris) with or without added P sources. The significant grain yield (quintol ha-1) with phosphate and seed inoculation ranged from 33.85 in maize, 26.33 in frenchbean, 22.41 in buckwheat, 20.71 in amaranth and 19.19 in fingermillet as compared to controls. The highest response was observed with frenchbean followed by fingermillet, buckwheat, amaranth and maize. Phosphate use efficiency was highest in frenchbean followed by maize and lowest and almost at par in buckwheat, amaranth and fingermillet. Available phosphate was also highest in frenchbean cultivated plot followed by amaranth, fingermillet, buckwheat and maize. The MPN count of phosphate solubilizing bacteria were also influenced by seed inoculation of strain PAS-2. Frenchbean exerted greaterrhizosphere effect followed by pseudocereals and cereals. Likewise, phosphate nutrition of crops were also improved through seed inoculation irrespective of added P sources. The study thus demonstrated that selection of efficient strain of PSB from acid soil and its seed inoculation in selected crop genotype is beneficial in boosting up crop yield in low productive hill soil. Seed inoculation also created greater rhizosphere effect over uninoculation which improved P-nutrition of crops and also available soil P.

Effects of pH on growth, photosynthesis, respiration, and copper tolerance of three Scenedesmus strains

Our objective was to investigate the sensitivity of growth to acid pH, and to compare acid-sensitivity of growth, photosynthesis, and respiration of copper-tolerant (B-4, X-Cu) and copper-intolerant (X-72) strains of Scenedesmus acutus f. alternans. Further, we wanted to assess the effects of pH on copper toxicity. We hypothesized that B-4 and X-Cu will be more acid tolerant than X-72, and that growth will be more acid-sensitive than photosynthesis and respiration. Optimal growth of the three strains occurred at circumneutral pHs where growth rates were similar. Growth rates responded differently to decreasing pHs. In B-4, the most acid-tolerant strain, growth was reduced by 50% at pH 4.42. X-Cu was less tolerant since it did not grow at pH 4.5, and showed 50% growth reduction at pH 4.81. X-72 was not acid-tolerant. Growth was reduced by 50% at pH 5.05, and no growth occurred at pH 4.8 or less. Photosynthesis was less inhibited by acid pHs than growth but the strain ranking was the same: B-4 was most acid-tolerant, followed by X-Cu and X-72. A reduction in respiration with declining pH was observed in X-72 and B-4 but not in X-Cu, where respiration was unaffected by pH. CO 2 compensation points for photosynthesis in all three strains showed a general decrease with pH of growth, and were lower in B-4 and X-Cu than in X-72. The inhibitory effect of copper on photosynthesis in X-72 and B-4 was reduced at lower pHs; copper toxicity in X-Cu was unaffected by pH. These findings suggest that algae such as X-Cu with a metal-exclusion mechanism can also exclude protons. In such algae, high H + concentration in the medium does not ameliorate copper toxicity. In contrast, in non-tolerant species (e.g. X-72) and in species that accumulate copper (e.g. B-4) high H + concentrations in the range suitable for growth ameliorate copper toxicity, possibly by competing with the metal for binding sites on the cell surface thereby decreasing copper uptake.

Cellular mechanisms of pH tolerance in Rhizobium loti

Seven strains of Rhizobium loti were tested for acid tolerance in yeast-extract mannitol (YEM) broth at pH values ranging from 4.0 to 8.0. The strains that grew at pH 4.0 showed the slowest generation time when grown at pH above 7.0 and also produced the most acid. The acid tolerance was related to the composition and structure of the membrane. pH influenced protein expression in acid-tolerant strains growing at pH 4.0 or 7.0. Acid tolerant strains showed one membrane protein of 49.5 kDa and three soluble proteins of 66.0, 58.0 and 44.0kDa; their expression increased when the cells grew at pH 4.0. It is suggested that acid tolerance in Rhizobium loti involves constitutive mechanisms, such as permeability of the outer membrane together with adaptive responses, including the state of bacterial growth and concomitant changes in protein expression.

Regulation of heavy metal toxicity in acid-tolerant Chlorella: Physiological and biochemical approaches

To explore the mechanism(s) of survival of the organism in acid- and metal-stressed environment and to test certain hypotheses, the growth rates, uptake of NH + 4, Na +, K +, Ca 2+, NO − 3, PO 3− 4, efflux of Na + and K +, activities of nitrate reductase, acid phosphatase and ATPase and internal pH of an acid-tolerant and an acid-sensitive strain of Chlorella vulgaris exposed to Cu and Ni at different pHs (pH 6.8, 5.0, 4.0 and 3.5) were compared. A general reduction in all variables was noticed with decrease in pH; however, the acid-tolerant strain was metabolically more active than the acid-sensitive strain. Reduced cation (NH + 4, Na +, K +, Ca 2+, Cu 2+ and Ni 2+) uptake coupled with an increased concentration of anions (NO − 3 and PO 3− 4) supported the hypothesis of the development of positive membrane/zeta potentials in the acid-tolerant strain. Further an accelerated accumulation of NO − 3 and PO 3− 4 in the cells and a significant increase in ATPase activity with decreasing pH also supported our postulates that these processes are responsible for regulating the internal pH at neutrality in the acid-tolerant strain either by exporting H + ions into the external medium or by producing OH − ions from NO − 3. The lower toxicity of Cu and Ni to the acid-tolerant strain compared to the acid-sensitive strain also supported our hypothesis of low metal uptake in the acid-tolerant strain due to cation repulsion by the membrane having low permeability and positive zeta and membrane potentials. The development of a superactive ATPase and a change in membrane potential and permeability not only offer protection against acidity by keeping internal pH neutral but also confer co-tolerance to metals.

The trout ( Salmo trutta L.) populations of the headwaters of the Rivers Severn and Wye, mid-Wales, UK

Brown trout ( Salmo trutta L.) populations were studied in two contrasting headstream systems in mid-Wales. An electrofishing survey in 1979 compared the long-term effects on trout populations in the upper Severn and upper Wye systems of mature conifer forest (Severn) and sheep pasture (Wye), respectively, and identified a previously unknown trout population in the upper Severn. In June 1979, brown trout were found in six out of seven stations in the Wye system. In the Severn system trout were found at only one station out of 10. Population densities and biomasses ranged from 0 to 0.32 fish m −2 and 0–5.6 g m −2 in the Wye and the age composition of the samples indicated good recruitment in most years. In the Severn station, population density was c.0.05 fish m −2 and biomass was c.3.1 g m −2. There was evidence of irregular recruitment. Growth in the Wye system could be described in terms of an asymptotic length of 21.5 cm, a von Bertalanffy [1] growth constant (K) of 0.34 and a I-group mean length in June of 7.9 ± 0.95 cm (95% C.L.). Growth in the Severn population appeared to be similar to but rather more rapid than, that in the Wye. The population of the upper Severn occupies a stream area of c.1700 m 2 and probably fluctuates between c.50 and c.400 individuals. Complete or partial isolation is likely to have occurred within the last 55 years. Acidification of upland streams often causes local extinction of trout. The occurrence of this small population in the Hafren may indicate that this is an acid tolerant strain, or that the water in the relevant portion of the Hafren is better buffered than previously supposed, or that the population is maintained by occasional incursions of spawners from further downstream.

Effect of nickel on certain physiological and biochemical behaviors of an acid tolerant Chlorella vulgaris.

This study concerns the inhibitory effects of acid pH and nickel on growth, nutrient (NO3- and NH4+) uptake, carbon fixation, O2 evolution, electron transport chain and enzyme (nitrate reductase and ATPase) activities of acid tolerant and wild-type strains of Chlorella vulgaris. Though a general reduction in all these variables was noticed with decreasing pH, the tolerant strain was found to be metabolically more active than the wild-type. A reduced cation (NH4+, Na+, K+ and Ca2+) uptake, coupled with a facilitated influx of anions (NH4+, PO4(3-) and HCO3-), suggested the development of a positive membrane potential in acid tolerant Chlorella. Nevertheless, a tremendous increase in ATPase activity at decreasing pH revealed the involvement of superactive ATPase in exporting H+ ions and keeping the internal pH neutral. A difference in Na+ and K+ efflux of the two strains at decreasing pH suggests there is a difference in membrane permeability. The low toxicity of Ni in the acid tolerant strain may be due to the low Ni uptake brought about by a change in membrane potential as well as in permeability. Hence, the development of superactive ATPase and a change in both membrane potential and permeability not only offers protection against acidity, but also co-tolerance to metals.

Acid pH tolerance in strains of Rhizobium and Bradyrhizobium, and initial studies on the basis for acid tolerance of Rhizobium tropici UMR1899

Acid pH limits the persistence of Rhizobium strains in soil, and the nodulation and nitrogen fixation of legumes. To identify acid-tolerant strains, we tested the ability of 45 Rhizobium, Azorhizobium, and Bradyrhizobium strains to produce isolated colonies on agar medium of pH 4.00 to pH 7.00. Only Rhizobium tropici UMR1899 (=CIAT899) grew at pH 4.00 in unbuffered medium, though 6 strains of R. tropici and 3 Bradyrhizobium strains grew at pH 4.25, and 15 strains grew at pH 4.50. Tolerance to acid pH in R. tropici UMR1899 was not an adaptive response, nor was it plasmid mediated, correlated with the production of extracellular polysaccharide, or related to synthesis of polyamines in the cell. When UMR1899 was grown in buffered medium at acid pH, it maintained a ΔpH (measured using 31P-NMR) of up to 1.7 pH units. However, when this strain was subjected to acid shock, it showed only limited ability to regulate cytoplasmic pH in the short term. Cells of UMR1899 accumulated glutamate under pH stress, and were markedly hydrophobic and resistant to the effects of crystal violet, the latter traits raising the possibility that outer membrane composition and structure could also be a factor in pH tolerance.Key words: Rhizobium, pH tolerance, glutamate, hydrophobicity, 31P-NMR.

Studies of the Physiological and Genetic Basis of Acid Tolerance in Rhizobium leguminosarum biovar trifolii

Acid-tolerant Rhizobium leguminosarum biovar trifolii ANU1173 was able to grow on laboratory media at a pH as low as 4.5. Transposon Tn5 mutagenesis was used to isolate mutants of strain ANU1173, which were unable to grow on media at a pH of less than 4.8. The acid-tolerant strain ANU1173 maintained a near-neutral intracellular pH when the external pH was as low as 4.5. In contrast, the acid-sensitive mutants AS25 and AS28 derived from ANU1173 had an acidic intracellular pH when the external pH was less than 5.5. The acid-sensitive R. leguminosarum biovar trifolii ANU794, which was comparatively more sensitive to low pH than mutants AS25 and AS28, showed a more acidic internal pH than the two mutants when the three strains were exposed to medium buffered at a pH of less than 5.5. The two acid-sensitive mutants had an increased membrane permeability to protons but did not change their proton extrusion activities. However, the acid-sensitive strain ANU794 exhibited both a higher membrane permeability to protons and a lower proton extrusion activity compared with the acid-tolerant strain ANU1173. DNA hybridization analysis showed that mutants AS25 and AS28 carried a single copy of Tn5 located in 13.7-kb (AS25) and 10.0-kb (AS28) EcoRI DNA fragments. The wild-type DNA sequences spanning the mutation sites of mutants AS25 and AS28 were cloned from genomic DNA of strain ANU1173. Transfer of these wild-type DNA sequences into corresponding Tn5-induced acid-sensitive mutants, respectively, restored the mutants to their acid tolerance phenotypes. Mapping studies showed that the AS25 locus was mapped to a 5.6-kb EcoRI-BamHI megaplasmid DNA fragment, whilst the AS28 locus was located in an 8.7-kb BglII chromosomal DNA fragment.

Physiological and biochemical studies on an acid-tolerant Chlorella vulgaris under copper stress.

This study compared the growth, photosynthesis, electron transport, ATP content, NO3- and NH4+ uptake, Na+ and K+ loss, nitrate reductase and ATPase activities of an acid-tolerant strain and a wild-type strain of Chlorella vulgaris exposed to Cu at varied pHs (pH 6.8, 5.0, 4.0 and 3.5). A general reduction in growth, NH4+ uptake, photosynthesis and nitrate reductase activity of both the strains was noticed at decreasing pH. In contrast, the acid-tolerant strain depicted an increased NO3- uptake, insignificant Na+, K+ loss and superactive ATPase at decreasing pH. Both pH and Cu produced a non-competitive inhibition of NO3- uptake. An increase in Vmax of ATPase of acid-tolerant Chlorella and inhibition of the Vmax of its wild-type strain at decreasing pH was noticed. Though the magnitude of Cu toxicity was significantly low in tolerant strain, a significant negative correlation (r=-0.99, p<0.01) of Cu uptake with acid pH was noticed. The acid-tolerant Chlorella was less sensitive to Cu and physiologically more efficient than the wild type. Presence of a superactive ATPase and change in membrane permeability are considered responsible for acid tolerance in Chlorella.

Persistence, productivity and seed yield of Medicago murex, M. truncatula, M. aculeata and Trifolium subterraneum on an acid red earth soil in the wheat belt of eastern Australia

The persistence, productivity, and seed yield of 4 annual legume species, Trifolium subterraneum L. var. subterraneum, Medicago murex Willd, M. truncatula Gaertn var. truncatula, and M. aculeata, were compared on an acid red earth soil at Temora. New South Wales. All species persisted for 4 years but there were significant differences in herbage yields and seed reserves between species, and between cultivars and lines within species. The recently released T. subterraneum cultivar, Junee, produced superior seedling densities in 2 of the 3 regenerative years and maintained similar summer seed reserves, but carried over a higher proportion of seed (hard seed) through each winter and was as productive as the older cultivar, Woogenellup, except in late spring. The 6 M. murex lines persisted and remained well nodulated on the moderately acid [pH(CaCl2) 4.81 soil when grown with a new acid-tolerant Rhizobium strain, WSM 419. Medicago murex demonstrated potential as an alternative species to T. subterraneum for the eastern wheat belt; it maintained high seed reserves and was more hardseeded than T. subterraneum cultivars, but this did not reduce its ability to produce large populations of seedlings in autumn. While the M. aceuleata and M. truncatula lines demonstrated that they could persist and regenerate adequately over the 4-year period, neither proved to be superior to either the T. subterraneum or M. murex lines in any of the parameters.

Construction of an Acid-Tolerant Rhizobium leguminosarum Biovar Trifolii Strain with Enhanced Capacity for Nitrogen Fixation

Strain ANU1173 is an acid-tolerant Rhizobium leguminosarum biovar trifolii strain that is able to nodulate subterranean clover plants growing in agar culture at pH 4.4 At pH 6.5, its symbiotic effectiveness in association with Trifolium subterraneum cv. Mt. Barker was 80% relative to that of strain ANU794, a Sm derivative of the commercial inoculant R. leguminosarum bv. trifolii TA1. Strain ANU1173 contained four indigenous megaplasmids, the smallest of these being the symbiotic (Sym) plasmid. The critical pH requirement for growth of strain ANU1173 in laboratory media was shown not to be associated with this plasmid. When the Sym plasmid of strain ANU1173(pSym-1173) was mobilized into a Nod strain of R. leguminosarum bv. viciae, the plasmid conferred to the transconjugant a level of symbiotic effectiveness in association with T. subterraneum that was similar to that observed with ANU1173. The symbiotic effectiveness of strain ANU1173 was improved by first curing pSym-1173 (generating strain ANU1184) and replacing it with another R. leguminosarum bv. trifolii Sym plasmid, pBR1AN. Subterranean clover plants inoculated with strain ANU1184 (pBR1AN) exhibited a 35 or 53% increase in acetylene reduction activity and a 20 or 17% increase in dry weight when grown at pH 6.5 and pH 4.4, respectively, compared with plants inoculated with strain ANU1173 and grown under the same pH conditions. It was further shown that pBR1AN was stably maintained in strain ANU1184 under free-living and symbiotic conditions. These results indicate that it is possible to construct an acid-tolerant strain of R. leguminosarum bv. trifolii with an enhanced capacity for nitrogen fixation.

Do Organic and Anthropogenic Acidity Have Similar Effects on Aquatic Fauna?

In Westland, New Zealand, there are many brownwater steams with naturally low pH (often around 4) brought about by high concentrations of organic acids. Up to 90% of dissolved aluminium in these humic streams is bound to organic matter and is therefore non-toxic. This condition enabled us to investigate the effects of long-term, natural acidity on aquatic biota without the compounding effects of high concentrations of toxic aluminium that are found in many anthropogenically-acidified, clearwater streams of the Northern Hemisphere. Most fish are absent from Northern Hemisphere clearwater habitats with pH < 5. However, in Westland 9 out of 14 native fish species were found in brown waters with pH below 5, and 7 species were taken from waters with pH < 4.5. Furthermore, 34 of the 37 most widespread aquatic insect taxa were recorded in Westland streams with pH < 5, and 24 were taken from sites with pH < 4.5. Clearly, many members of the freshwater fauna in Westland are well-adapted for life in waters of low pH. Physiological adaptations enabling this tolerance may be the same as those that evolved in response to the physicochemical variability associated with the unpredictable flow regimes of Westland streams. Tolerances of low pH have been documented in aquatic fauna from some naturally acidic, humic waters of the Northern Hemisphere, although degree of tolerance can vary within as well as between species. Colonisation of recently-acidified waters by acid-tolerant strains may cause more subtle changes in community structure than can be detected by conventional ecological techniques.

Maintenance of Intracellular pH and Acid Tolerance in Rhizobium meliloti

The development and function of the Rhizobium meliloti-Medicago sp. symbiosis are sensitive to soil acidity. Physiological criteria that can be measured in culture which serve to predict acid tolerance in soil would be valuable. The intracellular pH of R. meliloti was measured using either radioactively labeled weak acids (5,5-dimethyloxazolidine-2,4-dione and butyric acid) or pH-sensitive fluorescent compounds; both methods gave similar values. Six acid-tolerant strains (WSM419, WSM533, WSM539, WSM540, WSM852, and WSM870) maintained an alkaline intracellular pH when the external pH was between 5.6 and 7.2. In contrast, two Australian commercial inoculant strains (CC169 and U45) and four acid-sensitive strains from alkaline soils in Iraq (WSM244, WSM301, WSM365, and WSM367) maintained an alkaline intracellular pH when the external pH was >/=6.5, but had intracellular pH values of </=6.8 when the external pH was </=6.0. Four transposon Tn5-induced mutants of acid-tolerant strain WSM419, impaired in their ability to grow at pH 5.6, showed limited control over the intracellular pH. The ability to generate a large pH gradient under acid conditions may be a better indicator of acid tolerance in R. meliloti under field conditions than is growth on acidic agar plates.

Nitrogenase activity by diazotrophs grown on a range of agricultural plant residues

Six diazotrophs, each representative of a different genus, were grown on a range of plant residues in pure culture and also in co-culture with a cellulolytic bacterium, Cellulomonas sp. CS1-17. The residues were analysed for total nitrogen, and hot and cold water-extractable carbohydrate. In the absence of Cellulomonas sp., nitrogenase activity by all diazotrophs was low on most residues. Cereal straws (wheat, oat and rice), rice hulls and palmnut residues yielded the highest nitrogenase activities with Azospirillum sp., Pseudomonas sp., Azomonas agilis and Beijerinckia indica being the more active diazotrophs. When grown in co-culture with Cellulomonas sp., all diazotrophs showed substantial nitrogenase activity on all straws and. with the exception of Azotobacter chroococcum, on palmnut residues. The residues which were not readily utilized to support nitrogenase activity were sawdust, oil-extracted melaleuca leaves and sugar cane trash. Straw from five varieties of wheat, one variety grown under differing cultural conditions to yield straw with a range of N contents, showed significant variation in the nitrogenase activity recorded between both the wheat variety and the diazotroph strain. There was no significant correlation with nitrogenase activity and the N content of any of these residues, but for Pseudomonas sp., Derxia gummosa and B. indica there was a positive correlation between nitrogenase activity and the cold water-extractable carbohydrate component of the residue. Examination of the relationship between respiration and nitrogenase activity indicated that a respiratory threshold was essential for significant rates of nitrogenase activity, and that above this threshold, the respiratory efficiency of nitrogenase activity improved. However, some residue-diazotroph combinations achieved high rates of respiration without achieving significant rates of nitrogenase activity. Even with acid-tolerant strains such as B. indica, nitrogenase activity in co-culture with Cellulomonas was promoted at pH 6.5. and higher, relative to pH 5.5, but this may reflect a pH effect on the cellulolytic activity of the Cellulomonas.

Nitrogen Fixation and Biomass Accumulation in Plant Communities Dominated by Cytisus scoparius L. in Oregon and Scotland

(1) The nitrogenase activity of Cytisus scoparius (L.) Link measured at indigenous soil temperatures at intervals during the year, showed two apparent maxima, early and late spring, which coincided with flowering and with late fruit maturation, respectively. The final decline in nitrogenase activity in early summer coincided with a period of drought which increased soil moisture stress, as measured by an increase in pre-dawn xylem pressure potential, to about -5 bars. (2) Nitrogenase activity was eliminated by frosting during the winter of 1979, unlike 1978 when virtual absence of frost permitted nitrogenase activity throughout the winter. (3) The age structure of Cytisus scoparius (L.) Link plants on two study sites in Oregon and Scotland differed by about 1 year (predominantly 2-4and 5-6-year-old plants, respectively). Biomass accumulation in the former site was 44 000 and in the latter 84 000 kg dry weight top growth ha-'. (4) Plants in both areas were poorly modulated (1 2 to 2 9 g m-2 in Oregon and 0 3 g m-2 in Scotland) and analysis of the total N content of soil of the Scottish site showed an accretion of N only half that of an adjacent area colonized by Alnus glutinosa (L.) Gaertn. Low soil pH could limit the modulation of broom and inoculation of plants with acidtolerant strains of Rhizobium could be one way to improve nitrogen fixation on many sites. (5) Selection and use of non-flowering strains of Cytisus scoparius may also improve accretion of fixed nitrogen, by eliminating the mid-season trough in nitrogenase activity, and would reduce the nuisance value of the species due to its free-seeding habit.

Acid tolerance in the Rhizobium meliloti - Medicago symbiosis

Several strains of Rhizobium meliloti that originated from acid soils in Sardinia, Italy, were markedly superior in colonizing a moderately acid loamy sand (pH 5.0 in 1:5 0.01 M CaCl2) than two Australian commercial inoculant strains (U45 and CC169), and a group of strains that originated from alkaline soils in Syria and Iraq. Six Medicago hosts also varied greatly in their ability to achieve nodulation in this soil. M. polymorpha and M. murex were far superior in this respect to M. littoralis, M. truncatula and M. tornata. The most acid-tolerant strains of R. meliloti, WSM419 and WSM413, were able to nodulate a high proportion of plants of M. polymorpha and M. murex sown in the second year between 11 and 20 cm from the point of introduction of the rhizobia into the soil the previous year. It is suggested that these more saprophytically competent isolates of R. meliloti, combined with the species of Medicago more able to nodulate readily in acid soil, will extend the range of soils suitable for successful regenerative growth of these species.

Competitiveness amongst ecotypes of Rhizobium, specific to Cicer arietinum L. in west bengal soils

Summary Competitiveness amongst three ecotypes of Rhizobium (serologically distinct) for nodule sites on Cicer arietinum L. was studied under controlled laboratory conditions as well as under field conditions in three agro-climatic locations of West Bengal. From the results it is evident that “host-preference” to a particular strain of Rhizobium did not play any significant role in determining the relative competitiveness, but it was the relative virulence of the rhizobial strains that actually governed the competitiveness of different strains. For this reason, the amount of competition exhibited by a patricular pair of rhizobia was not so much affected, due to change of the host cultivars. It was also found that the rhizobial ecotypes actually governed the ultimate expression of competition in a particular ecological situation. Thus, acid-tolerant strain S22 came off better in acid soils against the otherwise most virulent strain S9; so also the salt-tolerant strain S30 in the saline soil.

Methanogenic Bacteria, Including an Acid-Tolerant Strain, from Peatlands

Five pure cultures of methanogenic bacteria were isolated from Minnesota peatlands by enrichment culture techniques. One strain, identified as a member of the family Methanobacteriaceae by antigenic fingerprinting, was acid tolerant and able to produce methane at pH 3.1. Growth could not be demonstrated at pH less than 5.3.

Nodulation of Lotus pedunculatus in acid rooting solution by fast- and slow-growing rhizobia

The nodulation of Lotus pedunculatus and the multiplication of three Rhizobium loti (fast-growing, acid-producing) and two Bradyrhizobium (slow-growing, alkali-producing) strains was investigated in acidified rooting solution. R. loti strains multiplied at pH 4.5 but Bradyrhizobium strains failed to multiply. No difference in growth rate between R. loti and Bradyrhizobium strains was apparent in rooting solution at pH 6.7. Similar responses to pH were observed in yeast extract-mannitol broth except that Bradyrhizobium strains multiplied more slowly than R. loti at pH 6.7. All strains nodulated L. pedunculatus growing in acid (pH 4.5) rooting solution when presented as single cultures. Following inoculation with 1:1 mixtures of R. loti and Bradyrhizobium strains, R. loti formed 93% of nodules at pH 4.5 and significantly fewer nodules (66%) at pH 6.7. These results demonstrate a competitive advantage for acid-tolerant strains over acid-sensitive strains in nodulation of their lost legume at pH 4.5.

Screening clover and Lotus rhizobia for tolerance of acidity and aluminium

Clover rhizobia (55 strains) were screened for tolerance of acidity and Al, using the technique of Keyser and Munns (1979). Assessment of visible turbidity after 14 days indicated three strains tolerant of pH 4.5 (although growth rate was reduced), 25 strains tolerant of 5μ m Al and no strains tolerant of 50 μ m Al at pH 5.5.50 μ mAl caused a decrease in the numbers of acid-tolerant strains at pH 4.5. Tolerance of acidity or Al was not associated with the pH or Al status of the soil from which a strain was isolated. Screening of eight strains of clover rhizobia and nine strains of Lotus rhizobia using turbidity assessment and viable counts indicated seven strains of clover rhizobia with different degrees of tolerance of 20 μ m Al but none tolerant of 50 μ m Al at pH 5.5. All Lotus rhizobia (both slow- and fast-growers) were tolerant of 20 and 50 μ m Al at pH 5.5, with 50 μ m Al causing a reduction in growth rate. Subculturing of strains in non-stressed and stressed media had no effect on the response to 50 μ mAl at pH 5.5.