Microbiological Synthesis of d-Cysteine and an Analogue

A biofertilizer, showing antagonistic activity against potato common scab in pot tests, was produced from swine feces with a newly isolated strain, CH-33, identified as Streptomyces albidoflavus. This strain characteristically grew on fresh swine feces at 20~35°C without sterili-zation or any additives, and produced an antibiotic substance against Streptomyces scabies, the common scab-pathogen, during composting. The addition of the biofertilizer at from 0.1 g to 1.6 g total nitrogen (N) per 600 g humic volcanic ash soil in a pot did not inhibit the growth of Brassica rapa var. perviridis but increased it, even at the highest nitrogen content tested. Common scab was completely inhibited when the biofertilizer was added at 0.1 g to 1.6g as nitrogen (N) per 4 kg of scab-infected soil in a pot. Thus a biofertilizer suppressing plant pathogenic microorganisms was developed.     

Designing a crop rotation strategy to manage Streptomyces scabies causing potato scab in north India

Common scab of potato caused by various species in the genus Streptomyces has assumed serious proportions in certain potato-growing regions of North India. Although it does not have significant effect on tuber yield, it downgrades quality and reduces marketability. Being soil-borne, this disease is often difficult to manage. Therefore, the present studies were conducted to evaluate different potato-based crop rotations prevalent in north India as a part of a strategy to manage Streptomyces scabies population in scab sick fields. Our results showed that S. scabies population in soil can be effectively reduced either by keeping the field fallow after potato or by including mungbean or sunhemp as green manure, in a single year cropping sequence over a period of four years. Three crop rotations, viz. fallow–rice–potato, mungbean–rice–potato and sunhemp–rice–potato, showed maximum decrease in the population of S. scabies in soil resulting in reduced scab severity. The mungbean- and sunhemp-based crop rotations also enhanced rhizospheric soil microflora especially fluorescent pseudomonads and Trichoderma spp.     

Distribution of rare actinomycetes in Japanese soils

The distribution of rare actinomycetes in 237 soil samples from various locations throughout Japan was investigated using a special isolation medium, HV agar.The populations (colony forming units) of these actinomycetes per gram of dried soil were Microtetraspora 6 × 10³, Saccharomonospora 1.7 × 10⁴, Dactylosporangium 5.4 × 10⁴, Streptosporangium 1.2 × 10⁵, Microbispora 1.4 × 10⁵, Nocardioforms 1.9 × 10⁵, and Micromonospora 6.8 × 10⁵. Streptomycetes 2.2 × 10⁶, and Unidentified actinomycetes 0.9 × 10⁶ were also observed.Their distributions seemed to be associated with environmental factors such as soil type (Land Use Classification), soil pH, humus content, and the characteristics of the humic acid. In general, the largest populations were found in soils of cultivated fields, which were rich in humus and had pH values between 6.5–7.0.However, the distribution of some genera in cultivated field soils (154 samples) was remarkable. The numbers of Microbispora and Streptosporangium were the largest in humus-rich acidic (pH 5.0–6.05) soils with low humic acid Δ log K values (black colored humic acid). Saccharomonospora was found most frequently in relatively humus-poor alkaline (pH 7.0–7.5) soils having higher Δ log K values (brown humic acid).Dactylosporangium and Microtetraspora, Saccharomonospora, and Micromonospora were most frequently isolated from mountainous forest soils, level-land forest or cultivated field soils, and pasture soils, respectively.     

Inoculum Density and Population Dynamics of Suppressive and PathogenicStreptomycesStrains and Their Relationship to Biological Control of Potato Scab

SeveralStreptomycesstrains are capable of suppressing potato scab caused byStreptomyces scabies.Although these strains have been successful in the biocontrol of potato scab in the field, little is known about how populations of pathogenicStreptomycesin the potato rhizosphere are influenced by inoculation of the suppressive strains. The effects of inoculum densities of pathogenic and suppressiveStreptomycesstrains on their respective populations on roots and in rhizosphere soil were examined during the growing season. The relationships between inoculum density or rhizosphere population densities and disease severity were also investigated. Populations of suppressiveStreptomycesstrain 93 increased significantly on roots with increasing inoculum dose. At its highest inoculum dose, the suppressive strain reached a population density greater than 10⁶CFU/g root 14 weeks after planting. The ability of the suppressive strain to increase its populations with increasing inoculum density was hindered at high inoculum doses of the pathogen, suggesting that density-dependent competitive interactions may be occurring between the two antagonists. Strain 93 was most effective at preventing scab early in the growing season (8 weeks after planting), when tubers were most susceptible to the scab disease. Population densities of the suppressive strain in soil were more highly negatively correlated with scab severity than were populations on roots, suggesting that rhizosphere soil rather than potato roots may be the primary source of inoculum of the suppressive strain for tubers.     

Estimation of the Number of Polyhydroxyalkanoate (PHA)-Degraders in Soil and Isolation of Degraders Based on the Method of Most Probable Number (MPN) Using PHA-Film

A new method to estimate the number of polyhydroxyalkanoates (PHA)-degraders in soil and to isolate degraders, called the film-MPN method, is proposed. The incubation time was measured by the first order reaction (FOR) model. This method was used to estimate numbers of poly (3-hydroxybutyrate-co-3-hydroxyvalerate)[P(3HB-co-3HV)]- and poly(3-hydroxyvalerate-co-4-hydroxybutyrate)[P(3HB-co-4HB)]-degraders in garden soil (4.30 × 10⁵ and 2.15 × 10⁵ aerobic degraders per gram of dry soil, respectively). The number of P(3HB-co-3HV)-degraders in paddy field soil was 5.06 × 10⁵ aerobic degraders per gram dry soil. Also, several P(3HB-co-3HV)-degraders were isolated directly from positive-growth tubes of high dilution.     

Higher Abundance of Ammonia Oxidizing Archaea than Ammonia Oxidizing Bacteria and Their Communities in Tibetan Alpine Meadow Soils under Long-term Nitrogen Fertilization

Increasing usage of nitrogen fertilizer for food production has resulted in severely environmental problems of nutrients enrichment. This study aimed to examine the response of ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) to a long-term nitrogen fertilization in Tibetan alpine meadow. The abundance and composition of both AOB and AOA were assessed using quantitative real-time PCR, cloning and sequencing techniques based on amoA gene under different fertilization gradient (0, 30, 60, 90, and 120 g m^?2 year^?1). Our results showed that, abundances of AOA amoA genes (ranging from 1.48 × 10⁹ to 2.00 × 10⁹ copies per gram of dry soil) were significantly higher than those of AOB amoA genes (1.25 × 10⁷ to 2.62 × 10⁸ copies per gram of dry soil) under fertilization scenario. The abundance of AOB amoA genes increased with increasing nitrogen fertilization, whereas fertilization had little effect on AOA abundance. Sequences of clone libraries of the different treatments revealed that AOB communities were dominated by representatives of Cluster 4, constituting 48.94–64.44% in each clone library. Sequences of Clusters 9, 1 and 2 were prevalent in soils under higher fertilization. All archaeal amoA sequences recovered were affiliated with the soil/sediment clade and marine sediment clade, and no significant difference was observed on the community structure among different fertilization treatments. Variations in the AOB community structure and abundance were linked to ammonium-N and soil pH induced by different fertilization treatments. These results showed that the abundance and structure of the AOB community respond to the fertilization gradient, not AOA.     

Decreased severity of potato common scab after foliar sprays of 3,5 -dichlorophenoxyacetic acid, a possible antipathogenic agent

Glasshouse tests on the potato cultivars Majestic and Maris Bard measured the effects of single early foliar sprays of ring-substituted phenoxyacetic acids (0.9 × 10–³ M) on the incidence of potato common scab, caused by soil-borne Streptomyces scabies. The most effective compound was 3,5-dichlorophenoxyacetic acid (3,5-D), which decreased scab by about 90%; its action was preventative, early sprays being more effective than late However, it slightly decreased yield, and increased the number of tubers per plant and the proportion of deformed tubers. The trichloro- and tetrachloro- acids containing the 3,5-dichloro- group also decreased scab, though to lesser extents; the most active of these was the 2,3,4,5-tetrachloro- acid, which had the same side effects as 3,5-D. The herbicide 2,4-D almost halved scab incidence, but seriously decreased yield. Other acids tested had little or no effect on scab. In tests against S. scabies in culture, 3,5-D was no more toxic than other disubstituted acids which were inactive, or weakly active, against the disease in vivo. 3,5-D may be an ‘antipathogenic agent’, preventing scab development by altering tuber metabolism.     

Effect of exchangeable aluminium on the reduction of potato scab

The severity of the incidence of the fungal disease, potato scab, varies with different soil groups at the same soil pH. At a soil pH of 5.3, potato scab is easily controlled in soils of western Hokkaido (soil group A) by simply decreasing soil pH, but in soils from eastern Hokkaido (soil group B) it is not so easily controlled. The difference appears to be due to higher levels and exchangeable aluminium in Group A.Addition of sufficient aluminium or ferrous sulfate to a group B soil decreased the incidence of potato scab in a field experiment. Higher levels of aluminium sulfate depressed crop production. It is concluded that aluminium ions control the incidence of potato scab in acid soils. It is suggested that, in soils with low exchange acidity Y1, potato scab can be controlled by adding sufficient aluminium to increase their exchange acidity Y1 to above 7–8.     

The fluorescence enhancement of quercetin–nucleic acid system and the analytical application

Nucleic acid can greatly enhance the fluorescence intensity of quercetin in HMTA‐HCl (pH 5.5) buffer. The enhanced intensity is in proportion to the concentration of nucleic acids in the range 5.0 × 10^?9 to 1.0 × 10^?6 g/mL for fsDNA, 5.0 × 10^?9 to 7.0 × 10^?7 g/mL for ctDNA and 5.0 × 10^?9 to 1.0 × 10^?6 g/mL for yRNA, and their detection limits (S/N = 3) are 3.5 × 10^?9, 7.8 × 10^?10 and 2.6 × 10^?9 g/mL, respectively. In comparison with most reported fluorescent probes for the determination of nucleic acids, the proposed probe has higher sensitivity and lower toxicity. The interaction investigation indicates that quercetin binds with double‐strand DNA in groove binding mode, resulting in fluorescence enhancement of this system. Copyright © 2009 John Wiley & Sons, Ltd.     

Determination of Factors Associated with Natural Soil Suppressivity to Potato Common Scab

Common scab of potatoes is a disease, which is difficult to manage due to complex interactions of the pathogenic bacteria (Streptomyces spp.) with soil, microbial community and potato plants. In Bohemian-Moravian Highlands in the Czech Republic two sites (Vyklantice and Zdirec) were selected for a study of common scab disease suppressivity. At both sites, a field with low disease severity occurs next to one with high severity and the situation was regularly observed over four decades although all four fields undergo a crop rotation. In the four fields, quantities of bacteria, actinobacteria and the gene txtB from the biosynthetic gene cluster of thaxtomin, the main pathogenicity factor of common scab, were analyzed by real-time PCR. Microbial community structure was compared by terminal fragment length polymorphism analysis. Soil and potato periderm were characterized by contents of carbon, nitrogen, phosporus, sulphur, calcium, magnesium, and iron. Quality of organic matter was assessed by high performance liquid chromatography of soil extracts. The study demonstrated that the suppressive character of the fields is locally specific. At Zdirec, the suppressivity was associated with low txtB gene copies in bulk soil, while at Vyklantice site it was associated with low txtB gene copies in the tuberosphere. The differences were discussed with respect to the effect of abiotic conditions at Zdirec and interaction between potato plant and soil microbial community at Vyklantice. Soil pH, Ca soil content or cation concentrations, although different were not in the range to predict the disease severity. Low severity of common scab was associated with low content of soil C, N, C/N, Ca and Fe suggesting that oligotrophic conditions may be favorable to common scab suppression.     

Trichoderma harzianum strain SQR-T37 and its bio-organic fertilizer could control Rhizoctonia solani damping-off disease in cucumber seedlings mainly by the mycoparasitism

Damping-off disease is caused by Rhizoctonia solani and leads to serious loss in many crops. Biological control is an efficient and environmentally friendly way to prevent damping-off disease. Optical micrographs, scanning electron micrographs, and the determination of hydrolytic enzymes were used to investigate the antagonism of Trichoderma harzianum SQR-T37 (SQR-T37) against R. solani. Experiments were performed in pots to assess the in vivo disease-control efficiency of SQR-T37 and bio-organic fertilizer. The results indicate that the mycoparasitism was the main mechanism accounting for the antagonistic activity of SQR-T37. In one experiment, the population of R. solani was decreased from 10⁶ internal transcribed spacer (ITS) copies per gram soil to 10⁴ ITS copies per gram soil by the presence of the antagonist. In this experiment, 45% of the control efficiency was obtained when 8 g of SQR-T37 hyphae per gram soil was applied. In a second experiment, as much as 81.82% of the control efficiency was obtained when bio-organic fertilizer (SQR-T37 fermented organic fertilizer, BIO) was applied compared to only 27.27% of the control efficiency when only 4 g of SQR-T37 hyphae per gram soil was applied. Twenty days after incubation, the population of T. harzianum was 4.12 × 10⁷ ITS copies per gram soil in the BIO treatment, which was much higher than that in the previous treatment (8.77 × 10⁵ ITS copies per gram soil), where only SQR-T37 was applied. The results indicated that SQR-T37 was a potent antagonist against R. solani in a mycoparasitic way that decreased the population of the pathogen. Applying BIO was more efficient than SQR-T37 application alone because it stabilized the population of the antagonist.     

Influence of agricultural practices and seasons on the abundance and community structure of culturable pseudomonads in soils under no-till management in Argentina

Background and aims

Members of the genus Pseudomonas are common inhabitants of rhizospheres and soils, and it is known that soil types and crop species influence their population density and structure. 20?×?10⁶ ha are cultivated under no-tillage in Argentina and there is a need to find new biologically-based soil quality indexes to distinguish between sustainable and non-sustainable agricultural practices. Pseudomonads abundance and community structure were analyzed in no-till soils with different agricultural practices, in productive fields along 400 km of Argentinean Pampas.

Methods

We sampled soils and root systems from agricultural plots in which sustainable or non-sustainable agricultural practices have been applied. Samples were collected in summer and winter during 2010 and 2011. Culturable fluorescent and total pseudomonads were enumerated by plating on Gould’s selective medium S1. Colonies from these plates served as DNA source to carry out PCR-RFLP community structure analysis of the pseudomonads-specific marker genes oprF and gacA.

Results

Abundance of total and fluorescent culturable pseudomonads in bulk soils was influenced by seasonal changes and agricultural practices. Rhizospheric counts from the same crop were affected by agricultural treatments. Also, crop species influenced pseudomonads density in the rhizosphere. Combined PCR-RFLP profile of both genes showed a seasonal grouping of samples.

Conclusions

Sustainable soil management seems to promote pseudomonads development in soils, favoring root colonization of crops from those plots. Crop species influence total pseudomonads load of rhizospheres and its community structure. Total or relative pseudomonads load could function as soil quality indicator of good agricultural practices.     

Development of emulsion from rhizobial fermented starch industry wastewater for application as Medicago sativa seed coat

Starch industry wastewater was efficiently employed for the production of Sinorhizobium meliloti and the concentrated culture was used for the development of a biofertilizer formulation. Tween‐80 (0.02 g/L) acted as the best emulsifier for a Sinorhizobium–canola oil emulsion. The stability of the emulsion and survival of the organism was enhanced by supplementation of xanthan gum at pH 8. The refrigerated condition was most favorable for stability and survival of the microorganism. The survival of microorganism at 4±1°C was 2.78×10¹⁰ and 2.01×10¹⁰ CFU (colony forming unit)/mL on storage for 1 and 2 months, respectively. The values were higher than the prescribed cell count (×10³ CFU/mL) for field application. At 40°C, the survival of bacteria reduced from 3×10¹⁰ CFU/mL to 8.1×10⁹ and 8.8×10⁶ CFU/mL in 1 and 2 months, respectively. Emulsion‐coated seed was incubated at different temperatures and a cell count of 10⁵ CFU/seed was observed after 2 months of storage at 4°C, which was equal to the highest level of the described requirement (10³–10⁵ CFU/seed). Emulsion supplemented with xanthan gum improved the shelf‐life under optimized conditions (Sinorhizobium concentrate – canola oil (1:1) emulsion with 0.02 g/L Tween‐80; storage at pH 8 and temperature 4±1°C) and this emulsion with the required cell count and prolonged viability was used for the pre‐inoculation of seed or for in situ soil application.     

Study of Microflora in Malaysian Dried Fishes and Their Decontamination by Gamma-irradiation

The distribution of microorganisms in 10 samples of salted dried fish and the effects of irradiation of them were studied. The total aerobic bacteria in commercial dried fish were determined to be from 2 × 10⁴ to 3 × 10⁶ per gram. Mold counts were 1 × 10² to 7 × 10³ per gram with a lower amount of yeasts. In spoiled dried fish, total aerobic bacteria were determined to be 4× 10⁶ or 1 × 10⁷ per gram with a few yeasts. Coliforms were not isolated on MacConkey agar plates from any of the samples. The predominant bacteria occurring in spoiled dried fish were Pediococcus halophilus, Vibrio costicola and Planococcus sp. More than 50% of the molds consisted of the Aspergillus niger group, whereas lower amounts of the A. flavus, A. fumigatus and A. ochraceus groups, Penicillium chrysogenum series, etc. were also isolated from many samples of dried fish. All kinds of putrefactive microorganisms were radiation sensitive, and a dose of ca. 500 krad appears to be sufficient for extension of the shelf-life of dried fish from 2 to 4 times.     

Impact of commonly used agrochemicals on bacterial diversity in cultivated soils

The effects of three selected agrochemicals on bacterial diversity in cultivated soil have been studied. The selected agrochemicals are Cerox (an insecticide), Ceresate and Paraquat (both herbicides). The effect on bacterial population was studied by looking at the total heterotrophic bacteria presence and the effect of the agrochemicals on some selected soil microbes. The soil type used was loamy with pH of 6.0–7.0. The soil was placed in opaque pots and bambara bean (Vigna subterranean) seeds cultivated in them. The agrochemicals were applied two weeks after germination of seeds at concentrations based on manufacturer’s recommendation. Plant growth was assessed by weekly measurement of plant height, foliage appearance and number of nodules formed after one month. The results indicated that the diversity index (Di) among the bacteria populations in untreated soil and that of Cerox-treated soils were high with mean diversity index above 0.95. Mean Di for Ceresate-treated soil was 0.88, and that for Paraquattreated soil was 0.85 indicating low bacterial populations in these treatment-type soils. The study also showed that application of the agrochemicals caused reduction in the number of total heterotrophic bacteria population sizes in the soil. Ceresate caused 82.50% reduction in bacteria number from a mean of 40 × 10⁵ cfu g⁻¹ of soil sample to 70 × 10⁴ cfu g⁻¹. Paraquat-treated soil showed 92.86% reduction, from a mean of 56 × 10⁵ cfu g⁻¹ to 40 × 10⁴ cfu g⁻¹. Application of Cerox to the soil did not have any remarkable reduction in bacterial population number. Total viable cell count studies using Congo red yeast-extract mannitol agar indicated reduction in the number of Rhizobium spp. after application of the agrochemicals. Mean number of Rhizobium population numbers per gram of soil was 180 × 10⁴ for the untreated soil. Cerox-treated soil recorded mean number of 138 × 10⁴ rhizobial cfu g⁻¹ of soil, a 23.33% reduction. Ceresate- and Paraquat-treated soils recorded 20 × 10⁴ and 12 × 10⁴ cfu g⁻¹ of soil, respectively, representing 88.89% and 93.33% reduction in Rhizobium population numbers. Correspondingly, the mean number of nodules per plant was 44 for the growth in untreated soil, 30 for the plant in the Cerox-treated soil, 8 for the plant in Paraquat-treated soil and 3 for the plant in Ceresate-treated soil. The study has confirmed detrimental effect of insecticide on bacterial populations in the soil. Total heterotrophic counts, rhizobial counts as well as the number of nodules of all samples taken from the chemically treated soils were all low as compared to values obtained for the untreated soil. However, the effect of the insecticide was minimal in all cases as compared to the effects of the herbicides on the soil fauna. Indiscriminate use of agrochemicals on farms can therefore affect soil flora and subsequently food production.     

Influence of soil reaction on diversity and antifungal activity of fluorescent pseudomonads in crop rhizospheres

The diversity and antifungal activity of fluorescent pseudomonads isolated from rhizospheres of tea, gladiolus, carnation and black gram grown in acidic soils with similar texture and climatic conditions were studied. Biochemical characterisation including antibiotic resistance assay, RAPD and PCR-RFLP studies revealed a largely homogenous population. At soil pH (5.2), the isolates exhibited growth with varying levels of siderophore production, irrespective of crop rhizospheres. Two isolates with maximum chitinase production showed antagonism. The bacterial populations in general lacked the ability to produce deleterious traits such as cellulase, pectinase and hydrogen cyanide. However, increased pH levels beyond 5.2 caused reduction in metabolite production with reduced antifungal activity. The homogeneity of the bacterial population irrespective of crop rhizospheres together with decreased secondary metabolite production at higher pH levels reinstated the importance of soil over host plant in influencing rhizosphere populations. The studies also yielded acid tolerant chitinase producing antagonistic fluorescent pseudomonads.     

CROWN GALL TUMORIGENESIS IN POTATO TUBER TISSUE

Potato tuber discs were examined as a possible quantitative bioassay for studying tumor induction by Agrobacterium tumefaciens. Discs from two potato varieties, Pontiac and Russett Burbank, were inoculated and cultured on water agar plates. Tumors appeared within 10 days. Both the number and weight of tumors per disc increased linearly for inoculum concentration between 1 × 10⁷–1 × 10⁹ cells per ml. Polarity, position of the disc relative to tuber epidermis, potato variety and the light conditions did not influence the tumor formation. The simplicity of the procedure and the homogeneity of the tissue, together with the fact that it is a quantitative bioassay, makes the potato tuber disc an ideal system for the investigation of biochemical step(s) associated with the transformation process.     

A method for the determination of the concentration of catalytic centres in low activity and unstable preparations of acetylcholinesterase.

A simple and rapid method has been developed for the titration of catalytic centres of acetylcholinesterase of low activity and stability in homogenates of larvae of the cattle tick Boophilus microplus. It is based on the difference in uptake of the labeled organophosphate inhibitor [¹⁴C]coroxon between substrateprotected and unprotected enzyme. The excess coroxon is removed rapidly by solvent extraction of the acidified enzyme medium with acetone and toluene. The method was validated by the use of bovine erythrocyte acetylcholinesterase, with only 6 × 10^?12 catalytic centre mole equivalents of this enzyme being required for a single accurate assay. The turnover number at pH 7.6 and 37°C was 1.22 × 10⁶ molecules of acetylcholine hydrolysed per min per active centre. The catalytic efficiency of enzyme of larvae of the cattle tick was markedly different, being onetenth of that of bovine erythrocyte enzyme. Advantages of the method are discussed.     

Abundance and Diversity of Sulfur-Oxidizing Bacteria along a Salinity Gradient in Four Qinghai-Tibetan Lakes,China

Sulfur-oxidizing bacteria (SOB) play important roles in the sulfur cycle and are widespread in a number of environments, but their occurrence and relationship to geochemical conditions in (hyper)saline lakes are still poorly understood. In this study, the abundance and diversity of SOB populations were investigated in four Qinghai-Tibetan lakes (Erhai Lake, Gahai Lake 1, Gahai Lake 2 and Xiaochaidan Lake) by using quantitative polymerase chain reaction (qPCR) and soxB gene- (encoding sulfate thiohydrolase) based phylogenectic analyses. qPCR analyses showed that in the studied lakes, the total bacterial 16S rRNA and soxB gene abundances in the sediments were distinctly higher than in the overlying waters. The 16S rRNA gene abundance in the waters ranged 5.27 × 10⁶–6.09 × 10⁸ copies per mL and 7.39 × 10¹⁰–2.9 × 10¹¹ copies per gram sediment. The soxB gene abundance in the waters ranged from 1.88 × 10⁴ to 5.21 × 10⁵ per mL and 4.73 × 10⁶–2.65 × 10⁷ copies per gram sediment. The soxB gene in the waters of the two hypersaline lakes (Gahai Lake 2 and Xiaochaidan Lake) was more abundant (2.97 × 10⁵ and 5.21 × 10⁵ copies per mL) than that in the two low-salinity lakes (1.88 × 10⁴ and 3.36 × 10⁴ copies per mL). Phylogenetic analysis showed that Alpha- and Betaproteobacteria were dominant SOB in the investigated lakes, and the composition of proteobacterial subgroups varied with salinity: in freshwater Erhai Lake and low-salinity Gahai Lake 1, the SOB populations were dominated by the Betaproteobacteria, whereas in hypersaline Lake Gahai 2 and Xiaochaidan Lake, the SOB populations were dominated by Alphaproteobacteria. Overall, salinity played a key role in controlling the diversity and distribution of SOB populations in the investigated Qinghai-Tibetan lakes.     

Further studies on the distribution of a nuclear-polyhedrosis virus of the fall webworm,Hyphantria cunea,in soil

The median infectious concentration (IC₅₀) of a nuclear-polyhedrosis virus of the fall webworm, Hyphantria cunea, in soil was 7.14×10⁷ polyhedra per gram of soil. The number of virus-killed larvae equivalent to IC₅₀ decreased with the advance of the larval instar from 22 first-instar to 0.04 seventh-instar larvae per gram of soil. The degree of virus contamination of the surface soil in an area where the virus disease had occurred naturally in host populations was estimated to be as high as IC₅₀ on the basis of soil bioassays. Nearly 1000 virus-killed seventh-instar larvae/m² were necessary to obtain such extensive soil contamination. The possibility of the occurrence of so many virus-killed cadavers in the study area was discussed in relation with the past records of epizootics and host population trends. The distribution of the fallen leaves of large trees favorable to the fall webworm, such as plane and poplar trees, correlated with that of the nuclear-polyhedrosis virus in the soil.