Cyanobacterial–Bacterial Complexes in Plant Syncyanoses

The morphology and ultrastructure of associative microsymbiont complexes (AMC) isolated from the ferns Azolla pinnata and Azolla sp. and the apogeotropic roots of the cycad Cycas revoluta were studied. The composition of the AMC obtained includes the cyanobionts (symbiotic cyanobacteria) and satellite bacteria (SB). It was found that two types of cyanobacteria that substantially differ in their morphological organization are likely present as cyanobionts in the coralloids of C. revoluta. The isolated cyanobiont strains exhibited the morphological traits and regularities of development typical of the genus Nostoc; they were characterized by the ability of their cells to divide in mutually perpendicular planes. When isolating AMC from different morphological zones of C. revoluta apogeotropic roots, SB growth was revealed only around the pieces corresponding to the coralloid apical zone. No AMC components were revealed around the segments of the basal growth zone. Pure cyanobiont cultures were obtained from the AMC of C. revoluta coralloids. The AMC isolated from the ferns A. pinnata and Azolla sp. are characterized by obligate mutual dependence of the partners (the cyanobiont and SB).     

Suppression of the root-lesion nematode (Pratylenchus penetrans) in alfalfa (Medicago sativa) by Streptomyces spp.

Strains of Streptomyces were tested for their ability to reduce population densities of the root-lesion nematode (RLN), Pratylenchus penetrans, in roots of alfalfa (Medicago sativa) in growth chamber assays. Previously, these strains were shown to suppress potato scab disease, caused by Streptomyces scabies, in field experiments and to inhibit in vitrogrowth of a wide range of plant-pathogenic fungi and bacteria. Inoculation with Streptomyces at planting significantly reduced RLN population densities in roots of both susceptible and resistant alfalfa varieties grown in either heat-treated or untreated soil. Reductions in RLN population densities were observed 6 weeks after nematode inoculation. Shoot dry matter was not affected by any treatment; root dry weight was reduced in Streptomycesplus nematode treatments compared to the nematode inoculation alone in some experiments but was not affected by Streptomyces when RLN was absent. Mutant strains not producing antibiotics in vitro also reduced RLN population densities in alfalfa roots and all strains maintained high population densities after inoculation into heat-treated soil and on alfalfa roots. These strains may be useful in multi-crop, multi-pathogen management programs to augment genetic resistance to plant diseases.     

Activity profiles of bacterioplankton in a eutrophic river

1. The significance of microbial diversity in processing dissolved organic matter (DOM) is largely unknown. We investigated the range of functional diversity in the bacterioplankton from a eutrophic river by profiling extracellular enzyme activities (EEA) and substrate-induced respiration (SIR) patterns.
2. The EEA profiles consisted of assays for 21 hydrolases, measured using fluorogenic substrates arrayed on 96-well microplates. Commercially available BiOLOG^® GN and ECO plates, which contained 120 different substrates, were used for the SIR profiles.
3. The EEA data were more dynamic than the SIR. Five enzymes, leucine aminopeptidase, alkaline phosphatase, alanine aminopeptidase, arginine aminopeptidase and β-glucosidase, showed consistently high activity; ten others were ubiquitous at lower activity levels; the remainder were detected intermittently. The SIR data showed less temporal variability. With one exception (citrate), the 20 substrates that generated the largest responses were all saccharides or their derivatives.
4. The EEA and SIR data did not generally correlate. Both methods were effective for ordinating bacterioplankton although, unlike the SIR, the EEA ordination followed a clear temporal trajectory.
5. Because the SIR profiles are based on a culture response, whilst the EEA profiles measure activity of the extant community, the latter appear to be more directly linked to the mechanics of DOM processing.     

Inhibition of the metabolism of nitrifying bacteria by direct electric current

The nitrifying bacteria in activated sludge and biofilms consisting of the bacteria immobilized on polypropylene packing were subjected to an electric current via two electrodes. In activated sludge, the metabolism of nitrifying bacteria was inhibited when the applied current was over 2.5 A m^–2, whilst in biofilms, inhibition began when the current reached 5 A m^–2. At 15 A m^–2, the nitrification rate of NH₄ ⁺-N in a biofilm with a bacterial density of 1.62 g total solids, dry wt m^–2 decreased to about 80% of its initial value. Ninety-two % of the initial biomass on the packing was retained after 36 h.     

Intracellular Burkholderia strain has no negative effect on the symbiotic efficiency of the arbuscular mycorrhizal fungus Gigaspora margarita

This study investigates the effects of bacteria occurring in thecytoplasm of some arbuscular mycorrhizal fungi (AMF) on their symbioticefficiency. Gigaspora margarita, Gigasporarosea and Glomus versiforme, containing orwithout intracellular bacteria, were compared for their efficiency instimulating growth of Lactuca sativa L. Biomass productionand nutrient contents were evaluated in plants grown on two substrates. Theefficiency of G. margarita harbouring a homogenouspopulation of Burkholderia was greater than that of theother two AMF, mainly G. rosea, which does not containintracellular bacteria. When plants were grown in poor soil, inoculation withG. margarita resulted in the best growth rates as well asthe highest N, P and K values. The significantly higher N content is ofparticular importance, since the genome of Burkholderiapossesses nif genes.     

Ammonia-oxidizing bacteria on root biofilms and their possible contribution to N use efficiency of different rice cultivars

Ammonia-oxidizing bacteria (AOB) populations were studied on the root surface of different rice cultivars by PCR coupled with denaturing gradient gel electrophoresis (DGGE) and fluorescence in situ hybridization (FISH). PCR-DGGE of the ammonium monooxygenase gene (amoA) showed a generally greater diversity on root samples compared to rhizosphere and unplanted soil. Sequences affiliated with Nitrosomonas spp. tended to be associated with modern rice hybrid lines. Root-associated AOB observed by FISH were found within a discrete biofilm coating the root surface. Although the total abundance of AOB on root biofilms of different rice cultivars did not differ significantly, there were marked contrasts in their population structure, indicating selection of Nitrosomonas spp. on roots of a hybrid cultivar. Observations by FISH on the total bacterial community also suggested that different rice cultivars support different bacterial populations even under identical environmental conditions. The presence of active AOB in the root environment predicts that a significant proportion of the N taken up by certain rice cultivars is in the form of NO₃ ^–-N produced by the AOB. Measurement of plant growth of hydroponically grown plants showed a stronger response of hybrid cultivars to the co-provision of NH₄ ⁺ and NO₃ ^–. In soil-grown plants, N use efficiency in the hybrid was improved during ammonium fertilization compared to nitrate fertilization. Since ammonium-fertilized plants actually receive a mixture of NH₄ ⁺ and NO₃ ^– with ratios depending on root-associated nitrification activity, these results support the advantage of co-provision of ammonium and nitrate for the hybrid cultivar.     

The Growth-promoting Effect of Beijerinckia mobilis and Clostridium sp. Cultures on Some Agricultural Crops

New strains of Beijerinckia mobilis and Clostridium sp. isolated from the pea rhizosphere were studied with respect to their promoting effect on the growth and development of some agricultural crops. Seed soaking in bacterial suspensions followed by the soil application of the suspensions or their application by means of foliar spraying was found to be the most efficient method of bacterization. The application of B. mobilis andClostridium sp. cultures in combination with mineral fertilizers increased the crop production by 1.5–2.5 times. The study of the population dynamics of B. mobilis by the method of genetic marking showed that this bacterium quickly colonized the rhizoplane of plants and, therefore, had characteristics of an r-strategist. At the same time, Clostridiumsp. was closer to K-strategists, since this bacterium slowly colonized the econiches studied. The introduction of the bacteria into soil did not affect the indigenous soil bacterial complex. The presence of Clostridium sp. slowed down the colonization of roots by the fungal mycelium. The possible mechanisms of the plant growth–promoting activity of B. mobilisand Clostridiumsp. are discussed.     

Modulation of the bacterial response to spectral solar radiation by algae and limiting nutrients

SUMMARY 1. The response of bacterial production (measured as [³H]TdR incorporation rate) to spectral solar radiation was quantified experimentally in an oligotrophic high-mountain lake over 2 years. Bacterial responses were consistent: ultraviolet-B (UVB) was harmful, whereas ultraviolet-A (UVA) + photosynthetically active radiation (PAR) and PAR enhanced bacterial activity. Full sunlight exerted a net stimulatory effect on bacterial activity in mid-summer but a net inhibitory effect towards the end of the ice-free period.
2. Experiments were undertaken to examine whether the bacterial response pattern depended on the presence of algae and/or was modulated by the availability of a limiting inorganic nutrient (phosphorus, P). In the absence of algae, [³H]TdR incorporation rates were significantly lower than when algae were present under all light treatments, and the consistent bacterial response was lost. This suggests that the bacterial response to spectral solar radiation depends on fresh-C released from algae, which determines the net stimulatory outcome of damage and repair in mid-summer.
3. In the absence of algae, UVB radiation inhibited bacteria when they were strongly P-deficient (mean values of N : P ratio: 46.1), whereas it exerted no direct effect on bacterial activity when they were not P-limited.
4. P-enrichment of lake water markedly altered the response of bacteria to spectral solar radiation at the end of ice-free period, when bacteria were strongly P-deficient. Phosphorus enrichment suppressed the inhibitory effect of full sunlight that was observed in October, both in whole lake water (i.e. including algae) and in the absence of algae. This indicates that the bacterial P-deficiency, measured as the cellular N : P ratio, was partly responsible for the net inhibitory effect of full sunlight, implying a high bacterial vulnerability to UVB.