Terminal electron transport system (ETS)-activity in the sediment of Lake Balaton,Hungary

Terminal electron transport system (ETS)-activity of the sediment and plankton of Lake Balaton, the largest shallow lake of Central Europe was measured by tetrazolium-reduction biweekly during 1989–1990 and in the spring of 1991. Sediment proved to be enzymatically active to 30-35 cm down in the hypertrophic Keszthely Bay and to 15–20 cm down in the meso-eutrophic Siófok Basin. Sediment ETS-activity exceeded planktonic activity 15 to 24 fold.The total activity m^–2 showed one or two order of magnitude higher respiratory potential in Lake Balaton than needed for complete oxidation of the planktonic primary production; most of this potential was detected in the upper 3–5 cm sediment layer in springs. Incubations of cell-free homogenates of sediment bacteria showed that ETS remains active days after death of organisms at low temperature. Accumulated postmortem ETS-activity derived from the benthic diatoms, bacteria, plankton deposit and dead summer macrophytes seems to be responsible for the high ETS-activity of the sediment in the warming periods in springs. These enzyme fractions may contribute to the rapid oxidation of the alkaline, well-aerated lake.     

Bacteria and algae in stream periphyton along a nutrient gradient

1. Stream riffles in southern Ontario and western Quèbec were sampled for biomass (58 stations from 51 streams) and production (22 stations from 21 streams) of algae and bacteria in periphyton to test the hypothesis that bacteria in benthic biofilms compete with algae for nutrients. 2. Algal and bacterial biomass were positively correlated, as were algal and bacterial production. Bacterial production was also positively correlated to algal and bacterial biomass, but the relationship was not significant. The ratio of algal to bacterial biomass did not vary with nutrients whereas algal production tended to increase with nutrients more rapidly than bacterial production. 3. Instream nitrogen concentrations explained 38–58% of the variability in algal biomass and production. Bacterial abundance explained an additional 9–29% of the residual variance in algal production and biomass. However, the relationship between bacterial abundance and algal production and biomass, once nutrients were taken into account, was positive, in contrast to the predicted effect of competition. 4. Hence, we reject our original hypothesis that bacteria in biofilms compete with algae for nutrients and instead suggest that bacteria and algae in biofilms coexist in an association that offers space and resources to sustain production of both groups of organisms.     

Phytoplankton–bacterioplankton interactions in a neotropical floodplain lake (Laguna Bufeos,Bolivia)

Laguna Bufeos is a floodplain lake of the river Ichilo, a tributary of the Amazon basin situated in Bolivia. Nutrient addition assays involving whole water (<200 μm) as well as fractionated water (<0.8 μm) treatments were carried out in incubation tubes to test whether bacterial growth is limited by the availability of inorganic nutrients and to test whether bacteria are able to utilize inorganic nutrients directly or are stimulated by inorganic nutrients through increased production of phytoplankton. The assays were carried out during two extreme hydrological conditions, the high-water and the low-water period. During the high-water period experiment, neither N or P limited bacterial growth rates. During the low-water period, bacterial growth was P limited. Bacterial growth was stimulated in the fractionated as well as in the whole water treatments, indicating that bacterial growth was directly stimulated by P. Bacterial growth corrected for grazing losses (determined by means of dilution experiments) was significantly higher in the fractionated water containing only bacteria when compared to the whole water containing also grazers and phytoplankton. This suggests that bacterial growth was suppressed by competition with phytoplankton rather than stimulated through the production of dissolved organic matter by phytoplankton.     

Compatibility and interaction between Bacillus thuringiensis and certain insecticides: perspective in management of Tuta absoluta (Lepidoptera: Gelechiidae)

Bacillus thuringiensis var. kurstaki (Berliner) (Bt) has been suggested as a biological control agent for Tuta absoluta (Meyrick). The objective of this study was to determine the interaction between abamectin, azadirachtin, indoxacarb, chlorantraniliprole, dichlorvos and metaflumizone with Bt. Effect of recommended doses of the chemical insecticides on colonisation of Bt was also investigated in culture medium. Except for metaflumizone, none of the chemicals tested reduced the colonisation of Bt compared with control. Interaction between Bt and the chemical insecticides on 2nd-instar larvae was also assessed. In interaction tests, Bt was applied at LC₅₀ level, 0, 12, 24 or 36 h after treating the larvae with LC₁₀ or LC₂₅ of the chemical insecticides. An antagonistic effect was observed in all treatments where Bt was applied immediately after the chemical insecticide. Also, antagonism was observed when treatment with Bt was done 12 h after azadirachtin and metaflumizone applications. Applying Bt 12 and 24 h after treatment with LC₂₅ of chlorantraniliprole, dichlorvos and abamectin resulted in synergism. But, synergism with LC₁₀ of dichlorvos and abamectin was observed only after 12 h. Additive effect was observed in the rest of the time and concentration combinations. Based on the results obtained, simultaneous use of the chemical insecticides tested and Bt is not recommended for T. absoluta control; and an appropriate time interval should be taken into consideration accordingly.     

Evaluation of Solanum xanthocarpum extract as a synergist for cypermethrin against larvae of the filarial vector Culex quinquefasciatus (Say)

Cypermethrin and crude extracts of Solanum xanthocarpum were both observed for their larvicidal activity against Culex quinquefasciatus. Petroleum ether extract with lethal concentration (LC)₅₀ and LC₉₀ of 41.28 and 111.16 p.p.m. after 24 h and LC₅₀ 38.48 and LC₉₀ 80.83 p.p.m. after 48 h, respectively, was found to be the most effective, followed by carbon tetrachloride and methanol extracts. LC₅₀ and LC₉₀ for cypermethrin were 0.0027 and 0.0097 p.p.m. after 24 h and 0.0013 and 0.0092 p.p.m. after 48 h of exposure, respectively. Combined formulations were evaluated for synergistic activity and a 1:1 ratio of cypermethrin and petroleum ether extract was observed to be more effective than 1:2 and 1:4 ratios. Combinations of S. xanthocarpum extracts and cypermethrin demonstrated higher larvicidal activity, indicating synergistic activity. These results demonstrate the need for further studies on the effectiveness and toxicity to humans and animals, particularly aquatic forms.     

Enclosures study of trophic level interactions in the mesotrophic part of Lake Balaton

Enclosures, open to the bottom sediments and to the atmosphere, containing about 17 m³ of lake water in the mesotrophic area of Lake Balaton, were used to elucidate the role of the benthivorous fish bream (Ambramis brama L.) in the lake during 1984–1986.Throughout the whole period water was less transparent in the enclosure containing fish, which strongly influenced the concentrations of suspended solids and chlorophyll a.Both phytoplankton biomass and production readily responded to nutrient increase in the enclosure with fish. In 1985 diatoms were replaced by cyanobacteria whereas in 1986, at a lower fish stocking, a shift in algal structure towards chlorophytes was observed.Egested organic substances and the resuspension of sediment particles by fish increased bacterial production.     

Interactions of the microsporidium Vairimorpha necatrix with a bacterium,virus, and fungus in Heliothis zea

Interactions between Vairimorpha necatrix and three other pathogens of Heliothis zea were evaluated with dose-mortality studies in order to find a synergistic combination which could be tested for field control of H. zea. The effect of the microsporidium combined with the bacterium Bacillus thuringiensis was at least additive, with indications of synergism. The interaction between V. necatrix and Heliothis nuclear-polyhedrosis virus was antagonistic except that the highest microsporidian dose overcame the antagonism with a resultant independent action. The interaction between V. necatrix and the fungus Nomuraea rileyi was additive, though response varied some-what with different proportions of the two pathogens. Even though none of these interactions is likely to be valuable in microbial control, V. necatrix has the potential to synergize or antagonize any biological or chemical agent that acts on the midgut epithelium of host insects.     

Uptake of cadmium and zinc by the alga Chlorella vulgaris: II. Multi-ion situation

Many microorganisms are capable of sequestering and concentrating heavy metals from their aqueous environment. While much research has beep carried out on the uptake of single species of metal ions, little attention seems to have been given to the study of multimetal ion systems. A mathematical model has previously been developed to describe the uptake of individual metal species by a microorganism. The model proposes two sequential processes: an initial rapid uptake due to cellular surface adsorption and a subsequent slow uptake due to membrane transport of the metal into the cells. This article extends the treatment by considering the uptake of two metal species together, cadmium and zinc, under different experimental conditions. The results are discussed in terms of possible mechanistic interactions.     

The extraction by soil and absorption by plants of applied zinc and cadmium

In five consecutive years lettuce, spinach, spring wheat, endive and maize were grown in pots and the effects of native and soil-applied Zn and Cd on plant Zn and Cd concentrations were studied. The normal interactive pattern was antagonistic, Zn reducing plant Cd uptake, and conversely, but less so. Only in loam soil Zn and Cd were synergistic to some extent, plant Zn uptake increasing with applied Cd.When relating total soil Cd/Zn to plant Cd/Zn separate sets of data could be distinguished for loam and sandy soil, each fitting a straight line. The use of 0.1 M CaCl₂ instead of total extractable soil Cd/Zn makes the two sets of data to coalesce around a single straight line. All crops were found to show a positive linear relationship between 0.1 M CaCl₂-extractable soil Cd/Zn and plant Cd/Zn.     

Ecological regime shifts in salinised wetland systems. II. Factors affecting the dominance of benthic microbial communities

This paper is the second in a pair investigating potential mechanisms for ecological regime change in salinising wetlands. The first paper in this series focused on the responses of the salt-tolerant submerged macrophyte community to salinity. In this second paper, we investigated some of the environmental conditions required for initiation and dominance of benthic microbial communities using a combination of experimental and observational data. Two experiments were carried out. One investigated the importance of prior establishment of benthic microbial communities on their ability to maintain prevalence over macrophyte colonisation (‘persistence’ experiment), while the other investigated hydrology and its effect on sediment perturbation, potential nutrient release and subsequent benthic microbial community establishment (‘flooding’ experiment). The ‘persistence’ experiment measured the biomass of benthic microbial communities and emergence of macrophytes from sediments kept either wet or dry for 4 weeks then flooded at a range of salinities. Benthic microbial biomass was similar across all of the salinities tested (15, 45 and 70 ppt), with a slight increase at higher salinities, suggesting that none of these limited benthic microbial community development. Pre-wetting of sediments usually increased benthic microbial community biomass and reduced macrophyte germination, but the latter was attributed to the presence of anoxic sediments rather than the increased benthic microbial community biomass. Germinating macrophytes emerged through benthic microbial communities or dense heterotrophic bacterial blooms, demonstrating that they could become dominant even when another community was already established. Field data supported these results, suggesting that the development of benthic microbial communities is not limited by salinity alone, but includes other factors, such as the water regime. In the ‘flooding’ experiment, the largest differences in nutrient concentrations ultimately lay between the pre-wet and pre-dry treatments (due to the greater release of nutrients and development of anoxia in the latter) rather than those subjected to fast versus slow flooding. In response to this, highest benthic microbial community biomass was in treatments with pre-wet sediment, corresponding with lower phytoplankton biomass.     

Interactions Between Nematodes and Other Factors on Plants

The distinction between qualitative and quantitative interactions is stressed as it helps to elucidate differences between two current definitions of the term synergism and how multifactorial experiments should be designed, analysed statistically, and their results interpreted. Factorial design and analyses are strongly advocated so that interactions can be detected. It is suggested that interactions involving nematodes are common in nature and should be included, where appropriate, in hypotheses. Methods for testing the hypothesis that environment influences tolerance are suggested.     

Microbial antagonism: a neglected avenue of natural products research

Competition amongst microbes for space and nutrients in the marine environment is a powerful selective force which has led to the evolution of a variety of effective strategies for colonising and growing on surfaces. We are particularly interested in the chemical ecology of marine epibiotic bacteria which live on the surfaces of marine algae or invertebrates. Over 400 strains of surface-associated bacteria from various species of seaweed and invertebrate from Scottish coastal waters were isolated and 35% of them shown to produce antimicrobial compounds. This is a much higher proportion than free living marine isolates or soil bacteria. In addition, many strains which did not normally produce antibiotics could be induced to do so by exposing them to small amounts of live cells, supernatants from other bacterial cultures or other chemicals. Thus the number of strains able to produce antibiotics appears to be much higher than previously thought. Induction of antibiotic production was elicited by other marine epibionts and also by terrestrial human pathogens such as Bacillus subtilis, Escherichia coli and Pseudomonas aeruginosa. An understanding of this type of chemical induction and the factors regulating non-constitutive secretion of antimicrobial compounds will allow more effective strategies for searching for new chemotherapeutic antibiotics to be designed.