A preliminary study of the growth of fungi and bacteria from temperate and antarctic soils in relation to temperature

The growth of fungi isolated from a lowland temperate site (Roudsea Wood National Nature Reserve), an upland temperate moorland (Moor House National Nature Reserve) and an oceanic Antarctic island (Signy, S. Orkneys) was compared at 1, 14 and 25°C. This showed that low temperatures caused greatest retardation of growth in fungi from the warmest site (Roudsea) and least from the coldest site (Signy Island). At Moor House, fungi which were isolated most frequently in winter were able to grow better at 1°C than summer forms. The fungal flora of Signy Island was restricted and consists of cold tolerant cosmopolitan species which have been selected by or become adapted to the prevailing low temperatures. Of fungi isolated from any two of the sites, Mortierella alpina and Mucor hiemalis showed temperature adaptation correlated with prevailing site temperature, while Trichoderma viride, Penicillium thomii, and P. frequentans showed no adaptation.     

Nitrifier dominance of Arctic soil nitrous oxide emissions arises due to fungal competition with denitrifiers for nitrate

Arctic soils emit nitrous oxide, which is a potent greenhouse gas and also represents an important loss of nitrogen to oligotrophic Arctic ecosystems. However, little is known about the temperature sensitivity of nitrous oxide release in Arctic soils or the organisms mainly responsible for it. We investigated controls on nitrous oxide emissions in an Arctic soil across a typical temperature range (between 4 and 13 °C) on Truelove Lowland, Devon Island, Canada (75°40′N 84°35′W) at two different moisture contents. When fertilized with ammonia or nitrate, nitrous oxide emissions and temperature dependence of nitrous oxide emissions were insensitive to soil moisture content but linked to nitrification rates. Stable isotope analysis revealed that nitrous oxide was predominantly released by nitrifiers. However, nitrous oxide emissions were not linked to nitrifier prevalence with an insignificant (P < 0.219) increase in amoA genes and a (P < 0.01) decrease in archaeal nitrifiers. In contrast, denitrifier nosZ prevalence was 10,000 times greater than that of nitrifiers and was related to nitrous oxide emission potential when soils were fertilized with nitrate. Manipulating water-filled pore space should have changed the pattern of N₂O emissions. We used selective inhibitors to further explore why denitrification did not occur under field conditions when we manipulated water-filled pore space or when we used ¹⁵N analysis. When fungi were inhibited in the soil, nitrous oxide emissions from denitrifiers increased with no change in nitrous oxide released by nitrifiers. When fungi were active in the soil, there was little available nitrate but when fungi were inhibited, available soil nitrate increased over the incubation period. The dominance of nitrifiers in nitrous oxide emissions from Arctic soils under field conditions is linked to the competition for nitrate between fungi and denitrifiers.     

Resistance of microbial populations in DDT-contaminated and uncontaminated soils

One DDT-contaminated soil and two uncontaminated soils were used to enumerate DDT-resistant microbes (bacteria, actinomycetes and fungi) by using soil dilution agar plates in media either with 150 μg DDT ml⁻¹ or without DDT at different temperatures (25, 37 and 55°C). Microbial populations in this study were significantly (p<0.001) affected by DDT in the growth medium. However, the numbers of microbes in long-term contaminated and uncontaminated soils were similar, presumably indicating that DDT-resistant microbes had developed over a long time exposure. The tolerance of isolated soil microbes to DDT varied in the order fungi>actinomycetes>bacteria. Bacteria from contaminated soil were more resistant to DDT than bacteria from uncontaminated soils. Microbes isolated at different temperatures also demonstrated varying degrees of DDT resistance. For example, bacteria and actinomycetes isolated at all incubation temperatures were sensitive to DDT. Conversely fungi isolated at all temperatures were unaffected by DDT.     

Soil microbial and nutrient dynamics in a wet Arctic sedge meadow in late winter and early spring

Microbial activity is known to continue during the winter months in cold alpine and Arctic soils often resulting in high microbial biomass. Complex soil nutrient dynamics characterize the transition when soil temperatures approach and exceed 0 °C in spring. At the time of this transition in alphine soils microbial biomass declines dramatically together with soil pools of available nutrients. This pattern of change characterizes alpine soils at the winter-spring transition but whether a similar pattern occurs in Arctic soils, which are colder, is unclear. In this study amounts of microbial biomass and the availability of carbon (C), nitrogen (N) and phosphorus (P) for microbial and plant growth in wet peaty soils of an Arctic sedge meadow have been determined across the winter-spring boundary. The objective was to determine the likely causes of the decline in microbial biomass in relation to temperature change and nutrient availability. The pattern of soil temperature at depths of 5-15 cm can be divided into three phases: below −10 °C in late winter, from −7 to 0 °C for 7 weeks during a period of freeze-thaw cycles and above 0 °C in early spring. Peak microbial biomass and nutrient availability occurred early in the freeze-thaw phase. Subsequently, a steady decrease in inorganic N occurred, so that when soil temperatures rose above 0 °C, pools of inorganic nutrients in soils were very low. In contrast, amounts of microbial C and soluble organic C and N remained high until the end of the period of freeze-thaw cycles, when a sudden collapse occurred in soluble organic C and N and in phosphatase activity, followed by a crash in microbial biomass just prior to soil temperatures rising consistently above 0 °C. Following this, there was no large pulse of available nutrients, implying that competition for nutrients from roots results in the collapse of the microbial pool.     

Celtis tala and Scutia buxifolia leaf litter decomposition by selected fungi in relation to their physical and chemical properties and lignocellulolytic enzyme activity

This study analyzes the relationships between the physical and chemical properties of Celtis tala and Scutia buxifolia leaf litter and their degradation by selected fungi. The litter was analyzed for physical, chemical and enzyme properties such as pH, reducing sugars, aromatic compounds, chromophores, polymerization/polydispersity index and the lignocellulolytic enzyme activity of their water soluble fraction (WSF) after incubating them with fungi for 30 days. C. tala and S. buxifolia leaves were chemically different, with C-to-N ratios of 27 and 17, respectively. Fungi degraded C. tala leaves to a greater extent than those of S. buxifolia, which was directly related to the pH of the WSF. In this regard, properties other than microbial growth affecting substrate N concentration, such as lignin content or lignin-to-N ratio and the availability of nutrients for regulating the expression and activity of depolymerizing enzymes, governed the decomposition. Whereas degradation of S. buxifolia leaves by a group of selected fungi was related to cellobiohydrolase and β-1,4-endoglucanase activities, that of C. tala was related to the β-glucosidase activity. However, the fungi studied showed negligible ligninolytic potential. Still, physical and chemical properties such as pH and reducing sugars or chromophores as well as cellulose-degrading fungal enzymes were reliable indices of decomposition of the C. tala and S. buxifolia leaf litter.     

Bacterial and fungal growth in soil heated at different temperatures to simulate a range of fire intensities

The intensity of a fire is an important factor determining the recovery of soil microorganisms after a forest fire, since it can alter the quality and quantity of carbon sources. Recovery of the microbial community was studied in a Mediterranean pine forest soil subjected to different temperatures to simulate the short-term effects of fire intensity on bacterial and fungal growth, estimated using leucine incorporation for bacteria and acetate incorporation into ergosterol for fungi. Soil samples were heated for 15 min at 50, 80, 120, 200, 300, 400 and 500 °C. After inoculation with fresh soil, and adding water to achieve 60% WHC, the soils were incubated at 20 °C for 21 days. Bacterial growth was initially inhibited in the samples heated above 50 °C (totally inhibited ≥ 200 °C), but recovered within days to levels much higher than the control, except for the samples heated at 500 °C, where growth remained low throughout the incubation period due to the destruction of most of the organic matter. After the first week of incubation, the bacterial response decreased to values close to, but still above, that of the control. Samples heated at 200 °C showed the highest cumulative bacterial growth. Fungal growth was initially lower than in the control in all the heated samples (totally inhibited ≥ 200 °C). Fungal growth recovered slowly during incubation in soils heated at ≤ 300 °C, but the cumulative growth in heated soils did not exceed that in the control. No fungal growth was observed in samples heated at the two highest temperatures. Soil respiration was initially totally inhibited in soil heated at ≥ 200 °C, but recovered rapidly in all soils; the highest respiration being observed already 1 day after inoculation. This is the first time both fungal and bacterial growth has been directly estimated in heated soils. High soil pH favouring bacteria can explain these results, but the differences in fungal and bacterial responses suggest a competitive interaction between these groups.     

Assessing the tolerance to heavy metals of arbuscular mycorrhizal fungi isolated from sewage sludge-contaminated soils

Different fungal ecotypes were isolated from soils which had received long-term applications of metal-contaminated sewage sludge with the aim of studying the degree of tolerance and adaptation to heavy metals of arbuscular mycorrhizal (AM) fungi. The development and structural aspects of AM colonization produced by the different fungal isolates were studied using two host plants, Allium porrum and Sorghum bicolor, which were grown in either contaminated or non-contaminated soils. Four different AM fungi were successfully isolated from the experimental field plots: (i) Glomus claroideum, isolated from plots receiving only inorganic fertilizer; (ii) another apparently similar ecotype of Glomus claroideum, but isolated from plots with 300 m³ ha⁻¹ year⁻¹ of contaminated sludge added, (iii) an unidentified Glomus sp., present only in the less contaminated plots (100 m³ ha⁻¹ year⁻¹ of unamended sludge) and (iv) Glomus mosseae, isolated from plots receiving 100 or 300 m³ ha⁻¹ year⁻¹ of amended or unamended sludge (intermediate rates of contamination). There were consistent differences in behaviour among the four AM fungi tested with regard to the colonization levels they produced in non-contaminated and contaminated soils. Both total and arbuscular colonization were affected by heavy metal contamination. The main conclusions of this study are that Glomus sp. and G. mosseae isolates are strongly inhibited by heavy metals, which acted mainly by interfering with the growth of the external mycelium, and also by limiting the production of arbuscules. Our results suggest that G. claroideum isolates, particularly the ecotype which was isolated from the plots receiving the highest dose of metal-contaminated sludge, shows a potential adaptation to increased metal concentration in soil.     

Effects of soil frost on growth, composition and respiration of the soil microbial decomposer community

Most climate change scenarios predict that the variability of weather conditions will increase in coming decades. Hence, the frequency and intensity of freeze-thaw cycles in high-latitude regions are likely to increase, with concomitant effect on soil carbon biogeochemistry and associated microbial processes. To address this issue we sampled riparian soil from a Swedish boreal forest and applied treatments with variations in four factors related to soil freezing (temperature, treatment duration, soil water content and frequency of freeze-thaw cycles), at three levels in a laboratory experiment, using a Central Composite Face-centred (CCF) experimental design. We then measured bacterial (leucine incorporation) and fungal (acetate in ergosterol incorporation) growth, basal respiration, soil microbial phospholipid fatty acid (PLFA) composition, and concentration of dissolved organic carbon (DOC). Fungal growth was higher in soil exposed to freeze-thawing perturbations and freezing temperatures of −6 °C and −12 °C, than under more constant conditions (steady 0 °C). The opposite pattern was found for bacteria, resulting in an increasing fungal-to-bacterial growth ratio following more intensive winter conditions. Soil respiration increased with water content, decreased with treatment duration and appeared to mainly be driven by treatment-induced changes in the DOC concentration. There was a clear shift in the PLFA composition at 0 °C, compared with the two lower temperatures, with PLFA markers associated with fungi as well as a number of unsaturated PLFAs being relatively more common at 0 °C. Shifts in the PLFA pattern were consistent with those expected for phenotypic plasticity of the cell membrane to low temperatures. There were small declines in PLFA concentrations after freeze-thawing and with longer durations. However, the number of freeze-thaw events had no effect on the microbiological variables. The findings suggest that the higher frequency of freeze-thaw events predicted to follow the global warming will likely have a limited impact on soil microorganisms.     

Reforestation of burned stands: The effect of ectomycorrhizal fungi on Pinus pinaster establishment

The area occupied by Pinus pinaster in Portugal is rapidly diminishing because of forest fires. Ectomycorrhizal fungi form obligate, mutually beneficial associations with P. pinaster which improve plant growth and resistance to adverse conditions. The aim of this work was to assess whether native ectomycorrhizal fungi could be a useful tool in the reforestation of burned areas. The work was conducted in a forest nursery greenhouse, where P. pinaster seedlings were inoculated with compatible ectomycorrhizal fungal isolates: Suillus bovinus, Pisolithus tinctorius, Rhizopogon roseolus, and a mixture of the three fungi, using burned and unburned forest soil as substrate. Inoculation significantly enhanced the growth of P. pinaster, with R. roseolus proving to be the most effective in burned soil, with an 8-fold increase in plant fresh weight. Overall, inoculation stimulated growth most in burned than in unburned soil.This study suggests that inoculation with selected ectomycorrhizal fungi in containerised nurseries can be an advantageous approach for the successful establishment of P. pinaster in burned soil. The obtained results point out to the interest of extending these studies into fire-impacted areas, using ectomycorrhizal fungi as a biological tool.     

The presence of ash as an interference factor in the estimation of the maximum temperature reached in burned soils using near-infrared spectroscopy (NIR)

The aim of this work was to assess the effect of the presence of ash on maximum temperature reached (MTR) estimation using near infrared reflectance (NIR) spectroscopy. The degree of combustion (ash produced by heating to 100, 300, 500 and 700 °C), the type (ash from Pinus halepensis and Rosmarinus officinalis), and different quantities of ash (0–20% in 2% interval) were evaluated in a soil heated at seven different temperatures (100 °C–700 °C). Results showed that the estimation of MTR on samples with ash, using partial least squares (PLS) models constructed with samples without ash, could be erroneous. Both, ash quantity and degree of combustion affected the estimation of MTR. However, using discriminant analysis, a good classification of samples (> 97% correctly classified) according to the heating temperature classes (unheated, 100, 200, 300, 400, 500, 600 and 700 °C) was obtained despite the presence of ash.     

Temperature and stubble management influence microbial CO2-C evolution and gross N transformation rates

Few studies have examined the kinetics of gross nitrogen (N) mineralization, immobilization, and nitrification rates in soil at temperatures above 15 °C. In this study, ¹⁵N isotopic pool dilution was used to evaluate the influence of retaining standing crop residues after harvest versus burning crop residues on short-term gross N transformation rates at constant temperatures of 5, 10, 15, 20, 30, and 40 °C. Gross N mineralization rates calculated per unit soil organic carbon were between 1 and 7 times lower in stubble burnt treatments than in stubble retained treatments. In addition, significant declines in soil microbial biomass (P=0.05) and CO₂-C evolution (P<0.001) were associated with stubble burning. Immobilization rates were of similar magnitude to gross N mineralization rates in stubble retained and burnt treatments incubated between 5 and 20 °C, but demonstrated significant divergence from gross N mineralization rates at temperatures between 20 and 40 °C. Separation in the mineralization immobilization turnover (MIT) in soil at high temperatures was not due to a lack of available C substrate, as glucose-C was added to one treatment to test this assumption. Nitrification increased linearly with temperature (P<0.001) and dominated over immobilization for available ammonium in soil incubated at 5 °C, and above 20 °C indicating that nitrification is often the principal process controlling consumption in a semi-arid soil. These findings illustrate that the MIT at soil temperatures above 20 °C is not tightly coupled, and consequently that the potential for loss of N (as nitrate) is considerably greater due to increased nitrification.     

Influence of growth-promoting bacteria on the growth of wheat in different soils and temperatures

Plant-growth-promoting bacteria isolated from the rhizosphere, phyllosphere and soil of the root zone in different climatic regions of Germany and Uzbekistan were analysed for plant-growth-promoting effects and nutrient uptake on winter wheat on different soils and under different temperature regimes. The investigations were carried out in pot experiments using loamy sand and sandy loam soils from Müncheberg, Germany and Calcisol soil from Tashkent, Uzbekistan. The temperature and soil types were found to influence growth-promoting effects. Inoculation with bacterial strains Pseudomonas fluorescens PsIA12, Pantoea agglomerans 050309 and Mycobacterium sp. 44 isolated from Müncheberg (semi-continental climate) was found to significantly increase the root and shoot growth of winter wheat at 16 °C compared to 26 °C in loamy sand. Mycobacterium phlei MbP18 and Mycoplana bullata MpB46 isolated from Tashkent (semi-arid climate) were found to significantly increase the root and shoot growth of winter wheat in nutrient-poor Calcisol at 38 °C as well as in nutrient-rich loamy sand at 16 °C. Bacterial inoculation also resulted in significantly higher N, P, and K contents of plant components. The bacteria isolates were able to survive in the rhizosphere and in the soil of winter wheat after root and shoot inoculation.     

A modified method based on p-nitrophenol assay to quantify hydrolysis activities of lipases in litters

Lipases are glycerol ester hydrolases (EC 3.1.1.3) produced by a wide range of microorganisms. They catalyse the hydrolysis of different esters depending on the water content of the reaction medium. Here, we developed a simple methodology to quantify lipase hydrolysis activities using two different litters: a litter of Quercus pubescens (QP) and a litter of both Q. pubescens and Q. ilex. Different p-nitrophenyl esters were used to test hydrolysis in a reaction medium with an organic solvent (heptane). We showed that these activities depended on the amount of litter, the incubation time and the substrate concentration and that they increased with temperature. Furthermore, the lipases from the studied litters were still active after 2 h at 70 °C. These activities showed common properties of lipases: the highest activities were obtained with a medium-acyl chain substrate, p-nitrophenyl laurate. Moreover abiotic hydrolysis with short-chain acyl substrates was observable. The following parameters are recommended to quantify hydrolysis activities of lipases in litters: 10 mM of p-nitrophenyl laurate in 2 ml of heptane, 1 g of litter, 2 ml of water incubated at 30 °C for 2 h.     

Permafrost microbial community traits and functional diversity indicate low activity at in situ thaw temperatures

Previously-frozen stores of organic carbon (C) are now subject to decomposition due to a warming Arctic climate and associated permafrost thaw; however, estimates of the amount of greenhouse gases (GHG) that may be released are not well constrained. Knowing more about the functions of the extant permafrost microbial community will inform this knowledge gap. The exploration of microbial functional traits may be useful to elucidate the relationship between microbial diversity and ecosystem function. We characterized the community traits and functional diversity of the bacterial and Archaeal component of the microbial community from three depths of permafrost, as well as the organic and mineral horizons of the seasonally-thawed active layer, by assessing ‘substrate-use richness,’ ‘substrate preference,’ ‘growth rate,’ ‘and substrate specific growth rate.’ We measured the microbial community response to 31 substrates with an EcoPlate (Biolog, Inc.) assay at three incubation temperatures (1, 10, and 20 °C) using a kinetic approach, and modeled the microbial response to each substrate with a modified logistic growth function. We hypothesized that the permafrost communities would be selected for high functional potential and activity at cold temperatures. Rather, we found that the permafrost community did not have a higher functional diversity or activity at 1 °C than the organic active layer soils. In addition, permafrost communities increased their growth rates with increasing temperature, indicating that the highest incubation temperature (20 °C) was below their temperature optimum for growth. As predicted, the permafrost communities did exhibit temperature dependent substrate preferences. Thus, permafrost microbial communities did not appear to be selected for higher metabolism and the ability to use a broad suite of substrates at low temperatures, which suggests that they may have limited function immediately following thaw when temperatures are near 0 °C. However, changes in community composition or additional permafrost warming will increase the functional capabilities of permafrost microbes to decompose the C stored in those soils.     

Isotopic fractionation by saprotrophic microfungi: Effects of species, temperature and the age of colonies

Saprotrophic fungi represent an important resource for a number of fungivorous and omnivorous soil animals, but little is known about the patterns of isotopic fractionation by soil fungi. We grew five common species of saprotrophic microfungi in laboratory cultures on simple artificial substrate based on carbohydrates derived either from C3 or C4 plants. Fungal cultures were kept at 15, 20 or 25 °C. Isotopic composition of carbon (¹³C/¹²C) and nitrogen (¹⁵N/¹⁴N) in bulk fungal tissue was determined after 11, 21 and 32 days. The fractionation of carbon and nitrogen stable isotopes was species-specific, but generally did not differ in C3- and C4-based growth media. The Zygomycete Mucor plumbeus did not differ in δ¹³C from the carbon source used, though Ascomycetes (Alternaria alternata, Cladosporium cladosporioides, Trichoderma harzianum and Ulocladium botrytis) were depleted in heavy carbon relative to the carbon source by 0.5-0.9‰. Three species were significantly depleted in ¹⁵N relative to the sodium nitrate that was used as a single source of nitrogen. In all species, δ¹⁵N but not δ¹³C tended to increase with the age of fungal colonies. The effect of temperature on δ¹⁵N was weak and inconsistent in different species. In contrast, all fungi except T. harzianum accumulated more ¹³С at 25 °C than at 15 °C. The overall variation in the isotopic signatures of saprotrophic fungi growing in identical conditions reached 8‰ for δ¹⁵N and 2.5‰ for δ¹³C due to species-specific differences in the isotopic fractionation and the age of individual fungal colonies. This variation should be incorporated into the interpretation of the isotopic composition of fungivorous soil animals.     

Effect of temperature on the rate limiting step in the methanogenic degradation pathway in rice field soil

Temperature is an important factor controlling CH₄ production in rice field soils. However, it is unknown which step in the methanogenic degradation of organic matter is the limiting one that is controlled by temperature. Soil slurries prepared from Italian rice field soil were anaerobically incubated in the dark at six different temperatures between 10 and 37 °C until quasi-steady state was reached. Then, the potential and actual rates of polysaccharide hydrolysis and of CH₄ production from different immediate (acetate, H₂) and distal (glucose, propionate) methanogenic substrates were determined. Potential activities of exo-glucanase and glucosidase were always higher than the actual rates of polysaccharide hydrolysis indicating that the availability of the polysaccharide substrate was limiting at all temperatures. The actual rates of CH₄ production were always lower than those predicted from glucose release during polysaccharide hydrolysis indicating that a substantial amount of the released glucose was assimilated into microbial biomass. Addition of the different methanogenic substrates stimulated CH₄ production at all temperatures >10 °C, but only at >20 °C to values higher than rates of polysaccharide hydrolysis. Under steady state conditions, however, hydrolysis of organic polymers was the rate-limiting step at all temperatures >10 °C.     

‘Decomposer’ Basidiomycota in Arctic and Antarctic ecosystems

Current knowledge concerning ‘decomposer’ Basidiomycota in Arctic and Antarctic ecosystems is based on two sources: (a) collections and surveys of basidiomata, which have resulted in high-quality catalogues of species, although much of the species’ distribution and ecology are tentative and (b) isolations from soils and plant litter which typically result in a “low incidence of basidiomycetes” [Dowding, P., Widden, P., 1974. Some relations between fungi and their environment in tundra regions. In: Holding, A.J., Heal, O.W., MacLean Jr., S.F., Flanagan, P.W. (Eds.), Soil Organisms and Decomposition in Tundra. Tundra Biome Steering Committee, Stockholm, Sweden, pp. 123–150], probably because of selectivity in isolation methods. In the few molecular studies carried out in Arctic and Antarctic soils to date, basidiomycetes, particularly yeasts, have been found. These techniques should give better estimates of the order of magnitude of fungal species richness in Arctic and Antarctic soils, although caution should be used concerning primer choice and amplification conditions. From collections in Arctic regions, species of basidiomycetes appear to be circumpolar in distribution with restricted endemism. Using culture-independent methods, it should be possible to test whether selected Arctic or Antarctic species are truly cosmopolitan, circumpolar, endemic, or are cryptic phylogenetic species.Particularly in Arctic ecosystems, potential ‘decomposer’ fungi in soils and roots may be from phylogenetically diverse taxa, and currently it is unclear whether ‘decomposer’ basidiomycetes are the fungi undertaking the majority of organic matter decomposition in Arctic and Antarctic ecosystems. For example, in some recent studies, wood decomposition in cold Arctic and Antarctic sites appears to proceed via ‘soft rot’ by anamorphic ascomycetes (e.g. Cadophora species), rather than by ‘white rot’ or ‘brown rot’ basidiomycete species. Additionally, it appears basidiomycetes and ascomycetes as ericoid and ectomycorrhizal fungi have the potential to be involved directly in decomposition.Given that profound changes are likely to occur in patterns of vegetation (Arctic and Antarctic) and size of soil carbon (C) pools (particularly in the Arctic) by the end of this century, it is necessary to know more about which species of ‘decomposer’ basidiomycetes are present and to try to define their potentially pivotal roles in ecosystem C (and N) cycling. One solution to characterise further the identity and roles of these fungi in a logical way, is to standardise methods of detection and ‘function’ at networks of sites, including along latitudinal gradients. Results of functional tests should be related to community structure, at least for ‘key’ species.     

Analysis of aggregative behavior of Pseudomonas sp. 30-3 isolated from Antarctic soil

Pseudomonas sp. 30-3, a toluene degrading microorganism isolated from oil-contaminated Antarctic soils, was shown to form aggregated flocs of cells when exposed to temperatures of 22 and 4 °C, with an increase in aggregation at 4 °C. This was speculated to be due to the secretion of an extracellular polymeric substance (EPS), thus protecting the organism from cold or frost damage. The flocs of cells were stained with the Live/Dead BacLight Bacterial Viability kit and found to be viable cells. The EPS was identified by lectin binding analysis to consist of N-acetyl-d-glucosamine and N-acetylneuraminic acid. An enzyme-linked lectinosorbent assay was also carried out to quantify the amount of EPS produced at 37, 22 and 4 °C. Results showed that at 37 °C the amount of EPS secreted was low, but there was little difference in the amount of EPS secreted at 22 and 4 °C by Pseudomonas sp. 30-3.     

Actinomycetes of Moroccan habitats: Isolation and screening for antifungal activities

During a search for non-polyenic antifungal antibiotics, 320 actinomycete strains were isolated from several Moroccan habitats. Antibiotic productions of the isolates have been tested at various temperatures and production media. Thirty-two isolates showed strong activity against yeast, moulds and bacteria. The production of non-polyenic antifungal metabolites by active isolates was investigated using some of their biological activities: antibacterial activity, spheroplast regeneration and ergosterol inhibition. Ten active selected isolates were identified as belonging to the genus Streptomyces.     

Unfrozen water content moderates temperature dependence of sub-zero microbial respiration

Abrupt increases in the temperature sensitivity of soil respiration below 0 °C have been interpreted as a change in the dominance of other co-dependent environmental controls, such as the availability of liquid-state water. Yet the relationship between unfrozen water content and soil respiration at sub-zero temperatures has received little attention because of difficulties in measuring unfrozen water contents. Using a recently-developed semi-solid ²H NMR technique the unfrozen water content present in seasonally frozen boreal forest soils was quantified and related to biotic CO₂ efflux in laboratory microcosms maintained at temperatures between −0.5 and −8 °C. In both soils the unfrozen water content had an exponential relationship with temperature and was increased by addition of KCl solutions of defined osmotic potential. Approximately 13% unfrozen water was required to release the dependence of soil respiration on unfrozen water content. Depending on the osmotic potential of soil solution, this threshold unfrozen water content was associated with temperatures down to −6 °C; yet if temperature were the predictor of CO₂ efflux, then the abrupt increase in the temperature sensitivity of CO₂ efflux was associated with −2 °C, except in soils amended with −1500 kPa KCl which did not show any abrupt changes in temperature sensitivity. The KCl-amendments also had the effect of decreasing Q₁₀ values and activation energies (Ea) by factors of 100 and three, respectively, to values comparable with those for soil respiration in unfrozen soil. The disparity between the threshold temperatures and the reductions in Q₁₀ values and activation energies after KCl amendment indicates the significance of unfrozen water availability as an environmental control of equal importance to temperature acting on sub-zero soil respiration. However, this significance was diminished when soils were supplied with abundant labile C (sucrose) and the influences of other environmental controls, allied to the solubility and diffusion of respiratory substrates and gases, are considered to increase.