Bengt Söderström

The use of phospholipid and neutral lipid fatty acids to estimate biomass of arbuscular mycorrhizal fungi in soil

Cucumber seedlings associated with the arbuscular mycorrhizal (AM) fungi Glomus WUM10 or G. caledanium (BEG 15) were grown in PVC tubes with a lateral root-free compartment and an identical compartment containing both hyphae and roots. The amounts of specific fatty acids, in the neutral lipid and phospholipid fractions, were measured in both compartments and compared with controls without mycorrhiza. The phospholipid fatty acids (PLFAs) 16:1 omega 5, 18:1 omega 7c, 20:4 and 20:5 were present in higher amounts in soil with mycorrhizal hyphae than in soil without mycorrhizal hyphae. The largest relative difference was found in 20:5, but a good correlation existed between 16:1 omega 5 and 20:5 in soil with hyphae. Amounts of these fatty acids were correlated both with length of mycorrhizal hyphae and with amounts of ATP in soil. Conversion factors to calculate hyphal length and AM fungal biomass carbon using the phospholipid fatty acids could thus be estimated; 38 nmol PLFA 16:1 omega 5 mg(-1) AM fungal biomass C (Glomus WUM10) and 22 nmol PLFA 20:5 mg(-1) biomass C. The fatty acid 16:1 omega 5 from the neutral lipid fraction, containing triglycerides, dominated in soils with mycorrhizal hyphae. The amount of 16:1 omega 5 in the neutral lipid fraction decreased during storage of soils, indicating a decrease in storage lipids, while the proportion of 16:1 omega 5 in the phospholipid fraction was almost unaffected. (Less)

Vital staining of fungi in pure cultures and in soil with fluorescein diacetate

Abstract A method for vital staining of fungal mycelium with fluorescein diacetate (FDA) is described. In experiments with pure cultures, good correlation was obtained between relative staining efficiency, growth rate, and respiration. FDA thus appears to be a true vital stain, in that it stains only metabolically active mycelia. In fresh soil suspensions stained with FDA, brightly fluorescent hyphae and portions of hyphae were observed. The applicability of the method for measurement of active mycelium in the soil is discussed.

Fungal biomass and nitrogen in decomposing scots pine needle litter

The development of fungal biomass and increase of amounts of N was studied in decomposing pine needle litter for about 3 yr. After a relatively rapid increase of the amount of mycelium the fungal biomass became rather constant after about 2 yr. The absolute amount of N in the needles increased between the 4th and the 16th months and this increase was correlated to the increase of fungal biomass in the needles.

PLS DISCRIMINANT PLOTS

A projection method for multivariate classification problems is presented, which is based on partial least squares in latent variables (PLS). PLS models the dependency between two multivariate blocks X and Y. For a pattern recognition problem we use a dummy matrix Y which describes the class-membership of the training-set objects. The multivariate characterization of the objects forms the X block. The PLS model describes the X-block with a principal component (PC) like model, where class separation is enhanced. The significance of the class separation is tested by cross-validation. Plotting the score vectors of the PLS model gives a PLS discriminant plot.

Evaluation of soil respiration characteristics to assess heavy-metal effects on soil-microorganisms using glutamic-acid as a substrate

Computerized continuous monitoring of soil respiration rates before and during glutamic acid decomposition in heavy metal polluted soils was used to determine four microbial parameters: basal respiration rate, substrate induced respiration rate, lag time before the exponential increase of the soil respiration rate and the specific respiration increment during the exponential phase. Both smelter- and laboratory-contaminated soils were studied. Basal respiration rate was the parameter most inhibited (54–77%) by heavy metal contamination. Increased soil moisture resulted in increased basal respiration rate, irrespective of pollution level. The substrate-induced respiration rate after the addition of glutamic acid was strongly correlated with the basal respiration rate (r = 0.85−0.95). The change in specific respiration increment was not related to metal contamination but increased with increasing soil moisture, with an optimum at about 250% H2O based on soil organic matter (oven-dried). Lag time was the parameter best correlated with smelter-induced metal contamination (r = 0.64 and 0.75). Unlike the three other parameters, the lag time was unaffected by soil moisture, irrespective of contamination level. (Less)

Reduction of decomposition rates of Scots pine needle litter due to heavy-metal pollution

Decomposition of unpolluted Scots pine needle litter was studied in two heavy-metal-pollution gradients in Sweden; one near a brass mill and the other around a primary smelter. In the latter area locally collected polluted Scots pine needle litter was also incubated. Decomposition rates were strongly influenced by the metal pollution and a decrease in the rate of mass-loss occurred. In the brass-mill gradient this occurred until about 1 km from the pollution source which corresponded to about 500 µg Cu and 1 000 µg Zn g−1 soil. Data are presented to indicate that lignin decomposition was more sensitive to pollution than decomposition of whole litter and affected further away from the pollution sources. At the smelter sites, the metal-polluted needle litter decomposed more slowly than the unpolluted needle litter, and this difference was enhanced close to the smelter. The results indicate that heavy metals accumulated in needles prior to shedding have a long-term impact on the subsequent decomposition of the litter. Both litter quality and soil factors thus contribute to the reduced litter decomposition rate in metal-polluted forests. A new non-linear model with decreasing decay rate was used in the statistical evaluation. The model can be used to characterize the effects of pollution on decomposition rate.

Seasonal fluctuations of active fungal biomass in horizons of a podzolized pine-forest soil in central Sweden

Metabolically-active fungal biomass determined by fluorescein diacetate (FDA) staining, was studied during a 27-month period in three horizons of a podzolized pine-forest soil. Recurrent biomass peaks were registered in autumn and early spring. Biomass increase was also noted during the winter with soil temperatures below 0 deg C. Only a minor fraction (2.4-4.3%) of the total fungal biomass was active. The FDA-active biomass was equally distributed between the organic (5 cm) and mineral (15 cm) soil horizons, and varied between 0.5 and 2.4 g d.w. m-2. The amount of FDA-active biomass was correlated with soil moisture content.

Effect of nitrogen and carbon supply on the development of soil organism populations and pine seedlings - microcosm experiment

Series of pots containing Scots pine seedlings and a humus-sand mixture were watered twice weekly for 398 d with different nutrient solutions (control treatment with complete plant nutrient solution less nitrogen; glucose addition; nitrogen addition; glucose and nitrogen addition). Analyses were made of organic and nitrogen contents in the substrate, plant weights and nitrogen contents, nitrogen fixation and respiration rates, abundance, biomass and in some cases species composition of different soil organism populations. The growth rate of pine seedlings was lowest in series supplied with glucose, which is most easily explained by a deficiency of nitrogen due to microbial immobilization. The fungi and yeast were stimulated by addition of an easily available carbon source whereas the bacteria needed both nitrogen and carbon to maintain high biomass. A positive correlation between fungal feeding soil organisms and amount of fungal mycelium was found while a more complex situation prevailed with regard to bacterial numbers and bacterial feeding nematodes. The systems are evaluated in relation to the different treatments and compared with the field situation. (Less)

Respiratory activity of intact and excised ectomycorrhizal mycelial systems growing in unsterilized soil

Ectomycorrhizal mycelial systems were grown in unsterile soil using specially designed growth chambers, and the respiratory activity of the mycelium was measured before and after its excision from the host roots. Approximately 30% of the total respiration in the intact system was attributable to the mycorrhizal mycelium in the soil. Severance of mycelial connections at the roots lead to a greater than 50% decrease in respiration rate within 24 h. This decline was much more rapid than that of root respiration rate which occurred when the shoot was excised from the intact system. The implications of these findings for the carbon balance and microbial ecology of forest systems are discussed.

Transcriptional responses of Paxillus involutus and Betula pendula during formation of ectomycorrhizal root tissue

In order to obtain information on genes specifically expressed in the ectomycorrhizal symbiosis, 3,555 expressed sequence tags (ESTs) were analyzed from a cDNA library constructed from ectomycorrhiza formed between the basidiomycete Paxillus involutus and birch (Betula pendula). cDNA libraries from saprophytically growing fungus (3,964 ESTs) and from axenic plants (2,532 ESTs) were analyzed in parallel. By clustering all the EST obtained, a nonredundant set of 2,284 unique transcripts of either fungal or plant origin were identified. The expression pattern of these genes was analyzed using cDNA microarrays. The analyses showed that the plant and fungus responded to the symbiosis by altering the expression levels of a number of enzymes involved in carbon metabolism. Several plant transcripts with sequence similarities to genes encoding enzymes in the tricarboxylic cycle and electron transport chain were down regulated as compared with the levels in free-living roots. In the fungal partner, a number of genes encoding enzymes in the lipid and secondary metabolism were down regulated in mycorrhiza as compared with the saprophytically growing mycelium. A substantial number of the ESTs analyzed displayed significant sequence similarities to proteins involved in biotic stress responses, but only a few of them showed differential expression in the mycorrhizal tissue, including plant and fungal metallothioneins and a plant defensin homologue. Several of the genes that were differentially expressed in the mycorrhizal root tissue displayed sequence similarity to genes that are known to regulate growth and development of plant roots and fungal hyphae, including transcription factors and Rho-like GTPases.