Changes in the Composition and Physiological and Biochemical Properties of Fungi during Passage through the Digestive Tract of Earthworms.

It was established that the biomass of fungal mycelium decreased by 30-50% during passage through the in- testine of the Aporrectodea caliginosa and Lumbricus terrestris earthworms, while its content in empty intes- tines was 40-60% less than in the soil. It was found that the amount of mycelium increases again in three-day-old coprolites due to the rapid growth of the species. It was demonstrated that the physiological activity of fungi (estimated according to the time of the appearance of colonies on the medium and probability of propagation) is lower in the intestine content, digestive tract, and fresh excrement of the worms than in the soil. It was noted that the activity and diversity of organic substrates (utilized by fungi), as well as proteolytic activity is lower in fungal isolates from the intestine than from the soil. It was registered that the death of a part of the fungi occurs in the worm intestine, while the physiological state changes in the animals withstanding the effect of the digestive medium.

[Sorption of humic acids by bacteria].

Capacity for sorption of humic acid (HA) from water solutions was shown for 38 bacterial strains. Isotherms of HA sorption were determined for the cells of 10 strains. The bonding strength between the cells and HA (k) and the terminal adsorption (Q(max)) determined from the Langmuir equation for gram-positive and gram-negative bacteria were reliably different. Gram-positive bacteria sorbed greater amounts of HA than gram-negative ones (Q(max) = 23 ± 10 and 5.6 ± 1.2 mg/m2, respectively). The bonding strength between HA and the cells was higher in gram-negative bacteria than in gram-positive: k = 9 ± 5 and 3.3 ± 1.1 mL/mg, respectively.

[Earthworms as modifiers of the structure and biological activity of humic acids].

Passage of humic acids (HAs) through the digestive tract of the earthworm, Eiseniafetida andrei, resulted in a decrease in molecular masses of the HAs. The effect of earthworm-modified HAs on individual bacteria and on bacterial communities as a whole is different from the effect of native HAs. Modified HA probably induces and regulates microbial successions in soils and composts in a different manner than the native HA, suppressing or stimulating different groups of microorganisms. These results may explain why the positive effects of commercial humates in real soil ecosystems, unlike model communities, attenuate rapidly.

[Response of bacteria to earthworm surface excreta].

Response of bacteria to the surface excreta of the Aporrectodea caliginosa earthworm was studied. The excreta were obtained by a 1 h incubation of the earthworms in petri dishes with subsequent collection of the slime. Both inhibition and stimulation of growth were revealed, as well as suppression of the respiratory activity of some bacterial species treated with A. caliginosa surface excreta. The organisms studied included various taxa of soil bacteria (19 strains), bacteria isolated from A. caliginosa intestine and excrements (82 strain), and 48 Bacillus thuringiensis strains. For the cultures of soil bacteria, the respiratory activity was determined using the formazan color reaction due to the activity of the respiratory cycle enzymes. Earthworm excreta caused a consistent 30-50% decrease of dehydrogenase activity in 13 out of the 19 cultures. Determination of the growth rates (derived from OD620 of cell suspensions) after 10 h of incubation revealed growth stimulation in 48 out of the 82 strains isolated from intestines and excrement. Other strains exhibited no reaction to the excreta. For 29 out of 45 B. thuringiensis strains, growth stimulation was observed, while growth of two strains was suppressed; other strains exhibited no reaction to the excreta. No relation was found between bacterial reaction to the excreta and their taxonomic position. These results correlate with the researches that demonstrated antibacterial and antifungal activity of the extracts from the earthworm body and digestive tract. Thus, earthworms, apart from their medium-forming function, affect the formation of soil microbial communities by direct stimulation or suppression of specific microbial populations.

[Reaction of microorganisms to the digestive fluid of the earthworms].

The reaction of soil bacteria and fungi to the digestive fluid of the earthworm Aporrectodea caliginosa was studied. The fluid was obtained by centrifugation of the native enzymes of the digestive tract. The inhibition of growth of certain bacteria, spores, and fungal hyphae under the effect of extracts from the anterior and middle sections of the digestive tract of A. caliginosa was discovered for the first time. In bacteria, microcolony formation was inhibited as early as 20-30 s after the application of the gut extracts, which may indicate the nonenzymatic nature of the effect. The digestive fluid exhibited the same microbicidal activity whether the earthworms were feeding on soil or sterile sand. This indicates that the microbicidal agents are formed within the earthworm's body, rather than by soil microorganisms. The effect of the digestive fluid from the anterior and middle divisions is selective in relation to different microorganisms. Of 42 strains of soil bacteria, seven were susceptible to the microbicidal action of the fluid (Alcaligenes.faecalis 345-1, Microbacterium sp. 423-1, Arthrobacter sp. 430-1, Bacillus megaterium 401-1, B. megaterium 413-1, Kluyvera ascorbata 301-1, Pseudomonas reactans 387-2). The remaining bacteria did not die in the digestive fluid. Of 13 micromycetes, the digestive fluid inhibited spore germination in Aspergillus terreus and Paecilomyces lilacinus and the growth of hyphae in Trichoderma harzianum and Penicillium decumbens. The digestive fluid stimulated spore germination in Alternaria alternata and the growth of hyphae in Penicillium chrysogenum. The reaction of the remaining micromycetes was neutral. The gut fluid from the posterior division of the abdominal tract did not possess microbicidal activity. No relation was found between the reaction of microorganisms to the effects of the digestive fluid and the taxonomic position of the microorganisms. The effects revealed are similar to those shown earlier for millipedes and wood lice in the following parameters: quick action of the digestive fluid on microorganisms, and the selectivity of the action on microorganisms revealed at the strain level. The selective effect of the digestive gut fluid of the earthworms on soil microorganisms is important for animal feeding, maintaining the homeostasis of the gut microbial community, and the formation of microbial communities in soils.

[Morphometric analysis of bacteria associated with soil Myriapoda].

Scanning electron microscopy revealed that the average cell size of bacteria associated with the digestive tract of soil Myriapoda was 0.65 microm in diameter, 1.36 microm in length, and 0.60 microm3 in volume. An example of Myriapoda illustrated that the intestinal tract bacteria of soil invertebrates shared the following features: (1) a high density level in this habitat; (2) existence mostly in the form of vegetative cells; (3) a cell size significantly smaller than that of bacteria functioning in soil; (4) a cell size closer to the lower limits of the size range characteristic for bacterial cultures grown in laboratory media. All this suggests that the bacterial community of the digestive tract differs from the typical soil community not only in composition, but also in a higher level of physiological activity.

[Interrelationships between yeast fungi and collembolans in soil].

The possibility of feeding on green and newly fallen leaves of the small-leaved lime Tilia cordata was studied for the collembolans Protaphorura armata and Vertagopus pseudocinereus. Young leaves grown under sterile conditions and almost free of yeast fungi were established to be toxic to the collembolan V. pseudocinereus: feeding on them led to the death of the animals. Leaves grown under natural conditions were nontoxic: when used by the collembolans as feed, they provided for collembolan growth and fecundity. Feeding preferences of the collembolans in relation to the yeasts attributed to different ecomorphs-epiphytes, litter saprophytes, pedobionts, and saccharobionts-were studied. Of the 24 yeast strains isolated from plant green parts, litter, and soil and assigned to eight species, no strain was revealed that was not used by the collembolans. However, certain yeast strains were preferable for the collembolans. The population of the V. pseudocinereus collembolans feeding on the yeast Rhodotorula glutinis (nss 31-4) exceeded that grown on Cryptococcus terricola (2044) 1.5-fold. Thus, the collembolans have feeding preferences in relation to yeast fungi, as was shown earlier for mycelial micromycetes. The possible mechanisms of the feeding preferences of the collembolans in relation to yeasts are discussed.

[Regulation of the biomass and activity of soil microorganisms by microfauna]. / Reguliatsiia mikrofaunoi biomassy i aktivnosti pochvennykh mikroorganizmov.

Microcosm experiments showed that the microbial biomass and the respiration activity in soil were regulated by nematodes. Depending on nematode number and plant residue composition, the trophic activity of nematodes can either stimulate or inhibit microbial growth and respiration as compared to soil containing no nematodes. The stimulating effect was observed when nitrogen-free (starch) or low-nitrogen (wheat straw, C:N = 87) organic substrates were applied. Inhibition occurred when a substrate rich in nitrogen (alfalfa meal, C:N = 28) was decomposed and the nematode population exceeded the naturally occurring level. A conceptual model was developed to describe trophic regulation by microfauna (nematodes) of the microbial productivity and respiration activity and decomposition of not readily decomposable organic matter in soil. The stimulating and inhibiting influence of microfauna on soil microorganisms was not a linear function of the rate of microbial consumption by nematodes. These effects are largely associated with the induced change in the physiological state of microorganisms rather than with the mobilization of biogenic elements from the decomposed microbial biomass.

[Structure of a primed microbial community as an integral method of evaluating the microbiologic state of soil]. / Struktura initsiirovannogo mikrobnogo soobshchestva kak integral'nyi metod otsenki mikrobiologicheskogo sostoianiia pochvy.

The paper describes a laboratory method of initiated microbial cenosis which makes it possible to evaluate the microbiological state of soil and to predict its change under the action of various anthropogenic factors. To this end, the structure and characteristics of a microbial cenosis initiated by a substrate are studied using a scanning electron microscope in combination with classical techniques of microbiology: the actual predominance and proportions of individual microbial groups (bacteria, actinomycetes, fungi, algae, protozoa and microscopic invertebrates); the interaction and interrelation between individual microbial populations in the cenosis; the biological properties of predominating microorganisms; the ratio between active and resting forms; the rate and character of growth of individual microbial populations in the cenosis and their succession. All these indices taken together may serve as a criterion for integral evaluation of the effect produced by various physico-chemical factors on the microbiological state of soil.