During the past few years, an increasing awareness concerning the emission of an unexpected high number of bacterial volatiles has been registered. Humans sense, intensively and continuously, microbial volatiles that are released during food transformation and fermentation, e.g., the aroma of wine and cheese. Recent investigations have clearly demonstrated that bacteria also employ their volatiles during interactions with other organisms in order to influence populations and communities. This review summarizes the presently known bioactive compounds and lists the wide panoply of effects possessed by organisms such as fungi, plants, animals, and bacteria. Because bacteria often emit highly complex volatile mixtures, the determination of biologically relevant volatiles remains in its infancy. Part of the future goal is to unravel the structure of these volatiles and their biosynthesis. Nevertheless, bacterial volatiles represent a source for new natural compounds that are interesting for man, since they can be used, for example, to improve human health or to increase the productivity of agricultural products.

Bacterial volatiles and their action potential

Soil is undoubtedly the most complex of all microbial habitats. Primarily because of this complexity, there is insufficient information on how and where most microbial events occur in soil in situ and which microbes are the major and, physiologically, the most important participants in these events. Soil differs from most other microbial habitats in that it is dominated by a solid phase consisting of particulates of different sizes and which is surrounded by aqueous and gaseous phases that fluctuate markedly in time and space. The solid phase is a tripartite system composed of finely divided minerals (both primary and secondary); plant, animal, and microbial residues in various stages of decay; and a living and metabolizing microbiota. These particulates exist as both independent entities and mixed conglomerates. The aqueous phase surrounding the particulates is normally discontinuous, except when soil is saturated, and this restricts the movement of microbes, especially of bacteria and other nonfilamentous forms, and results in local accumulations of nutrients and toxicants, escape of cells from grazing predators, a low probability for genetic transfer, etc. These particulate-aqueous associations constitute the "microhabitats" wherein microbes reside and function in soil. The abiotic components of soil have been relatively well defined, both qualitatively and quantitatively. However, the microgeographic distribution and the geometric relations of abiotic components to each other-and to the microbiotic components-and the interactions among and between the abiotic and microbiotic components are not clearly defined. Most of what is known about the composition of the abiotic components has been obtained by dispersing soil-either chemically or physi-

Influence of Soil Mineral Colloids on Metabolic Processes, Growth, Adhesion, and Ecology of Microbes and Viruses

Volatile compounds are usually associated with an appearance/presence in the atmosphere. Recent advances, however, indicated that the soil is a huge reservoir and source of biogenic volatile organic compounds (bVOCs), which are formed from decomposing litter and dead organic material or are synthesized by underground living organism or organs and tissues of plants. This review summarizes the scarce available data on the exchange of VOCs between soil and atmosphere and the features of the soil and particle structure allowing diffusion of volatiles in the soil, which is the prerequisite for biological VOC-based interactions. In fact, soil may function either as a sink or as a source of bVOCs. Soil VOC emissions to the atmosphere are often 1-2 (0-3) orders of magnitude lower than those from aboveground vegetation. Microorganisms and the plant root system are the major sources for bVOCs. The current methodology to detect belowground volatiles is described as well as the metabolic capabilities resulting in the wealth of microbial and root VOC emissions. Furthermore, VOC profiles are discussed as non-destructive fingerprints for the detection of organisms. In the last chapter, belowground volatile-based bi- and multi-trophic interactions between microorganisms, plants and invertebrates in the soil are discussed.

https://onlinelibrary.wiley.com/doi/pdf/10.1111/pce.12340

Biogenic volatile emissions from the soil

(1972). Activity, Ecology, and Population Dynamics of Microorganisms in Soil. CRC Critical Reviews in Microbiology: Vol. 2, No. 1, pp. 59-137.

Activity, ecology, and population dynamics of microorganisms in soil.

SUMMARY

1. Fungistasis in soil is a widespread phenomenon affecting most fungal propagules, though some are insensitive. In most instances, it is coexistent with the presence of living microorganisms, and is annulled by energy-yielding nutrients. Fungistasis with characteristics similar to that in soil may also occur on leaves of plants.



2. Germination and growth of bacteria and actinomycetes is also restricted in soils. The characteristics of their inhibition appear to be the same as those for fungi. Therefore, the concept of a widespread microbial inhibition in soil can be applied to all three groups of microorganisms.



3. Fungistasis can be detected by various direct methods, or indirectly by methods involving the use of porous or permeable carriers. It may be expressed as a restriction on the final amount of germination (the usual parameter), germination rate (with time), and rate of germ-tube or hyphal growth. Since the expression of fungistasis is often complete in soil, titration with nutrients may be required to distinguish between the sensitivities of different fungi.



4. Fungistasis generally is expressed most strongly at soil moisture contents somewhat less than saturation. Its expression usually is maximal in neutral or slightly alkaline soils. In acidic conditions fungistasis may be lessened because of suppression of bacterial and actinomycete activity. Increased sensitivity of some fungi in soils of pH > 7.0 may be caused by a directly unfavourable effect of pH on the fungus.



5. Fungal species with small spores tend to be highly sensitive to fungistasis. These spores tend to germinate slowly and to require exogenous nutrients for germination. By contrast, species with larger spores and sclerotia often do not require exogenous nutrients for germination. The larger spores tend to germinate rapidly and to exhibit low sensitivity, as compared with small spores. A few nutrient-independent spores are insensitive to fungistasis. At least a part of the difference in sensitivity is related to germination time; spores which germinate slowly compete poorly with the soil micro-flora for their nutrients.



6. Fungistasis is often temporarily annulled by enriching the soil with energy-yielding nutrients. Usually, complex materials such as plant residues are most effective. A few weeks after such treatment, the level of fungistasis may, however, be increased. Annulment of fungistasis by compounds not utilized as energy sources has not yet been demonstrated.



7. Several soils naturally suppressive to Fusarium wilt diseases were more fungistatic to Fusarium than soils conducive to wilt. Potential means by which fungistasis may be manipulated to control root-infecting fungi are (a) through stimulation of germination with nutrients, thus exposing the germ tube to lysis, and (b) by increasing the fungistatic level of soil through appropriate amendments.



8. Volatile substances identified in soils, some of which are potentially inhibitory to fungi include (a) ammonia, which apparently is evolved from ammonium salts in some arid soils of high pH, (b) ethylene, which has been identified in some soils of pH < 7.0 (though high levels of this gas seem to be tolerated by most fungi), (c) allyl alcohol, and (d) other unidentified substances. Non-volatile inhibitors include high molecular weight substances revealed by molecular sieve chromatography of soil extracts. Microbial metabolites such as those present in staled fungal cultures also have been proposed to account for fungistasis. In a few soils fungistasis persists after sterilization because of the presence of inhibitory concentrations of calcium carbonate, iron or aluminium. Inherent in the proposition that inhibitory substances provide the primary mechanism of fungistasis is the concept of a highly complex phenomenon, involving various highly specific inhibitory and counteracting stimulatory substances, with the outcome for the fungus depending on the kinds and relative amounts of each present.



9. By the nutrient-deficiency hypothesis, the level of available nutrients in soil is insufficient to support germination of nutrient-dependent propagules, except in nutrient-rich microsites. Inhibition of nutrient-independent propagules is explained by loss of endogenous nutrients required for germination, through microbial nutrient competition. Evidence for this hypothesis is (a) the imposition of fungistasis on numerous nutrient-independent propagules during incubation on leaching model systems designed to simulate microbial nutrient competition in soil, (b) similar losses of endogenous nutrients occurring on soil and the leaching system, and (c) the fact that soils are chronically deficient in energy in relation to the microbial populations present, with the consequence that enforced inactivity is imposed upon most of the population at any given time for this reason alone, regardless of the presence or absence of fungistatic substances.



Journal series article no. 7747 from the Michigan Agricultural Experiment Station.

Fungistasis in soils

Volatile metabolites produced by eight bacterial species greatly inhibited growth and sporulation of all fungi tested, namely Fusarium oxysporum f. conglutinans, Gelasinospora cerealis, Penicillium...

Inhibition of fungal growth and sporulation by volatile metabolites from bacteria

Fusarium oxysporum f. conglutinans, Gelasinospora cerealis, Penicillium viridicatum, Trichoderma viride, and Zygorhynchus vuilleminii were grown on slide cultures supported above cultures of seven ...

Elizabeth Moore-Landecker

Papers

Inhibition of fungal growth and sporulation by volatile metabolites from bacteria

Effects of concentration of volatile metabolites from bacteria and germinating seeds on fungi in the presence of selective absorbents.