60 Mic~Jbial adaption to extreme env!ronments

will finish with the environmental extremes existing when life started to evolve and the cycle of trace elements and compounds in the

air. But we will start down to earth with a quite common environment , soil.

SOIL: A COMPLEX ENVIRONMENT

F.A. SKINNER

Many micro-organisms exist in soil, but they constitute only a very small part of the total soil volume. In non-rhizosphere soil, most bacteria occur as small cocci attached to particles and fungi as a sparse network of hyphae or as spores.

In soil-agar films, as used for the direct counting of micro-organisins in soil, bacteria are distributed in aggregates or colonies ac- cording to the terms of a negative logarithmic series. The probability of finding colonies with 1, 2, 3 cells etc. is given by the successive terms x, x2/2, x3/3 etc, where x is less than 1 and frequently equals 0.7-0.8. Thus, though single celis and micro-colonies are abundant, there is little chance of finding a colony as large as, say, one with 100 cells. This distribu- tion has been compared with that of colrpnies of soil bacteria growing in plates of a mineral salts agar with no added carbon source, in~~u- lated with soil dilutions of 1 O- 3 and 10 - Q. These nutrient conditions are poor compared with those normally obtaining in laboratory culture. After 7 days the agar was stained, examined microscopically and the volumes of colonies estimated. The most abundant of the microscopic colonies (ca 23%) were those of IO-20 ,prn diameter with, volumes of ca 500 and 4WQ (J,@~ respectively. As in soil-agar films, the frequency of occurrence of colonies decreased with increrqging colony size but t scale of dimension was quite ifferent.

Though bacterial growth was so poor in the agar plates it could still be considered as luxu- riant compared with that of the bacteria in soil-agar films. This, and other evidence de- rived from soil sections and direct observa- tions of soil particles, indicates that soil mi- cro-organisms grow under severe constraint determined by lack of nutrients, presence of inhibitors, or by unfavourable physical condi- tions.

Many soil microbes, especially species of Streptomyces, produce antibiotics in rich me- dia but little is likely to be formed in un- amended soils where nutrients, though varied, are scarce and subjects of competition. More- over, many antibiotics are adsorbed by clays, especially montmorillonites, and by organic matter. Dramatic antibiotic effects are unlike- ly in soils but an accumulation of different inhibitors is probable.

special case of microbial inhibition in soil is fungistasis which may be defined as the fail- ure of fungal spores to germinate in contact with soil even when conditions of tempera- ture and moisture favour germination. Fungis- tasis is widespread, tends to be stronger in warmer seasons and is more marked in surface than in deeper soil layers. It is probably mi- crobial in origin because

Microbialudnption to extreme environments 61

nutrients or by presence of inhibitors: proba- bly both mechanisms operate.

Though micro-organisms grow poorly in soil they survive well there; survival may well be linked with the properties of soil colloids, especially clays, which absorb cells, metabo- lites and water, and affect cell respiration. Microbes remain dormant as spores or as pro- tected vegetative cells, or grow just as fast as the limited supplies of nutrients and water, and the temperature permit.

The soil environment, which (for temper- ate, arable soils) is porous, aerated, moist, cool, of even temperature and contains a wide variety of substrates, is not a good medium for micro-organisms. It should, perhaps, be re- garded as a system in which a very varied inoculum is maintained, ready to decompose the many different substrates that continually become incorporated in it.

W. NEINEN

For the interaction of microorganisms with non-physiological elements in their natural or artificial environment, we can distinguish four different levels. Both the selectivity and the involvement of special enzymes increase from the first to the fourth level.

The simplest kind of interaction is the transfer #j,f elements in and out of the cell. During this “neui z il interaction” the elements mainly act as charge carriers to maintain elec- troneutrality. Many of the elements taken up function within the cell as cofactors for cer- tam enzymes. If an element is accumulated against a concentration gradient in an energy- requiring process, it can later be exchanged for others in an ion-exchange reaction that has no energy requirement. Some elements are accumulated in large amounts, and are of

The second level is that of “charge-transfer interaction”. In these reactions the elements serve as electron-donors or -acceptors at some place of the respiratory chain. The number of elements that can partake in such reactions ranges from the well-known N- and S-com-

unds to OS-, Se-, Te-, and ny of these elements can

reciuced to their zero valency state and depos- ited in this form either inside or outside the cell. An important side-effect is the alteration of the solubility as a result of charge transi- tion, which can have a drastic effect on the

bility of that particular element. an “incorporative interaction” the ele-

ment is built into a small cellular compound, and here we can distinguish between two sub-

n the first case the unfa