Environmental Factors and Microbial Growth
We have already considered the growth of microorganisms. It can be seen that this growth is very rapid and the high growth rates require suitable control of nutrients and physical and chemical conditions of the environment. The physicochemical factors of the natural environment determine the rates of microbial growth and the nature and size of the indigenous population.
1. Temperature. Environmental temperature is one of the most important factors affecting the growth rate of microbes. There is a minimum temperature, below which growth does not occur. As we rise above the minimum, the rate of growth increases in accordance with the laws governing the effect of temperature on the chemical reactions that make up growth.
These reactions are mostly enzyme-catalyzed. However, a point is reached at the optimum temperature when there is also a very rapid increase in the rate of inactivation of heat-sensitive cell components, like enzymes, ribosomes, DNA, membranes, etc. Above an optimum temperature, this heat denaturation will occur so rapidly that there is a corresponding rapid drop in the rate of growth to give a maximum temperature for growth for that particular microorganism.
Most microbes are capable of growth in a temperature range of 20-30° C. Most microorganisms have a growth optimum between 20 and 40° C and are called mesophilic. Those inhabiting cold environments such as polar areas can grow at much lower temperatures. There is a rich microbial growth on the surface of glaciers, where they may also cause visible red or green color. Such microbes are called psychrophilic, which may cause problems in food storage in refrigerators over longer periods. (Growth & Distribution of Microorganisms Study Material)
There are also thermophiles, that are able to inhabit environments such as compost heaps or hot springs. They can grow at temperatures as high as 80-100° C when the vast majority of live organisms die. It appears that microorganisms can grow at any temperature as long as the water is in a liquid state. How do they do this? Perhaps, by increased stability of most cell components coupled with active repair mechanisms of heat-denatured components. Growth should not be confused with survival.
2. pH. Microbes grow at a wide pH range. Most microbes grow best at pH near neutrality, bacteria usually slightly on the alkaline side and algae and fungi on the acid side. However, some can grow at extreme values of low or high environmental pH. The inability of most bacteria to grow below a pH value of 3-4 is utilized in the food industry where pickling is a common method of preservation. Acetic acid as vinegar added to food or bacteria themselves due to fermentation may lower the pH.
3. Oxygen and redox potential. The presence or absence of oxygen divides the organisms into the following three main classes. The chief criterion of this grouping is the nature of the energy-producing systems: (1) the microbes which require O2 as a terminal electron acceptor for oxidation, and if this is the only means of energy production, the organism will be a strict aerobe. (ii) If, in addition, a microbe can obtain energy in the absence of O2, it will be a facultative anaerobe.
Here growth is usually more abundant in presence of O2 than in its absence. (iii) strict anaerobes have an energy-producing system that does not require O2, and in addition, they are actually poisoned by oxygen.
The aerobic or anaerobic nature of a microbe is related to the normal natural environment of that organism. Thus, methanogenic (methane-producing) microbes are strict anaerobes and live in an environment such as the lower gut of animals, swamps, sewage, lake sediments, etc. Methanogenic bacteria belong to the genera Methano-bacterium, Methanococcus, Methanosarcina, and Methanospirillum.
On the other hand, methane-utilizing bacteria are strict aerobes, and must occur in environments where oxygen is available from the air but also supplemented by methane as the result of the action of methane-producing bacteria in related anaerobic areas. As a result in the stratified lake, we find methane producers in the anaerobic sediments, whilst methane utilisers are concentrated at the interface between the aerobic and anaerobic layers where methane is diffusing upwards and oxygen is diffusing downwards.
Methane-utilisers are the species of Methylosinus, Methylocystis, Methylomonas, Methylobacter, and Methylococcus. Another bacterium, Hyphomicrobium cannot utilize CH4 as a carbon source but can utilize other compounds containing one or more methyl groups. These bacteria, whose carbon and energy sources are methane or other compounds having one or more methyl groups are known as methylotrophs.
4. Osmotic pressure. Most microbes grow within a wide range of environmental osmotic pressure. Their ability to survive O.P. lower than those of cytoplasm is generally related to the tough cell wall or to the contractile vacuole (water-excreting mechanisms). Some microbes are able to tolerate higher levels of O.P. and are known as osmophiles or halophiles. They occur in salt lakes, salt pans, and oceans. Most microbes are unable to grow at such high O.P. and this fact is used in the preservation of food with salt or sugar.
5. Hydrostatic pressure. The only natural environment with high hydrostatic pressure is the depth of oceans. Here pressure may be as 1000 times that on the surface. The microbes present in such depths are called barophiles.
6. Radiation. Most microbes are killed by high doses of electromagnetic radiation, particularly in the UV range, and by smaller doses of ionizing radiation. Visible light is essential for photosynthetic forms.
From the above account, two overall groups of principles can be clearly seen.
(1) A microbial species usually has a fairly wide range of environmental conditions in which it will grow. This range is generally higher in prokaryotes than eukaryotes, as is in microbes in general when compared with cells of higher plants and animals. The amplitude of this range may reflect either a less sensitive cellular mechanism, or a capacity for controlling the cell wall composition in presence of environmental extremes, in other words, a capacity for homeostasis. Both possibilities can be considered.
(2) Microbial world as a whole possesses an extraordinary capability to occupy extreme environmental niches. Again, this is much marked in prokaryotes. Thus in the Dead Sea or near a hot spring, there will be exclusively prokaryotes. There is not much competition for available nutrients. Moreover, the simpler structure of the prokaryotic cell is more suited for evolutionary adaptation.
Eukaryotic microbes are more suited to evolutionary change in direction of multicellular differentiated life forms. There are thus very few places on the earth’s surface where Physico-chemical conditions prevent microbial growth.
As a group, microbes have tremendous metabolic versatility and are capable of degrading any natural organic material, and also most synthetic compounds. They have the potential to sequester nutrients present at very low concentrations. In addition, endospores of bacteria are the most resistant biological structures known and spores of fungi and actinomycetes are the most highly evolved dispersal structures.
Distribution of Microorganisms
The environment may be looked upon as a vast depository of microbes interacting with the environment, with each other, and with higher plants and animals. We would briefly examine the three specific environments for microbial cells and populations in particular.
[1] Atmosphere
This is the simplest, as it is relatively uniform and constant in composition. There is a lack of nutrients, water, etc., thus its major role for microbes is as a medium for dispersal. There is a decrease in the number of microbes with increasing altitude. Most are present as spores (mostly fungal).
Microbiology of the atmosphere has the following three main applications for man.
(1) in hospitals, bed-making and floor-sweeping may create aerosols of pathogens, giving rise to variation in the size and species composition of the airborne population. (Growth & Distribution of Microorganisms Notes Study Material)
(2) germ warfare, where the aim is to maximize spore viability and infectivity and to control when and where the cells come to the ground.
(3) spread of plant pathogens, most being fungal spores.
In all the above cases it is important to know the mechanisms creating aerosols, factors that cause them to settle, and duration periods over which airborne organisms remain viable and infective.
[II] Aquatic environments
These are the major site for microbial growth, with more than 70% of the earth’s surface covered by oceans. Growth is generally slow, because of low temperature (90% of the world’s seawater is always below 4° C) and low concentrations of organic matter. The oceans contain 0.4–10 mg organic carbon/ml, suspended microbes, and 10-1000/ml. Microbes are said to be oligo-trophic.
Photosynthetic forms are stratified according to their wavelength requirements. Some minerals, particularly nitrate, phosphate, and sulphate often limit the growth of algae in the water. Various factors cause an increased addition of such nutrients to inland waters. This process of enrichment is called eutrophication and it may be caused by sewage, industrial waste, or by drainage from agricultural land which is being intensively farmed. Due to eutrophication, there develop algal blooms.
[III] Soil
Soil represents the most varied and heterogeneous environment for microbes. It has solid, liquid as well as gaseous phases. The solid phase has complexes of clay minerals and organic matter. The remaining volume (pore space) is filled with water (containing soluble organic and inorganic material), and the soil atmosphere, which is saturated with water, with less O2 and more CO2 than the atmosphere above the soil. The environment of a microbial cell in the soil is in fact a microhabitat, where conditions may be much different from those in the bulk soil.
For instance, a cell growing on a piece of decaying root in soil may have an excess of carbon and nitrogen while the soil as a whole may be nutrient deficient. Different conditions may prevail in microhabitats. For example, aerobes may grow on the outside of soil crumb or microbial film whereas anaerobic processes occur in the center. (Growth & Distribution of Microorganisms Notes Study Material)
The number of microbes decreases with soil depth. About 1 g of garden soil will contain 107 bacteria and 5 meters of fungal hyphae. About 1 hectare of a typical agricultural field contains almost 6,000 kg wet weight of microbial biomass, which is the weight of 80 sheep. What is the significance of this microbial biomass? What form of this biomass? Cells, spores, hyphae, alive, dead, active, or dormant? Most soil activity comes from enzymes, such as ureases, phosphatases, and dehydrogenases. They are important in degradation.
Biogeochemical Cycling of Elements and Matter
The main building elements of organisms are carbon, hydrogen, oxygen, nitrogen, sulphur, and phosphorus. The growth of an organism involves the conversion of these elements present in an inorganic form to the organic compound that make up the living matter. The energy for this elemental conversion is ultimately derived from solar sources of photosynthesis.
If this were the only process, life would soon cease as the inorganic forms of elements, particularly carbon and nitrogen would be locked into organic matter. In fact, the reverse process-mineralization must also occur and is brought about by the activity of living organisms so that cycles of elements and matter occur.
The situation is complicated by the existence in nature of oxidized and reduced states of most of the essential elements: organisms may only be able to use one or another form and further cycles, therefore, exist between them.
The microbes interconvert these compounds for their own growth and each process is energy-consuming. While the different elements are cycled, energy flows through the ecosystem and is ultimately lost as heat. Let us consider in some detail how the major elements undergo cyclic changes in nature.
Carbon cycle
Carbon dioxide, either in the atmosphere or in solution in surface waters is the major source of carbon for living organisms. It is converted to organic form by the autotrophs which use it as the sole carbon source. The most important in this conversion is the photosynthetic autotrophs (seed plants on land and algae in water) that carry out the oxygen-producing type of photosynthesis. Photosynthetic and chemosynthetic autotrophs, chiefly bacteria also play role in this conversion.
A small part of the inorganic carbon is also present in the reduced form of methane, in some specific habitat conditions. Methane can be utilized as the sole source of carbon and energy by a special group of aerobic bacteria, methane-utilizing bacteria which convert it into organic carbon and CO2.
The total CO2 in the atmosphere would be completely exhausted with this rate of conversion of inorganic to organic carbon. However, the reverse process of mineralization of organic to inorganic carbon by the activity of heterotrophs prevents this exhaustion. The major end product of mineralization is CO2, though some methanogenic bacteria also produce CH4 by anaerobic respiration and fermentation.
Thus, a carbon cycle is constructed to give an equilibrium with equal rates in both directions.
The conversion of inorganic to organic carbon by plants and by autotrophic microbes is relatively straightforward. However, the reverse process by the animals and heterotrophic microbes is more complex and will be considered in some detail.
Mineralization of organic carbon. Heterotrophs-animals or microbes obtain their carbon and energy by the metabolism of an organic source provided by another form of life. (Growth & Distribution of Microorganisms Study Material)
As a result, some of the organic carbon is mineralized whereas the rest is converted to further organic form in the form of new growth of the heterotroph or as end-products of metabolism. The efficiency of this conversion will vary according to the organism and to the aerobic or anaerobic conditions. The particular heterotroph and any organic carbon produced by it then serve as a food source for other heterotrophs and so on along a food chain. At each trophic stage of the chain, a percentage of the original organic carbon is mineralized until the process is complete.
Microbes play an exceedingly important part in the process. Any compound that is a component of a living organism must be susceptible to mineralization so that it does not accumulate, and all the carbon remains unavailable in that form. Microbes have a very wide ability to break down organic compounds. Dead plant and animal matter on and in soil and in water is rapidly degraded. A sequence of microbes does this, each kind of microbe utilizing one or more components.
Available oxygen is an important factor in mineralization. In water-logged soils (anaerobic) acids and other end-products of fermentation accumulate that prevent further microbial growth and metabolism. In this way, layers of partly decomposed plant matter accumulate as in peat deposits. Such a phenomenon of partial breakdown is used in silage production. A suitable plant material grasses are packed into cylindrical towers or pits called silos, where conditions become readily anaerobic.
Carbohydrate fermentation by bacteria leads to the accumulation of lactic acid in a silo. The pH falls to a level that prevents the growth and activity of organisms that cause deterioration. Soon a stable stage is reached in which the silage can be stored for long periods before its use as animal fodder.
Microbial activities are thus of vital importance in the carbon cycle and all naturally-occurring carbon compounds are eventually mineralized to CO2 and CH4. However, man has been continuously adding synthetic organic compounds-herbicides, pesticides, etc. into the environment. Most of these are non-degradable and thus accumulate. They are also toxic, and thus do not allow the food chains to operate normally. The chains are likely to break up. Care is being taken to design biodegradable synthetics, such as detergents.
Moreover, in big cities, there is a problem with sewage disposal. If untreated sewage is simply discharged into nearby waters, two main problems result; (i) a public health hazard caused by contamination with pathogenic microbes, and (ii) the polluted waters may be made anaerobic by the action of microbial aerobic metabolism on the dissolved organic compounds; animal life thus destroyed due to lack of oxygen. Therefore, some treatment of raw sewage is desirable to decrease the level of organic compounds.