Madical brain Notes 10

  Microorganisms in the Environment   First, however, we must turn our attention again to the subject of energy relations in living things. We looked at the different ways in which microorganisms can derive and utilise energy from various sources. We now need to put these processes into a global perspective. All organisms may be placed into one of three categories with respect to their part in the global flow of energy:   ·                 (Primary) Producers : autotrophs that obtain energy from the sun or chemical sources(e.g. green plants, photosynthetic bacteria, chemolithotrophic bacteria). They use the energy to synthesise organic material from carbon dioxide and water.   ·                 Consumers : heterotrophs that derive energy through the consumption of other organ-isms (producers or other consumers). They ma...

  The carbon cycle A more detailed scheme of the carbon cycle is shown in Figure 16.2. Both aerobic and anaerobic reactions con-tribute to the cycle. The numbers in parentheses in the following description refer to those in Figure 16.2. Atmospheric CO 2  is fixed into organic compounds by plants, together with phototrophic and chemoau-totrophic microorganisms ( 1 ). The organic compounds thus synthesised undergo cellular respiration and CO 2  is returned to the atmosphere ( 2 ). The carbon may have been passed along a food chain to consumers before this occurs. Carbon dioxide is also produced by the decomposition of dead plant, animal and microbial material by heterotrophic bacteria and fungi.   Methanogenic bacteria produce methane from organic carbon or CO 2  ( 3 ,  4 ). This in turn is oxidised by methanotrophic bacteria; carbon may be incorporated into organic material or lost as CO 2  ( 5 ,  6 ).

  The nitrogen cycle   Nitrogen is essential to all living things as a component of proteins and nucleic acids. Although elemental nitrogen makes up three quarters of the Earth’s atmosphere, only a handful of life forms are able to utilise it for metabolic purposes. These are termed nitrogen-fixing bacteria, and incorporate the nitrogen into ammonia (Figure 16.3, reaction 1):     The nitrogenase enzyme complex responsible for the reaction is very sensitive to oxygen, and is thought to have evolved early in the Earth’s history, when the atmosphere was still largely oxygen-free. Many nitrogen-fixing bacteria are anaerobes; those that are not have devised ways of keeping the cell interior anoxic.  Azotobacter  species, for example, utilise oxygen at a high rate, so that it never accumulates in the cell, inactivating the nitrogenase. Many cyanophytes (blue-greens) carry out nitrogen fixation in thick-walled heterocysts which help maintain anoxic conditions. ...

  The sulphur cycle   Sulphur is found in living organisms in the form of compounds such as amino acids, coenzymes and vitamins. It can be utilised by different types of organisms in several forms; Figure 16.4 shows the principal components of the sulphur cycle.   In its elemental form, sulphur is unavailable to most organisms; however, certain bacteria such as  Acidithiobacillus  are able to oxidise it to sulphate ( 1 ), a form that can be utilised by a much broader range of organisms:   2S  +  3O 2 +  2H 2 O −−−−−−−→ H 2 SO 4   Powdered sulphur is often added to alkaline soils in order to encourage this reaction and thereby reduce the pH.   Sulphate-reducing bacteria convert the sulphate to hydrogen sulphide gas ( 2 ) using either an organic compound or hydrogen gas as electron donor:   8H + +  SO 4 2 − −−−−−−−→ H 2 S  +  2H 2 O  +  2OH −     These bacteria are obligate anaerobes, and the pr...

  Phosphorus Phosphorus exists almost exclusively in nature as phosphate; however, this is cycled between soluble and insoluble forms. This conversion is pH-dependent, and if phosphate is only present in an insoluble form, it will act as a limiting nutrient. This explains the sudden surge in the growth of plants, algae and cyanobacteria when a source of soluble phosphate (typically fertiliser or detergent) enters a watercourse. Unlike the elements discussed above, phosphorus hardly exists in a gaseous form, so its main ‘reservoir’ is in the sea rather than the atmosphere.

  The microbiology of soil   In the following section it will be necessary to generalise, and treat soil as a homogeneous medium. In fact, it is no such thing; its precise make-up is dependent upon the un-derlying geology, and the climatic conditions both past and present. In addition, the microbial population of a soil will vary according to the amount of available wa-ter and organic matter, and different organisms colonise different strata in the soil.   The organic content of a soil derives from the re-mains of dead plants and animals. These are broken down in the soil by a combination of invertebrates and microorganisms (mainly bacteria and fungi) known as the  decomposers . Their action results in the release of substances that can be used by plants and by other mi-croorganisms. Much organic material is easily degraded, while the more resistant fraction is referred to as  hu-mus , and comprises lignin together with various othermacromolecules. The humus con...

  The microbiology of freshwater   The microbial population of freshwater is strongly influenced by the presence or absence of oxygen and light. A body of water such as a pond or lake is stratified into zones (Figure 16.5), each having its own characteristic microflora, determined by the     availability of these factors. The  littoral  zone is the region situated close to land where the water is sufficiently shallow for sunlight to penetrate to the bottom. The  limnetic  zone occupies the same depth, but is in open water, away from the shore. The  profun-dal  zone occupies deeper water, where the sun is unable to penetrate, and finally the  benthic  zone comprises the sediment of mud and organic matter at the bottom of thepond or lake.   Oxygen is poorly soluble in water (9 mg/l at 20  ◦  C), so its availability is often a limiting factor in determining the microbial population of a body of water. Oxygen availab...

  The microbiology of seawater   The world’s oceans cover some 70 per cent of the Earth’s surface and have a fairly constant salt content of 3.5 per cent (w/v). The depth to which light can penetrate varies, but is limited to the first 100 metres or so. A world of permanent darkness exists at greater depths, however in spite of the absence of photosynthesis, oxygen is often still present. This is because the generally low levels of mineral nutrients in seawater limit the amount of primary production, and therefore heterotrophic activity. At extreme depths, however, anoxic conditions prevail.   Compared to freshwater habitats, marine ecosystems show much less variability in both temperature and pH, although there are exceptions to this general rule. A more pertinent issue in marine environments is that of pressure; this increases progressively in deeper waters, and at 1000 metres reaches around 100 times normal atmospheric pressure. Concomitant with this increase in pressu...

  Beneficial effects of microorganisms in the environment   The central role played by microorganisms in the recycling of essential elements on a global scale has already been stressed. Many of their natural activities are exploited by humans for their own benefit. Some form the basis of industrial processes   such as those used in the food and drink industries and while the application of others is essentially environmental. Notable among these is the harnessing of natural processes of  biodegradation  to treat the colossal volumes of liquid and solid wastes generated by our society. These are reviewed briefly in the following section.     Solid waste treatment: composting and landfill   We in the modern Western world are often described as living in a ‘throwaway society’. On average, each of us generates around 2 tonnes of solid waste material per year, and all of this must be disposed of in some way! Most of it ends up in landfill sites, huge h...