Introductory Microbiology

Microbial effect on organic and inorganic matter

 
• There are two major groups of bacteria, the “eubacteria” and the recently discovered “archaebacteria”. The eubacteria contain most of the common bacteria such as Escherichia coli, and the cyanobacteria (blue green algae). The archaebacteria are found mainly in the deep ocean near hydrothermal vents. What is striking from the stand point of the divergence of genetic material is that these two group of bacteria are more different than are animals and plants . In other words, these two groups of bacteria have evolutionary diverged further from one another than animals have diverged from plants.
• Microbes must acquire certain elements to grow and reproduce these elements compose their protoplasm (52.4% protein, C,H,N,O and S, 19.9% nucleic acid 16.6% polysaccharide and 9.4% phospholipid). In addition, they must produce ATP in order to use the stored energy in this molecule to operate various cellular processes. Assimilative processes are used to bring needed elements into the cell and to incorporate them into the cell protoplasm. Dissimilative processes donot incorporate elements into the cell, but instead they use the energy gained in the process to form ATP.

Important impacts of microbes on ecosystems

1.Generate oxygen in the atmosphere:
Almost all of the production of oxygen by bacteria on earth today occurs in the ocean by the cyanobactera or “blue green algae”.

2. Recycle nutrients stored in organic matter to an inorganic form.
Decomposition releases the mineral nutrients (e.g. N, P, K) bound up in dead organic matter in an inorganic form that is available for primary producers to use. Without this recycling of inorganic nutrients, primary productivity on the globe would stop.
On land most of the decomposition (also called ‘mineralization’) of dead organic matter occurs at soil surface, and the rate of decomposition is a function of moisture and temperature.
Fungi are important in terrestrial systems, but not in aquatic. They are present even before the leaves and twigs enter the soil and so decomposition starts in the living or scenescent plant material. Fungi are the most important decomposers of structural plant compounds(cellulose and lignin-but not that lignin is not broken down when oxygen is absent). The fungi invade the organic matter in soils first and are then followed by bacteria.
In water, the decomposition of organic matter is mostly oxic in streams and in the ocean and anoxic in the bottoms of lakes or in swamps. Oxic decomposition proceeds faster than decomposition in environments where these is no oxygen. In the open ocean, the water is so deep (average 3900 m) and contains so much oxygen, that most of the algal formed organic matter at the surface decomposes aerobically before it reaches the bottom. For example, only 2 % of the primary productivity in the upper ocean sinks to a depth of 3500m. Most of the world is ocean, and most of the ocean is deep, so most of the aquatic decomposition must be aerobic. But, in shallow waters, coastal oceans lakes and estuaries 25-60 % of the organic matter produced may settle out of the upper waters rapidly and be decomposed anaerobically.
Of course another important impact of decomposition besides generating inorganic nutrients is to produce CO2 and CH4 that is released to the atmosphere.

3) Fix nitrogen from the atmosphere into a useable form:
The only organisms capacity of removing N2 gas from the atmosphere and “fixing” it into usable nitrogen form (NH3) are bacteria. The specific bacteria that can perform N fixation are scatterd throughout the groups including the cyanobacteria. All organisams that fix nitrogen use same mechanisms and the same enzymes. This ability probably involved only once and early in the history of life. Symbotic nitrogen fixation costs the plant photosynthate to support the fixation and the NH3 assimilation; this cost could be from 15-30% of the total carbon assimilated by the plant. In fact, to fix one molecule of N2 requires about 25 molecules of ATP, so it is expensive from the bacterial stand point and that means that the plant must support that energy requirement. Inturn the plant receives nitrogen, which may otherwise be a limiting nutrient.
Another difficulty for the bacteria is that one of the enzymes necessary for N2 fixation is destroyed by oxygen (which is necessary for efficient ATP formation). One solution to this problem is to form symbiotic relationships with other organisms that can provide carbohydrates; these include diatoms, the fungi of certain lichens, shipworms, termites and certain plants especially in nodules of the roots.

4) Allow herbivores to consume poor quality food:
In the ocean, most of the primary productivity is consumers by herbivores. In contrast in terrestrial systems most of the primary productivity is not consumed by the herbivores.
The reason for this difference are :
1) Animals lack digestive enzymes capable of using cellulose and lignin and other structural plant compounds
2) Plants often have anti-grazing toxins, aromatic resins, or thorns
3) Most land plant tissue is poor in mineral nutrients compared to the tissue in the herbivore.
In a ruminant animal (cattle, dear giraffe) the ingested food, passibly regurgitated and re-chewed, passes into the rumen together with saliva. The rumen is really a continuous fermenter where the complex carbohydrates of the plant material are fermented into methane, carbon dioxide and fatty acids. The biota of the rumen are found in about equal biomasses of bacteria (1011/ml), protozons (105/ml to 106/ml) and fungi (poorly known biomass). About 60-65 % of the total energy removed from the plant food that is ingested by the animal comes from rumen fermentation. Plant tissues passing from the resume undergo secondary fermentation in the caecum and large intestine where an additional 8-30 % of the total energy is provided. In addition, many termites conntain protozoans and bacteria in their guts that perform similar operations. The protozoans are capable of digesting cellulose and bacteria in the gut generate CH4 from the organic compounds released from the cellulose degradation. Finally, some termites also have bacteria in their guts that are capable of fixing nitrogen from the atmosphere, providing a useable nitrogen source for the termite.

5) Give plant roots access to nutrients in the soil:
Plant roots create a zone of a nutrient depletion around themselves. To have access to new sources of nutrients, a plant can either grow more roots and small root hairs or form an association with a fungus whose hyphae provide an even more efficient absorptive structure. Most vascular plants can form such associations, which are called “mycorrhizae”. Mycorrhizal fungi include those living on the surface of plants (ectotrohic or sheathing) and those which enter the host (endo trophic or vesicular-arbuscular or simply “V-A”).
The added advantage to the plant is that the hyphae can secrete enzymes that break down organic molecules and make inorganic nutrients available. While the plants gain nutrients, the fungi gain carbohydrate food from the plant. There is also a cost to the plant in this association; one study reported that mycorrhizal biomass was only 1% of a fir forest ecosystem but used 15 % of the net primary production.
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