Everything you need to know about the history of Microbiology. Some of the most frequently asked questions are as follows:-
Q.1. What is metal cycling?
Ans. The heavy metals like mercury can exist in a variety of inorganic and organic forms. Some microorganisms can form methyl mercury and can also form methylated forms of arsenic, selenium, tin and some other metals.
Q.2. What reactions are mainly involved in biogeochemical cycling of cycling of iron?
Ans. These are the oxidation reduction reaction which reduced ferric iron to ferrous iron and oxidizes ferrous iron to ferric iron (Fig. 4.1). The ferric (Fe3+) and ferrous (Fe2+) irons possess highly different solubility properties. The ferric form of iron precipitates in alkaline conditions as ferric hydroxide.
However, the ferric iron may be reduced under anaerobic conditions into more soluble ferrous iron. If the conditions are anaerobic sufficient H2S may be evolved to lead the precipitation of iron as ferrous sulphide. In aerobic habitats in the well drained soils the iron mainly exists in ferric form.
Q.3. How most bacteria have been evolved and gifted by nature to fulfil their iron requirements, by secreting iron chelating agents?
Ans. All microorganisms with an exception to been some lactobacilli require iron employed as a cofactor by many metabolic enzymes and regulatory proteins because of its characteristic, two stable oxidation states Fe2+ (ferrous form) and Fe3+(ferric form). Although the iron is abundant in environment yet the iron is a limiting factor for bacterial growth because it forms insoluble ferric hydroxide complexes under aerobic conditions and neutral pH.
The bacteria have developed specialized high affinity transport system to acquire sufficient amount of iron. Most bacteria bear the ability to produce and secrete molecules called siderophores (means iron bearers, in Greek). The siderophores are special iron chelating agents which help in solubilization and uptake of iron. Siderophores are water soluble low molecular weight (500 – 1500 Da) molecules which bind with ferric iron with great affinity.
There are two main categories of siderophores which have been studied extensively. Phenol catechol derivatives are synthesized by enterobacteria and have been given common names enterochelin and enterobactin. These systems carry iron into the cell. There is system to monitor the intracellular level of iron in the bacterial cell. The iron deficiency leads to excretion of siderophores. The shortage of iron in bacterial cell will bring down the growth while high concentrations of iron are toxic to bacterial cells.
Q.4. What are manganese nodules?
Ans. Like iron, the manganese is also oxidized and reduced. The Bacterial activity in some cases result in precipitation of manganese, and lead to formation of manganese nodules.
Q.5. How do microorganisms form deposits of diatomaceous earth?
Ans. Like other elements, silicon is also cycled between soluble and insoluble forms by microbial populations. Diatoms have silicon dioxide in their frustules and thus form deposits of diatomaceous earth.
Q.6. How are coral reefs formed?
Ans. Hard red, pink or white substances are built on the sea bed by small creatures, as a result of consumption of carbon dioxide by photosynthetic microorganisms, which shift the equilibrium away from the soluble bicarbonate to less soluble carbonate, resulting the formation of coral reefs in marine regions. Sometimes coral islands are also formed (Fig. 4.2).
Q.7. How can nitrogenize activity be measured?
Ans. The enzyme complex nitrogenize is responsible for nitrogen fixation. Nitrogenize activity can be measured by acetylene reduction assay.
Q.8. How do microorganisms lead to rock-weathering and formation of soil?
Ans. The fungi, cyanobacteria and lichens on and within the siliceous rocks in harsh environments quickly dissolve silica by releasing Carboxylic acids. In this regard 2-ketogluconic acid, citric and oxalic acids are mainly associated in dissolution of siliceous rocks. Solubilization of siliceous rocks by chelating action of the said organic acids result rock weathering and soil formation.