Question

1. Describe the roles that microorganisms have in the global cycles of nitrogen (N) or sulphur (S)?

Answer 1

Nitrogen cycle has five steps:

1. Nitrogen fixation (N2 to NH3/ NH4+ or NO3-)
2. Nitrification (NH3 to NO3-)
3. Assimilation (Incorporation of NH3 and NO3- into biological tissues)
4. Ammonification (organic nitrogen compounds to NH3)
5. Denitrification(NO3- to N2)

Nitrogen is a key component of the bodies of living organisms. It exists in the atmosphere as gas..The nitrogen cycle is a series of processes that convert nitrogen gas to organic substances and back to nitrogen in nature. It is a continuous cycle that is maintained by the decomposers and nitrogen bacteria. Nitrogen-fixing microorganisms capture atmospheric nitrogen and  convert  it to ammonia ,which is  taken by plants and used to make organic molecules. The nitrogen compounds are passed through the food chain as other organisms feed on the plants and each other.Waste products (undigested food, urine and faeces) and dead organisms which contain nitrogen compounds are added to the soil.

Two kinds of nitrogen-fixing bacteria are there The first free-living (nonsymbiotic) bacteria, includes the cyanobacteria (or blue-green algae) Anabaena and Nostoc and genera such as Azotobacter, Beijerinckia, and Clostridium. The second one is comprises the mutualistic (symbiotic) bacteria; examples include Rhizobium, associated with leguminous plants (e.g., various members of the pea family); Frankia, associated with certain dicotyledonous species (actinorhizal plants); and certain Azospirillum species, associated with cereal grasses.

Symbiotic nitrogen-fixing bacteria invade the root hairs of host plants, where they multiply and stimulate formation of root nodules, enlargements of plant cells and bacteria in intimate association. Within the nodules the bacteria convert free nitrogen to ammonia, which the host plant utilizes for its development. To ensure sufficient nodule formation and optimum growth of legumes (e.g., alfalfa, beans, clovers, peas, soybeans), seeds are usually inoculated with commercial cultures of appropriate Rhizobium species, especially in soils poor or lacking in the required bacterium.Nitrogen fixing bacteria fix 60% of nitrogen gas.

Nitrification

Nitrification is a two-step process , that is carried out by the nitrifying bacteria. Ammonia or ammonium ions are oxidized first to nitrites and then to nitrates.First, the soil bacteria Nitrosomonas and Nitrococcus convert NH3 to NO2-, and then another soil bacterium, Nitrobacter, oxidizes NO2- to NO3-. These bacteria gain energy through these conversions, both of which require oxygen to occur.

Assimilation

Assimilation is the process by which plants and animals incorporate the NO3- and ammonia formed through nitrogen fixation and nitrification. Plants take up these forms of nitrogen through their roots, and incorporate them into plant proteins and nucleic acids. Animals are then able to utilize nitrogen from the plant tissues. Plant roots themselves can affect the abundance of various forms of nitrogen by changing the pH and secreting organic compounds or oxygen.This influences microbial activities like the inter-conversion of various nitrogen species, the release of ammonia from organic matter in the soil and the fixation of nitrogen by non-nodule-forming bacteria.

Ammmonification

  Ammonification is the process by which the organically bound nitrogen of microbial, plant, and animal biomass is recycled after their death. Ammonification is carried out by a diverse array of microorganisms that perform ecological decay services, and its product is ammonia or ammonium ion. Ammonium is a suitable source of nutrition for many species of plants, especially those living in acidic soils is the conversion of organic nitrogen into ammonia. The ammonia produced by this process is excreted into the environment and is then available for either nitrification or assimilation.The decomposers - certain soil bacteria and fungi e.g. ammonifying bacteria - break down proteins in dead organisms and animal wastes, releasing ammonium ions.

Denitrification

Denitrification is a bacterial process, carried out by a relatively wide range of species. In denitrification, nitrate is reduced to either nitrous oxide or dinitrogen, which is then emitted to the atmosphere. Example is Pseudomonas stutzeri. This bacterial species has almost 50 genes that are known to have a direct role in denitrification. The process of denitrification occurs under conditions where oxygen is not present, and its rate is largest when concentrations of nitrate are large.This process only occurs where there is little to no oxygen, such as deep in the soil near the water table.

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Sulphur Cycle

Sulfur is a component of a couple of vitamins and essential metabolites and it occurs in two amino acids, cysteine and methionine. The microbes can transform sulfur from its most oxidized form (sulfate or SO₄) to its most reduced state (sulfide or H₂S). In sulfur cycle, in particular, involves some unique groups of procaryotes and procaryotic processes. Two unrelated groups of procaryotes oxidize H₂S to S and S to SO₄. The first is the anoxygenic photosynthetic purple and green sulfur bacteria that oxidize H₂S as a source of electrons for cyclic photophosphorylation. The second is the "colorless sulfur bacteria" (now a misnomer because the group contains many Archaea) which oxidize H₂S and S as sources of energy.

Sulfur-oxidizing procaryotes are frequently thermophiles found in hot (volcanic) springs and near deep sea thermal vents that are rich in H₂S. They may be acidophiles, as well, since they acidify their own environment by the production of sulfuric acid.

Since SO₄ and S may be used as electron acceptors for respiration, sulfate reducing bacteria produce H₂S during a process of anaerobic respiration analogous to denitrification. The use of SO₄ as an electron acceptor is an obligatory process that takes place only in anaerobic environments. The process results in the distinctive odor of H₂S in anaerobic bogs, soils and sediments where it occurs.

Sulfur is assimilated by bacteria and plants as SO₄ for use and reduction to sulfide. Animals and bacteria can remove the sulfide group from proteins as a source of S during decomposition. These processes complete the sulfur cycle.

Steps of Sulphur Cycle

Following are the important steps of the sulphur cycle:

Decomposition of Organic Compounds

Protein degradation releases amino acids that contain sulphur. Sulphates are reduced to H₂S by the action of Desulfotomaculum bacteria.

Oxidation of Hydrogen Sulphide to Elemental Sulphur

Hydrogen sulphide oxidises to produce elemental sulphur. Certain photosynthetic bacteria from the families Chlorobiaceae and Chromatiaceae initiate the oxidation process.

Oxidation of Elemental Sulphur

Elemental sulphur present in the soil cannot be utilized directly by the plants. Therefore, it is converted into sulphates by chemolithotrophic bacteria.

Reduction of Sulphates

Sulphates are reduced to hydrogen sulphide by Desulfovibrio desulfuricans. This occurs in two steps:

• Firstly, the sulphates are converted to sulphites utilizing ATP.
• Secondly, the reduction of sulphite to hydrogen sulphide