Biogeochemical Cycling (Exam Questions) | Microbiology

Everything you need to know about biogeochemical cycling. Some of the most frequently asked exam questions are as follows:-

Q.1. What is carbon cycle?

Ans: It mainly includes the transfer of CO₂ and organic carbon between the atmosphere (the source of inorganic CO₂) and hydrosphere (the aqueous envelope of the earth as bodies of water and aqueous vapours) and lithosphere (solid part of the earth). The carbon dioxide (CO₂) in lithosphere and hydrosphere reacts with water to form carbonate and bicarbonate which are the main inorganic forms of carbon occurring there.

Q.2. What is greenhouse effect?

Ans: Burning of fossil fuels increases carbon dioxide in the atmosphere leading to rise in concentration of carbon dioxide in the atmosphere (the mass of air surrounding the earth and a unit of pressure approximately 1 × 10⁶ dynes/cm²). This rise in concentration of CO₂ and the consequent rise in global temperature are referred to as greenhouse effect.

Q.3. How does production of methane by a specialized group of methanogenic archaea represent a diversion to normal cycling of carbon?

Ans: It is due to the fact that methane produced by the archaea cannot be used by most of heterotrophic organisms and in this way is lost from biological community to the atmosphere, and hence the methanogens are diversists of the system of normal cycling of CO₂.

Q.4. What are primary producers in the ecosystem?

Ans: Autotrophs, the organisms whose growth and reproduction are independent of external sources of organic compounds, the required cellular carbon is supplied by reduction of CO₂ and the needed cellular energy is supplied by the conversion of light energy to ATP or the oxidation of inorganic compounds to provide the free energy for the formation of ATP; are the primary producers.

Q.5. What is gross primary production?

Ans: It is the total amount of organic matter produced by an autotrophic biological community of a niche or suitable natural home.

Q.6. What is net primary production?

Ans: It is gross primary production minus oxidative metabolism of biological community which removes organic carbon and energy stored in such compounds. It may be regarded as decay of energy stored in a given niche.

Q.7. What is trophic structure?

Ans: The feeding relationship between organisms or the routes by which energy and materials are transferred within an ecosystem (living andrion living components of self-supporting systems).

Q.8. What is food web?

Ans: The food web may be regarded as transfer of energy stored in organic compounds between the organisms in the community. At the base of food web come primary producers which manufacture the organic matter for the system. Grazers are the organisms which depend for their nutrition on primary producers. However, in a phytoplankton-based food web algae and cyanobacteria (blue green algae) are the primary sources of food for grazers.

In a detrital food web, the microbial biomass produced from growth on dead organic matter, called detritus, is the primary food source for the grazers. The grazers in the chain are eaten by predators and the predators are in turn are preyed on by larger predators. In this way carbon and stored energy are moved to the higher levels of food web. Of course, some of the carbon and energy are lost in each transfer in the process of respiration. (Fig. 25.1)

Q.9. What is biogeochemical cycling of nitrogen or nitrogen cycle?

Ans: The molecular nitrogen is one of the most abundant substances in the air. The nitrogen moves from atmosphere through biological taxa (biotas) living in the soil and aquatic habitats. Thus involves nitrogen fixation in the form of (NH₄⁺), nitrification (NO₂^– and N0₃^–) and assimilation into plants as (R – NH₂), passed on to animals, leading to ammonification on death of the animal and again fixation of NH₃ to N0₂^– and NO₃^–, NH₃^– and NH₄⁺ in soil may be broken down to molecular nitrogen by the microorganisms.

Q.10. How is nitrogenase protected from oxygen?

Ans: The enzyme nitrogenase is protected from oxygen in the root nodule where some bacteria fix nitrogen and by the red pigment leghaemoglobin which supplies oxygen to organism for respiration without denaturing the enzyme nitrogenase.

Q.11. Give the composition of nitrogenase which is an oxygen labile enzyme.

Ans: The nitrogenase an oxygen labile enzyme is composed of dinitrogenase (MoFe protein) and dinitrogenase reductase (Fe protein). Dinitrogenase is made of two dissimilar polypeptides, α₂, β₂, the ∞ polypeptides in it are encoded by nif D while β polypeptides by nif K genes. There are two active metallo clusters in dinitrogenase protein.

These are:

(i) P cluster having 8 iron and 7 to 8 sulphur atoms (Fe₈ S_7-8) and

(ii) Iron-molybdenum cofactors (FeMoCO) having 7 iron and 9 sulphur, one molybdenum atom and one molecule of homocitrate (Fe₇S₉ Mo-homocitrate).

The P cluster works as an intermediate electron acceptor and perhaps transfers electrons to FeMOCO cluster. It is believed that FeMOCO cluster functions as the site of nitrogen reduction. The dinitrogenase reductase (Fe protein comprises of two identical polypeptides y₂ encoded by the nif H gene, and each polypeptide has two iron atoms.

The four Fe atoms are organized into a Fe₄S₄ cluster. The major function of Fe protein is to bind and hydrolyse Mg ATP and to transfer electrons from Fe₄S₄ cluster to the P cluster of the MOFe protein. Both proteins are folded in such a manner as to bring the active centres of each close to each other.

Q.12. Explain the enzymatic catalysis for reduction of nitrogen to ammonia.

Ans: In enzymatic catalysis electrons are systematically transferred in a sequence one at a time from the iron centres of dinitrogenase reductase to the iron molybdenum cofactors of dinitrogenase. In this way the electrons are finally transferred to the substrate i.e. nitrogen leading to its reduction. Many rounds of electron transfer must take place before nitrogen is reduced to ammonia and at least two Mg – ATP molecules are hydrolyzed for each electron that is transferred.

Subsequently Mg – ATPs bind to the Fe protein, dinitrogenase reductase, but ATP is not hydrolysed until the Fe protein is complexed to MoFe protein or dinitrogenase. Distincdy speaking dintrogenase is one of the two proteins that build or comprise nitrogenase; protein of nitrogenase which has attached iron and molybdenum or vanadium cofactor; enzyme composed of two dissimilar polypeptides, α₂, β₂, that function as place or site of nitrogen fixation.

Q.13. Name the common free living Nitrogen – fixing bacteria. Also mention their habitats.

Ans: There are large numbers of free living nitrogen-fixing bacteria depending upon the type of soil or habitat. These are:

(1) Azotobacter, Azomonas and Derxia in temperate parts in neutral or alkaline soils and waters.

(2) Beijerinckia are more acid tolerant.

(3) Frankia is actinomycetes which are important symbiotic free living bacteria.

(4) Azospirillum lipoterum and Azobacter paspali are nitrogen fixing soil bacteria associate with rhizosphere (or the ecological niche making the surface of plant roots) of some tropical grasses.

(5) Cyanobacteria or blue green algae (BGA) like Anabaena and Nostoc are found in aquatic habitats.

Q.14. Which are the common symbiotic nitrogen fixing bacteria?

Ans: Rhizobium and Bradyrhizobium make association with leguminous plants. These invade the roots and form root nodules.

Q.15. Which are major steps leading to establishment of symbiotic association between leguminous plants and Rhizobia?

Ans: These are:

(i) Attraction of bacteria to the plant roots by amino acids secreted by roots.

(ii) Binding of bacteria to receptors, lectins on plant roots which are receptor specific proteins.

(iii) Activity of plant growth substances causing curling of rootlets.

(iv) Entry of bacteria into root hairs.

(v) Development of infection thread.

(vi) Transformation of root/plant cells to form a tumrous outgrowth.

(vii) Multiplication of bacteria within nodule.

(viii) Transformation of invading bacteria into variable pleomorphic forms.

Q.16. Give the genera of nitrifying bacteria.

Ans: These are named below:

Q.17. Name the bacterium which reduces nitrite to ammonia, called nitrite ammonification.

Ans: Clostridium species reduce nitrite to ammonium ions and this process is called nitrite ammonification.

Q.18. Name a denitrifying bacteria reducing nitrate to nitrite.

Ans: Escherichia coli.

Phosphorus (P) Cycle:

Q.19. What is a phosphate solubilizing microorganisms? Define.

Ans: Any microorganisms having capability of transforming insoluble organic or inorganic (in mineral form) phosphate into soluble orthophosphate (Pi) in a manner which may be shown to make a pronounced significant contribution to the phosphate nutritional status of a specific plant or plant population within the microorganism’s native soil ecosystem. The soil phosphorus mobilization by microorganisms is of great significance as microorganisms assimilate inorganic phosphates and mineralize organic phosphate compounds and. microbial activities are involved in the solubilization and mobilization of phosphate compounds.

Q.20. Give the common terminologies for microorganisms participating in phosphate solubilization mechanism.

Ans: A.H. Goldstein and P.U. Krisnaraj in 2007 recognised following common terminologies for microorganisms participating in phosphate solubilization mechanisms:

1. Phosphate solubilizing microorganisms (PSMs).

2. Mineral phosphate solubilizing microorganisms (MPSMs).

3. Organic phosphate solubilizing microorganisms. (OPSMs).

4. Phosphate mobilizing microorganisms (PMMs).

5. Mineral phosphate mobilizing microorganisms (MPMMs).

6. Organic phosphate mobilizing microorganisms (OPMMs).

Q.21. What is eutrophication?

Ans: The eutrophication is the enrichment of natural waters (streams, rivers and ponds) with inorganic materials, specially nitrogen and phosphorus compounds that support the excessive growth of micro and microorganisms, including photosynthetic organisms like cyanobacteria, algae and hydrophytes belonging to different groups of the plants, water ferns and angiosperms.

Phosphorus is essential to the growth of organisms and can be the nutrient which limits the primary productivity of a water body. The discharge of raw or treated waste water, agricultural drainage, and certain industrial wastes to that water may stimulate the growth of photosynthetic aquatic micro and macro organisms in nuisance quantities. During the subsequent decomposition of accumulated organic matter, water column can be severely depleted of oxygen causing major fish kills.

Q.22. How does microbial transformations of sulphur establish the sulphur cycle in nature?

Ans: The microorganisms remove sulphur from organic compounds, in the process of desulphurisation by forming sulphate under aerobic conditions, while under anaerobic conditions hydrogen sulphide (H₂S) is normally produced from mineralisation of organic sulphur compounds.

Hydrogen sulphide can also be formed by sulphate reducing bacteria which utilize sulphate as a terminal electron acceptor in anaerobic respiration. Hydrogen sulphide can accumulate in toxic concentrations in area of rapid protein decomposition. It can react with metals to form insoluble metallic sulphides. The reduction of sulphates to H₂S is done by sulphate reducing bacteria as Desulfovibrio, in biogeochemical cycling of sulphur.

Q.23. Which thermal vent communities are involved in sulphur cycling?

Ans: The thermal vents are the deep sea areas of volcanic activity and support the growth of unique biological communities that depend on chemoautotrophic metabolism for primary production. These include high densities of clams, mussels, vestimentiferan worms (marine tubeworms which lack digestive system), and other invertebrates. It is interesting to note that energetically, entire vent community is supported by the chemoautotrophic oxidation of reduced sulphur mainly by Beggiatoa, Thiomicrospora and other sulphide and sulphur oxidisers.

The methane produced from reduction of CO₂ with geothermally produced hydrogen by extremely thermopile Methanococcus species has been detected from anoxid hydrothermal fluids, is also oxidized by methanotrophic bacteria which supply additional carbon and energy input for the vent ecosystem.

Q.24. What are tube worms?

Ans: The tube worms are Riftia pachypthil about a metre in length which grow extensively near deep – sea thermal vents have no guts. The red brown colour of the worms is due to form or haemoglobin which provides oxygen and hydrogen sulphide to chemoautotrophic bacteria which grow inside the tissue of tube worms. Begiatoa grow between the strands of tube worms and other chemoautotrophic bacteria on trophosome (a spongy tissue) of tube worms.