Welcome to the COMPETITIVE EXAM MCQs SERIES of ENVIRONMENTAL SCIENCE for UGC-NET/JRF, SLET, GATE, and other entrance tests: Environmental Chemistry of Biogeochemical Cycles.
Syllabus outline
- Definition and significance of biogeochemical cycles.
- Types of biogeochemical cycles.
- Carbon cycle – photosynthesis, respiration and combustion.
- Nitrogen cycle: nitrogen fixation, nitrification, and denitrification.
- Phosphorus cycle – Weathering, absorption, and assimilation.
- Sulphur cycle – Sulphur sources and sinks.
- Role of Microorganisms in Biogeochemical Cycles
- Deforestation and its effects on the carbon cycle.
- Fertiliser use and its effects on nitrogen/phosphorus cycles.
- Industrial activities and their impact on the sulfur cycle.
- Climate change and its impact on biogeochemical cycles
Quick Study Guide
Biogeochemical cycles represent the closed-loop pathways through which essential elements continuously circulate between the living biosphere and the abiotic lithosphere, atmosphere, and hydrosphere. These cycles are strictly driven by solar energy, geothermal forces, and microbially mediated chemical reactions. Understanding these pathways requires analysing oxidation-reduction kinetics, environmental nutrient limitations, and the specific atmospheric and sedimentary phases of each element.
- Nitrogen Cycle and Microbial Kinetics: Nitrogen cycling relies almost entirely on specialised bacteria to change the element’s oxidation state. Nitrogen fixation converts inert atmospheric N2 into bioavailable ammonia (NH3). Nitrification is an aerobic oxidation process: Nitrosomonas bacteria convert ammonium to nitrite (NO2–), and Nitrobacter oxidises it to nitrate (NO3–). In oxygen-depleted soils, anaerobic denitrifying bacteria reduce nitrate back into N2 gas.
- Carbon Cycle and Oceanic Buffering: The global carbon cycle operates on both rapid biological timescales (photosynthesis) and slow geological timescales (carbonate-silicate weathering). A critical component is the oceanic carbonate buffer system. Atmospheric CO2 dissolves into seawater to form carbonic acid, which dissociates into bicarbonate and carbonate ions. This acts as a massive carbon sink, though excessive CO2 aggressively lowers ocean pH.
- Phosphorus Cycle (Sedimentary Limitation): Unlike carbon or nitrogen, phosphorus completely lacks a significant atmospheric, gaseous phase. It is a strictly sedimentary cycle driven by the slow weathering of terrestrial rocks. Because of its slow mobility, phosphorus is naturally scarce and acts as the primary limiting nutrient in most freshwater ecosystems. Excess phosphorus from agricultural runoff directly triggers severe eutrophication.
- Sulfur Cycle and Biogenic Gases: Sulfur operates through both massive sedimentary sinks and active atmospheric phases. In anaerobic environments, sulfate-reducing bacteria convert sulfate (SO42-) into toxic hydrogen sulfide (H2S). In marine surface waters, phytoplankton emit dimethyl sulfide gas, which oxidises in the atmosphere to form sulfate aerosols. Sulfate aerosols act as critical cloud condensation nuclei.
- Stoichiometry and Anthropogenic Disruption: Ecological nutrient demand is strictly governed by the Redfield Ratio (106 C: 16 N: 1 P in marine phytoplankton). Human activities severely disrupt this natural stoichiometry. The industrial Haber-Bosch process has artificially doubled global nitrogen fixation, while fossil fuel combustion rapidly releases ancient, sequestered carbon, bypassing the slow geological cycle and overloading atmospheric sinks.
Test Your Knowledge
This quiz contains 25 concept-based MCQs of “Environmental Chemistry of Biogeochemical Cycles “. Each question has a single correct/most appropriate answer.
1. Which of the following is not a way in which sulfur dioxide emissions affect the environment?
a) Smog formation
b) Global warming
c) Respiratory problems
d) Acid rain
2. What is the possible path of nitrogen movement?
a) Air-animals-soil-plants-air
b) Air-soil-plants-animals-soil-air
c) Soil-air-plants-air-animals-soil
d) Soil-animals-plants-air-soil
3. In which environmental compartment are metals most likely to undergo redox reactions?
a) Soil
b) Aquatic sediments
c) Oceans
d) Atmosphere
4. Which of the following is not a way in which phosphorus is lost from ecosystems?
a) Erosion
b) Harvesting of crops
c) Uptake by plants
d) Leaching
5. Which of the following is a major source of phosphorus in aquatic ecosystems?
a) Phosphorus fixation by bacteria
b) Human sewage and agricultural runoff
c) Weathering of rocks and minerals
d) Atmospheric deposition
6. Which of the following is an example of sulfate-reducing bacteria?
a) Thiobacillus.
b) Desulfovibrio.
c) Nitrobacter.
d) Pseudomonas.
7. Which of the following is not a way in which nutrient cycling affects ecosystem productivity?
a) Nutrient cycling regulates the pH and salinity of soils and water.
b) Nutrient cycling increases the availability of essential elements for plant growth.
c) Nutrient cycling reduces the competition for resources among different species.
d) Nutrient cycling promotes the decomposition of organic matter.
8. Which metal is often associated with acid mine drainage pollution?
a) Copper
b) Arsenic
c) Selenium
d) Zinc
9. Which of the following forms of nitrogen can be used by microorganisms in place of molecular oxygen?
a) Nitrate
b) Organic Nitrogen
c) Ammonium ion
d) Nitrogen
10. Which of the following organisms can transform nitrite to nitrate?
a) Nitrobacter.
b) Rhizobium.
c) Pseudomonas.
d) Nitrosomonas.
11. The legume root releases ______________ that is absorbed by the rhizobial symbionts, initiating the expression of plasmid-encoded nod (nodulation) genes.
a) Glycolipids
b) Leghemoglobin
c) Flavonoids
d) Chitolipooligosaccharides
12. In the root nodules of leguminous plants, the level of __________ is controlled by Leghemoglobin.
a) Water
b) Cellular respiration
c) Nitrogen
d) Oxygen
13. Which of the following nutrient cycles is a sedimentary cycle?
a) Sulphur
b) Hydrogen
c) Phosphorus
d) Nitrogen
14. ________ is a glycoprotein found in milk that binds to iron and is known to exhibit antimicrobial activity against a broad spectrum of bacteria and viruses.
a) Ovotransferrin
b) Lactoferrin
c) Serotransferrin
d) Melanotransferrin
15. Which of the following organisms can transform ammonium ions to nitrite?
a) Pseudomonas.
b) Nitrobacter.
c) Nitrosomonas.
d) Rhizobium.
16. Which of the following is not a step in the sulfur cycle?
a) Sulfur oxidation by bacteria
b) Sulfur assimilation by plants
c) Sulfur deposition in soils and sediments
d) Sulfur release from volcanic eruptions
17. Which of the following is not a way in which climate change is affecting biogeochemical cycles?
a) Decreasing soil erosion
b) Changing ocean currents
c) Increasing atmospheric carbon dioxide concentrations
d) Altering precipitation patterns
18. Assertion (A): Metal hyperaccumulating plants are adapted to grow in soils with high metal concentrations.
Reasoning (R): These plants use metal hyperaccumulation as a defence mechanism against herbivores.
a) Both A and R are true, and R is the correct explanation of A.
b) A is true, but R is false.
c) Both A and R are true, but R is an incorrect explanation of A.
d) Both A and R are false.
19. The phosphorus cycle differs from the water, carbon, and nitrogen cycles in that
a) Phosphorus is primarily found in a mineralised state within rocks.
b) Phosphorus is far more abundant than water, carbon, or nitrogen.
c) Phosphorus is less important to biological systems than water, carbon, or nitrogen.
d) Phosphorus, once used by an organism, does not cycle back to the environment.
20. The wood roaches died when they were placed in a bell jar containing wood chips and a high concentration of oxygen because
a) Oxygen is toxic to flagellates present in the guts of wood roaches
b) Lack of food availability
c) Oxygen is toxic to wood roaches
d) Oxygen has an inhibitory effect on cellulose oxidation
21. What is the primary mechanism of bioaccumulation of metals in aquatic ecosystems?
a) Sedimentation
b) Bacterial activity
c) Decomposition
d) Trophic levels
22. The phenomenon of metal hyperaccumulation in plants is most closely associated with:
a) Chromium
b) Nickel
c) Cadmium
d) Lead
23. What role do ligands play in transporting trace metals in aquatic environments?
a) Precipitation
b) Oxidation
c) Chelation
d) Inhibition
24. Some bacteria can fix nitrogen. This means
a) They convert ammonia into nitrites and nitrates.
b) They convert nitrate into nitrogen gas.
c) They convert atmospheric nitrogen gas into biologically useful forms of nitrogen.
d) They break down nitrogen-rich compounds and release ammonium ions.
25. In enhanced biological phosphate removal, Acinetobacter, Pseudomonas or Nocardia utilize the stored organics and take up phosphorus to produce ___________ to store energy under aerobic conditions.
a) High-energy carbon compound
b) Inorganic Phosphorus (Pi)
c) ATP
d) Polyphosphate
Previous: Inorganic and organic components of soils
Next: Toxic chemicals
References
- Erach Bharucha (2017). Environmental Studies, Universities Press, 4th Edition.
- Manahan, Stanley E. (2008). Fundamentals of Environmental Chemistry, CRC Press, 3rd Edition.
- De, Anil Kumar and De, Arnab Kumar (2024). Environmental Chemistry, New Age International, 11th Edition.
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