Environmental and Agricultural Microbiology
Domain 9 tests the roles of microbes in nutrient cycling, soil fertility, pollution control and plant disease. Expect nitrogen-cycle conversions, biofertilizer examples and water-quality indicators.
Core rules and concepts
- Carbon cycle: photosynthesis fixes CO2; decomposers release it. Nitrogen cycle: nitrogen fixation (N2 to NH3), ammonification, nitrification (NH4+ to NO2- by Nitrosomonas, NO2- to NO3- by Nitrobacter), denitrification (NO3- to N2).
- Symbiotic nitrogen fixers include Rhizobium in legume root nodules; free-living fixers include Azotobacter and cyanobacteria such as Anabaena. Mycorrhizae improve phosphorus uptake.
- Biofertilizers use live microbes; compost and manure improve soil structure. Bioremediation uses microbes to degrade pollutants.
- Water quality is assessed by coliform counts (MPN or membrane filtration) and BOD; high BOD indicates organic pollution.
Worked example
A farmer rotates a legume with a cereal. Reasoning: Rhizobium in legume root nodules fixes atmospheric nitrogen into ammonia, which is assimilated into plant protein. When the legume residues decompose, nitrogen is released into the soil, reducing the need for synthetic nitrogen fertilizer for the next cereal crop. The benefit depends on effective nodulation and the right Rhizobium strain.
Mark-losing traps
- Nitrification is not nitrogen fixation; it converts ammonium to nitrate.
- Denitrification removes nitrogen from soil as N2 gas and can reduce fertility.
- Coliforms are indicators of faecal contamination, not necessarily pathogens themselves.
- Biofertilizers are not chemical fertilizers and do not supply all nutrients instantly.