Environmental Microbiology and Biotechnology: Kinetics, Indicators and Bioremediation
This domain tests microbial metabolism, growth kinetics, pathogen indicators and engineered biological processes. Questions are typically quantitative on growth and substrate utilisation, or conceptual on redox zones and bioremediation strategy.
Core rules and concepts
- Monod growth: specific growth rate u = umax S/(Ks + S); net growth = u - kd; doubling time = ln2/u.
- Yield and substrate: substrate utilisation q = u/Y; observed yield is lower than true yield because of decay and maintenance.
- Electron acceptor ladder: O2 > NO3- > Mn(IV)/Fe(III) > SO4^2- > CO2. This sequence defines aerobic, anoxic and anaerobic zones and determines which bioprocess applies.
- Anaerobic digestion: hydrolysis, acidogenesis, acetogenesis, methanogenesis; methanogens are sensitive to pH (optimum 6.8-7.4) and temperature.
- Indicators: total coliforms, faecal coliforms, E. coli and enterococci indicate faecal contamination; CFU counts are culturable cells, MPN is a statistical estimate.
- Tools: 16S rRNA sequencing, qPCR and DGGE identify and quantify communities; cometabolism explains degradation of non-growth substrates such as TCE.
Worked example
A culture has umax = 0.5 h^-1, Ks = 20 mg/L and substrate S = 80 mg/L. Then u = 0.5 x 80/(20 + 80) = 0.40 h^-1, so doubling time = 0.693/0.40 = 1.73 h. If the decay coefficient kd is 0.05 h^-1, net growth is 0.35 h^-1, and the observed doubling time lengthens to 1.98 h. Recognising that Monod gives gross growth, not net growth, is the key reasoning step.
Traps that cost marks
Reporting MPN as a direct cell count; using umax when substrate is limiting; confusing anoxic (nitrate present) with anaerobic; calculating sludge age without the decay term; and forgetting that nitrification consumes alkalinity and oxygen.