🧫 1. Iron and Nitrogen Oxidation (Chapter 14)

Microbes don’t just eat sugar — some “eat” inorganic compounds like iron, ammonia, or nitrite! These are chemolithotrophs.

⚙️ Ferrous Iron (Fe²⁺) Oxidation

• Example: Acidithiobacillus ferrooxidans oxidizes Fe²⁺ → Fe³⁺ for energy.
• Electrons flow through cytochromes and rusticyanin, making a proton motive force to generate ATP.
• It must oxidize tons of Fe²⁺ to grow — which causes rust-colored Fe(OH)₃ precipitates.
• Some bacteria can oxidize Fe²⁺ even without oxygen (anoxygenic phototrophs).

💡 Think of it like “breathing” iron instead of air!

💨 Nitrification (Ammonia & Nitrite Oxidation)

Two steps, two teams:

1. Ammonia oxidizers (like Nitrosomonas or Nitrosopumilus):
   • NH₃ → NH₂OH → NO₂⁻
   • Enzymes: AMO (ammonia monooxygenase) and HAO (hydroxylamine oxidoreductase).
2. Nitrite oxidizers (like Nitrobacter):
   • NO₂⁻ → NO₃⁻
   • Enzyme: Nitrite oxidoreductase.

Both use reverse electron flow to make NADH for CO₂ fixation via the Calvin cycle. They’re crucial in soils, oceans, and wastewater — turning toxic ammonia into plant-usable nitrate. 🌾

🔴 Anammox (Anaerobic Ammonia Oxidation)

A superstar in anoxic wastewater zones!

• NH₄⁺ + NO₂⁻ → N₂ + 2H₂O
• Bacteria: Brocadia, Kuenenia, Scalindua (Planctomycetes).
• Occurs inside a unique organelle: the anammoxosome (has dense “ladderane” lipids to trap toxic intermediates).
• Uses hydrazine (N₂H₄) — yes, rocket fuel! — as an intermediate 🚀

✅ Environmental benefit: Removes ammonia without oxygen, saving energy in wastewater plants.

🌿 2. Anaerobic Respiration & Denitrification (Still Chapter 14)

When there’s no O₂, microbes use other electron acceptors — like nitrate (NO₃⁻), sulfate (SO₄²⁻), or even metals.

🔁 Nitrate Reduction → Denitrification

• NO₃⁻ → NO₂⁻ → NO → N₂O → N₂
• Enzymes: nitrate reductase, nitrite reductase, nitric oxide reductase, nitrous oxide reductase.
• Example microbes: Pseudomonas stutzeri, Paracoccus denitrificans.

🌍 Good in sewage (removes nitrate pollution). 🌾 Bad in farms (removes fertilizer N). 🌤 Ugly for the atmosphere — produces N₂O, a greenhouse gas ~300× stronger than CO₂!

🔁 3. The Nitrogen Cycle (Chapter 21)

All nitrogen transformations connect in one big global loop 🌎:

🧩 The key insight: microbes move nitrogen between oxidation states, controlling soil fertility, water quality, and even climate.

🚰 4. Wastewater Treatment (Chapter 22)

Goal: remove organic matter, nitrogen, and phosphorus to protect ecosystems and health.

🧱 Primary Treatment

• Physical removal (screens, sedimentation tanks).
• Solids → anaerobic digesters → methane energy recovery 🔥
• Liquid effluent → secondary treatment.

🌬 Secondary Treatment

Uses aerobic microbes to degrade organics and reduce BOD (biochemical oxygen demand).

Two main systems:

1. Activated sludge – aerated tanks with microbial flocs (esp. Zoogloea ramigera).
2. Trickling filters – wastewater drips over rocks coated in biofilms.

Result: up to 95% BOD reduction.

🧪 Tertiary Treatment

Polishing stage — removes phosphorus and nitrogen.

🟣 Enhanced Biological Phosphorus Removal (EBPR)

• Bacteria: Accumulibacter and Tetrasphaera.
• Alternate between anaerobic (store PHA) and aerobic (absorb phosphate).
• Recycles phosphorus without toxic chemicals.

💨 Nitrogen Removal

1. Classical nitrification–denitrification (NH₄⁺ → NO₃⁻ → N₂).
2. Advanced nitritation–denitrification (stop at NO₂⁻ to save energy 💡).
3. Anammox & partial nitritation (PNA) – best combo! No organic carbon or O₂ needed, less sludge, low emissions.

⚗️ Sludge Treatment

• Anaerobic digesters turn sludge → CH₄ + CO₂ (biogas).
• Side streams (rich in NH₄⁺, NO₂⁻) often treated by anammox reactors.
• Produces renewable energy & recycles nutrients.

🧴 Contaminants of Emerging Concern

Even after full treatment, new pollutants sneak through — 💊pharmaceuticals, ☀️sunscreens, 🧴personal care products. They’re xenobiotics that resist degradation or act via cometabolism. Modern research targets microbes that can break them down sustainably.

💧 5. Drinking Water Purification & Distribution

From raw water → potable water, the key steps are:

1. Sedimentation: solids settle.
2. Coagulation & flocculation: alum + anionic polymers make flocs that settle out.
3. Filtration: sand and charcoal remove remaining particles & microbes.
4. Disinfection:
   • Chlorination (HOCl or NH₂Cl residuals) 💦
   • or UV radiation (260 nm) — kills microbes, no chemicals needed.

🚰 Clean water is then piped to cities — but biofilms can still grow in pipes, especially where chlorine levels drop.

🦠 Microbiology of Distribution Systems

Even treated water can host:

• Biofilms on pipes,
• Opportunistic pathogens (like Legionella),
• Nutrient-dependent growth (where chlorine fades).

Hence, ongoing monitoring is vital for safety.

🎯 Key Takeaways

• Microbes drive nitrogen and carbon cycling through diverse redox metabolisms.
• Wastewater treatment relies on microbial cooperation — both aerobic and anaerobic.
• Anammox and EBPR make modern treatment energy-efficient and eco-friendly.
• Safe drinking water depends on both purification and careful maintenance of pipelines.