---
title: "Short note: Bacteria-algal symbiosis"
subject: "Sanitary Engineering"
chapter: 7
key: "semester_6/sanitary_engineering:bacteria-algal-symbiosis"
source: https://www.bctengineer.com/q/semester_6/sanitary_engineering/bacteria-algal-symbiosis
site: BCT Engineering
---

# Short note: Bacteria-algal symbiosis

Sanitary Engineering · Chapter 7: Wastewater Treatment

- Asked in: 2068 Bhadra (old course), 4 marks
- Video: [Aerobic Wastewater Treatment: Oxidation Pond](https://www.youtube.com/watch?v=Xmf4Df9kVrM) — Biotechnology by Dr. Swati Upadhyay
- Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14
- Page: https://www.bctengineer.com/q/semester_6/sanitary_engineering/bacteria-algal-symbiosis
- Whole chapter: https://www.bctengineer.com/md/semester_6/sanitary_engineering/chapter-7

## Question

Write a short note on the bacteria-algal symbiosis process.

## Short answer

Bacteria oxidize sewage organics with oxygen, releasing CO2, ammonia and phosphate; algae use these nutrients and sunlight to produce oxygen, which bacteria reuse. This symbiosis supplies oxygen naturally, eliminating mechanical aeration, and removes 70–90% BOD. Oxygen production stops at night and in cloudy weather, so shallow depth and proper loading are required.

## Answer

**Bacteria-algal symbiosis** is the mutually beneficial relationship between aerobic bacteria and algae that makes natural purification possible in oxidation ponds (waste stabilisation ponds).

#### Process
1. **Bacteria** oxidise the organic matter of sewage using dissolved oxygen:
$$
\text{organics}+O_2\xrightarrow{\text{bacteria}}CO_2+H_2O+NH_3+PO_4^{3-}+\text{new cells}
$$
2. **Algae** use the CO₂, ammonia and phosphates released by the bacteria as nutrients and, by **photosynthesis** in sunlight, produce oxygen:
$$
CO_2+H_2O+\text{sunlight}\rightarrow\text{algal cells}+O_2
$$
3. This oxygen is used by the bacteria to oxidise more organic matter, and the cycle repeats.

```text
         sunlight
            |
        +---v----+   O2   +----------+
        | ALGAE  |------->| BACTERIA |<-- organic matter
        |        |<-------|          |
        +--------+ CO2,   +----------+
                   NH3,PO4
```
#### Importance
- Each group supplies what the other needs, so oxygen is supplied free of cost and mechanical aeration is not needed.
- Organic matter is converted into algal and bacterial cells and stable products, so the BOD of the sewage is reduced by 70-90%.
- Oxygen is produced only in the daytime with enough sunlight; at night the algae also respire and the DO falls, and in cloudy weather the oxygen supply may be insufficient. So a shallow depth and a proper loading rate are used.

## Check yourself

The answer's key points as multiple-choice questions, with the correct option and why.

1. Bacteria-algal symbiosis means
   A. algae kill the bacteria to clean the water
   B. bacteria and algae compete for oxygen
   C. a mutually beneficial relation between aerobic bacteria and algae in oxidation ponds
   D. bacteria digest algae anaerobically
   Answer: C. Each group supplies what the other needs. (Key point: Meaning)

2. What does each partner supply to the other?
   A. Bacteria supply O2 and algae supply organic matter
   B. Algae supply O2 and bacteria supply CO2, NH3 and phosphates
   C. Both supply methane
   D. Algae supply sunlight and bacteria supply water
   Answer: B. Bacteria release CO2, ammonia and phosphates; algae produce O2 by photosynthesis. (Key point: Exchange of materials)

3. What is the main practical benefit of the symbiosis?
   A. Oxygen is supplied free, so mechanical aeration is not needed
   B. It produces biogas
   C. It removes sludge completely
   D. It works equally well at night
   Answer: A. Algal photosynthesis provides the oxygen for bacterial oxidation. (Key point: Benefit)

4. What is a limitation of the process?
   A. It needs a high power supply
   B. It works best in cloudy weather
   C. It gives no BOD removal
   D. At night algae respire and DO falls; cloudy weather reduces oxygen supply
   Answer: D. Without sunlight there is no photosynthesis, so DO may fall. (Key point: Limitation)