Estimating Algae Biomass
Lesson plan
How much algae is actually in that water? It is a harder question than it sounds, because the cells are tiny and most of them will not filter. This lab has students answer it three different ways and compare the answers.
The concept
- Beer's Law. Optical density is directly proportional to the amount of dissolved or suspended material. That is the principle a spectrophotometer runs on, and the principle a Secchi Stick runs on.
- Comparing methods. Three techniques can measure the same thing with very different cost, precision and effort. Students see the trade-off directly.
- Why spirulina. Most algae cells are too small to filter easily. Spirulina is a cyanobacterium large enough to catch on a simple screen, so separation is fast and the lab finishes in a class period.
Materials
- 3 litres of spirulina culture
- A harvesting screen
- A Secchi Stick
- Aluminium weigh boats
- A balance, and a drying oven or a warm place
- A spectrophotometer if you have one. Not required.
Method
1. Make a dilution series
Measure the culture and the clear media volumetrically into eight bottles. Every bottle is a known fraction of the original.
| Bottle | mL algae culture | mL clear media |
|---|---|---|
| 1 | 50 | 0 |
| 2 | 45 | 5 |
| 3 | 40 | 10 |
| 4 | 35 | 15 |
| 5 | 30 | 20 |
| 6 | 25 | 25 |
| 7 | 20 | 30 |
| 8 | 15 | 35 |
2. Estimate biomass optically
- Take a Secchi Disk Depth reading on every bottle.
- If you have a spectrophotometer, read absorbance on every bottle as well.
3. Get the real answer
- Filter each bottle through the screen.
- Rinse the salts off. This matters. If you skip it you weigh the salt, not the algae.
- Dry the biomass and weigh it immediately.
4. Plot it
- Plot dry weight against Secchi reading on paper first, by hand.
- Then plot it again on a computer and determine the slope.
- If you ran the spectrophotometer, plot that too and compare all three.
The point of the lab: a $2 stick and a $3,500 instrument should give you the same shape of answer. Students discover that the expensive method is more precise, not more correct.