2023/11/06
Skip to content

Activity

Activity 2 - How to count?

Objective:

To introduce techniques used for quantification of algal cells.

Materials:

- compound microscope
- microscope slides / cover glass
- counting cells
- algal culture e.g. Chlorella sp.

Quantification:

Algal species vary in sizes. So choosing a suitable magnification on counting the different species becomes very important. The table below shows the recommended microscope working magnification for counting different size classes of microalgae.

 

  Size Magnification
  0.2 – 2.0 um (picoplankton)
2.0 – 20.0 um (nanoplankton)
> 20.0 um (microplankton)
1000x**
100 – 400x
100x
** With oil immersion

 

The method for counting microalgae depends on the sensitivity that observers want to achieve. Different methods for counting microalgae are shown below.

Summary of methods for quantification of microalgae using compound microscope

 

  Methods for quantification of microalgae
Volume (ml) of sample required
Sensitivity (cells/l) Preparation time
  Drop on slide
0.02
50000 to 100000 1 min
  Sedgewick-Rafter cell (counting cell)
1
1000 15 mins
  Palmer – Maloney cell (counting cell)
0.1
10000 15 mins

 

Sedgewick-Rafter cell

Palmer – Maloney cell
Sedgewick-Rafter cell
Palmer – Maloney cell

 

Method 1 – Drop on Slide

This is the simplest method to estimate the density of microalgae. One drop of seawater is approximately equal to 0.02ml, if you find one algal cell in a drop of sample, the estimated density is 50,000cells/L.

Place a drop of sample on the slide
Mix the sample by covering the lid and shaking gently. Use a dropper to add a drop of sample on to a glass slide. Cover the drop of sample with a glass slid by lowering the slid. Place the slide on the stage of microscope and observe slowly.
Place a drop of sample on the slide.
 

 

Method 2 - Sedgewick-Rafter cell

Sedgewick-Rafter cell is widely used for quantification of phytoplankton with a volume of 1mL [50mm(length) x 20mm (width) x 1mm (height)]. The bottom plate of the Sedgewick-Rafter cell is divided into 50 columns and 20 rows forming 1000 squares altogether.

Algal cells per mL = Number of cells counted x 1000 / Number of squares observed

Remark: Sedgewick-Rafter cell can be purchased from biological supplies stores at about HK$200

 

Filling the cell
 
Step 1 Step 1

The cover glass placed diagonally across the cell.
   
Step 2 Step 2

Add the sample at one corner into the cell with plastic dropper
   
Step 3 Step 3

Turn the cover glass slowly to cover the cell under. The cell contains 1ml of sample. Do not leave air bubbles in the counting cell.
   

 

Counting of algal culture

Observe the sample and count the density of algal cells under microscope. Try to compare the results of cell density of the same algal sample by using different counting methods.

 

Back  Back to Top