1 Algae vary from small, single-celled forms to complex multicellular forms, such as the giant kelps of the eastern Pacific that grow to more than 60 metres in length

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Algae are a very diverse group of organisms that vary widely in size, shape and colour. They are photosynthetic organisms that occur in most habitats, ranging from marine and freshwater to desert sands and from hot boiling springs to snow and ice.1 Algae vary from small, single-celled forms to complex multicellular forms, such as the giant kelps of the eastern Pacific that grow to more than 60 metres in length. They exhibit a wide range of reproductive strategies, from simple, asexual cell division to complex forms of sexual reproduction.

Algae are important as primary producers of organic matter at the base of the food chain thus, provide oxygen for other aquatic life 2. They also contribute to economic well being in the form of food, medicine and other products. Current research is also on going to use algae as an alternative to fuel as it produces no carbon dioxide.

But, most importantly, in what ways can the growth of algae be made more efficient? Factors such as temperature, light intensity and different wavelengths of light all contribute to the effectiveness of algal growth. We decided to consider only one of these factors. The aim of this experiment was to use red, yellow, blue and green light to investigate under which of these algae photosynthesise best.


 To investigate the effect of wavelength of light on algal photosynthesis.


 Different wavelengths of light have differing effects on the photosynthesis of algae. It was hypothesised that red light will be best for algal photosynthesis.



  • Air pump

  • Centrifuge

  • 1 litre conical flask

  • Air stone

  • Light bank

  • 5x 250cm3 conical flasks

  • Measuring cylinder

  • 2 Beakers (100ml)

  • Clamp stand

  • 2.5 cm3 sodium alginate solution

  • 2% 20 cm3 solution of calcium chloride

  • Tea strainer

  • 10 cm3, 5cm3 syringe

  • Red, green, blue & yellow plastic filter papers

  • Glass rod

  • 50 cm3 hydrogen bicarbonate indicator

  • Thermometer


The algae are very small and difficult to work with directly in the water so the first part of the practical involved ‘immobilising’ them. This effectively trapped large numbers of algal cells into concentrated ‘jelly like’ balls. The size of ball was kept as constant as possible using the same volume of droplet to ensure only wavelength was the altered variable. Equal volumes were taken to as an indication that they contained a very similar number of algae cells which would be otherwise unpractical to count due to their microscopic size. It was sodium alginate that was used to make the jelly and was of no harm to the algae.3

By centrifuging 50 cm3 of dark green algal suspension for a few minutes, a concentrated algal culture of about 10 cm3 was obtained as figure one shows a changing concentration.

Figure 1 A culture of algae that has been grown for about three weeks which has been centrifuged, to obtain a more concentrated suspension.

Using a 5cm3 syringe, 2.5 cm3, 2-3% solution of sodium alginate solution and 5cm3 of the concentrated algae where added into a beaker. Since there are different types of alginate available, which have different concentrations and thicknesses’, a viscous mixture that will drip steadily through the syringe was used. A glass rod was used to stir the solution to ensure an even distribution of algae within the beaker. Figure three below shows both solutions in a syringe which have been put into a 100ml beaker and mixed.

Figure 2: 5 cm3, 2-3% solution of sodium alginate solution and 5cm3 of the concentrated algae into a 100ml beaker.

The mixture made above was then put into a 20cm3, 2 % solution of calcium chloride drop by drop, using a syringe. This volume of calcium chloride was in excess so that all the algal balls could be fully immersed. The same size of drops where used to ensure the size of the algal beads was kept constant and only one variable was being changed. The beaker was then left untouched for 15 minutes so that the balls “hardened” after which, a tea strainer was used to remove the algal balls from the solution. The balls where then washed and ready for use. Figure five shows this sequence.

Figure 3: Pictures- during the immobilising of algal balls

The algal balls were then used to indicate the link between the wavelengths of light, corresponding to a particular colour, on the effect of photosynthesis. Since hydrogen carbonate indicator is very sensitive to changing concentrations of carbon dioxide, a change in carbon dioxide concentration can be easily monitored. As only algae are within the McCartney bottles, any change is directly due to the photosynthesising algae. As algae photosynthesise, they use up carbon dioxide which lowers the available carbon dioxide concentration thus, the indicator will show the corresponding colour change.

Hydrogen carbonate indicator

Increasing C02 concentration atmospheric concentration Decreasing C02 concentration

Figure 4 Hydrogen carbonate indicator scale showing varying ph’s

Five Mccartney bottles where set up to contain each of the following: 20 algal balls, 5cm3

Bicarbonate indicator and blue, red, green and yellow plastic filter paper. A control without a plastic filter was also set up. The set up looked like figure five below.

Figure 5: Set up of initial experiment
A light bank was used as a bright white light source as only one colour of light corresponding to the plastic filter paper got through. All variables such as light intensity (same distance from the same light source), number of algal balls, and volume of hydrogen carbonate indicator, temperature and size of container where kept constant and only the variable being measured was changed. The experiment was carried out at room temperature; about 23C using a thermometer. A control with no filter was also set up.
The first McCartney bottle which showed a colour change of purple (from the original atmospheric red) indicated a low concentration of carbon dioxide. This was an indication of stopping the experiment and checking the colour of all other bottles as clearly if the bottle that shows this colour can change in that period of time then all the other ones have the potential to do so but to varying extents. Thus, the colour of each McCartney bottle was compared to the carbonate indicator scale and the identical pH identified. A Colorimeter could also be used to double check the pH values. This measures the absorbance of different wavelengths of light in a solution. It is used to measure the concentration of a known solute. Thus, we could determine which of the bottles showed the lowest carbon dioxide concentration and thus highest photosynthesis.

Initially, the hydrogen carbonate indicator in which the algal balls where placed, was red (this is the colour for atmospheric concentration). But, as the algae photosynthesised the concentration of carbon dioxide began to fall, represented by a colour change. Figure six shows an average of the results.



pH 1: highest ---> pH 9 : lowest











Less carbon dioxide means an increased uptake by algae for photosynthesis. Thus, 1 shows least photosynthesis and 9 shows the most.

Figure 6: results table


Figure 7: set up

All of the McCartney bottles contained hydrogen carbonate indicator which was initially red which is the normal colour for atmospheric concentration. But as the algae gradually photosynthesised, the indicator colour began to change as shown below.


Graph of Average Results

The above scale shows colour of indicator at varying levels of carbon dioxide concentration. Less carbon dioxide means an increased uptake by algae for photosynthesis. Thus, 1 shows least photosynthesis and 9 shows the most.


Algae photosynthesise with all colours of light but to varying degrees. Our results show that red light is best for photosynthesising algae followed by blue. Green, yellow and the control (without a filter paper) showed the least photosynthesis. Thus, our conclusion is what we had hypothesised. If a longer period of time was used for carrying out the experiment then all would have shown purple; a low carbon dioxide concentration; since photosynthesis is a continuous process despite varying degree’s of efficiency affected by various factors. Our graph below illustrates this conclusion.

Discussion/further plans

It can be concluded from our research that algae photosynthesise at all wavelengths to some varying extent. But, our results show that optimum photosynthesis occurs using red light.

Since algae are very important to humans and a source from which many of the products used today are made from, making algae growth more efficient is very important as this will allow for a greater yield of algae and therefore, more of the desired end product.

Other than wavelength of light, there are many other factors which can increase algae yield. This includes temperature, light intensity and carbon dioxide concentration.

However, the direct link between the above factors and photosynthesis remains to be concluded by us, from further experiments in the near future.


  • 1: www.wikipedia.org wiki/Algae (accessed on 1st January 2010) This page was last modified on 29th December 2009 at 16:05.

  • 2: http://www.forestry.gov.uk/ (accessed on 4th January 2010)

  • 3: Cartwright.chem.ox.ac.uk/hsci/chemicals/sodium_alginate.html (accessed on 2nd January 2010)-update: Oxford, March 31, 2006-
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