Introduction
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.
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Aim
 To investigate the effect of wavelength of light on algal photosynthesis.
Hypothesis

Different wavelengths of light have differing effects on the photosynthesis of algae. It
was hypothesised that red light will be best for algal photosynthesis.
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Method
Apparatus:
 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
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Instructions:
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
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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
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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
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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.
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Results
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.
COLOUR OF FILTER PAPER
CO2 CONCENTTRATION
pH 1: highest
---> pH 9 : lowest
Clear
6.5
Green
6.5
Yellow
6.5
blue
8.0
red
9.0
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
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Initially:
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.
Finally:
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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.
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Conclusion
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.
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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.
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Bibliography
 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, 2006Comenius - European Cooperation on School Education
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Hutchesons’ Grammar School
Generic Risk Assessment Form
Location
Science
Block
Department
Ref No
Biology
Operation/Activity
Photosynthesis of immobilised algal balls
Hazards
1.
2.
3.
4.
Glassware - Can cut you if broken.
Sodium Alginate Solution - Irritation in eyes and harmful if too much swallowed.
Calcium chloride Solution - Irritation in eyes and harmful if too much swallowed.
Centrifuge – Danger of test tubes inside whirling out if centrifuge is opened too early.
Individual or groups exposed
Current Control Measures
(1) Good Lab practice
(2,3) Safety goggles
(4) Make sure centrifuge is locked properly, test tubes are properly placed inside
and wait for test tubes to completely stop spinning before opening centrifuge.
With these controls
the risk is (tick)
LOW
MEDIUM HIGH UNACCEPTABLE
(See over for guidance on risk ratings)
Further control measures required
Cautious at all times!
Eye contact with sodium alginate or calcium chloride solution: Immediately flush the eye with
water. If irritation persists, call for medical help.
If sodium alginate or calcium chloride is swallowed: Wash out the mouth with water if the
person is conscious. If the quantity that has been swallowed is large, call for medical help.
Signed
Dated
Review
Dates
Use a new box each time this assessment is reviewed. Assessments should be
reviewed once a year or whenever equipment or circumstances change.
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