Algal Ball Photosynthesis




  • In eukaryotic cells, specialised organelles facilitate biochemical processes of photosynthesis, cellular respiration, the synthesis of complex molecules (including carbohydrates, proteins, lipids and other biomacromolecules), and the removal of cellular products and wastes (ACSBL049)

  • Cells require inputs of suitable forms of energy, including light energy or chemical energy in complex molecules, and matter, including gases, simple nutrients, ions, and removal of wastes, to survive (ACSBL044)
  •  The cell membrane separates the cell from its surroundings and controls the exchange of materials, including gases, nutrients and wastes, between the cell and its environment (ACSBL045)
  • Photosynthesis is a biochemical process that in plant cells occurs in the chloroplast and that uses light energy to synthesise organic compounds; the overall process can be represented as a balanced chemical equation (ACSBL052)  


During photosynthesis, radiant energy is converted into organic substances that can be stored within the plant to support growth, reproduction, and metabolism. Plants metabolise the sugars that form as a result of sunlight, carbon dioxide, and water reacting. Reliably studying photosynthesis first hand can be difficult and require precise setting up. Algal ball photosynthesis kits offer an easier, more accurate method of studying photosynthesis that is perfect for use within the classroom. Algal balls are stored in small vials filled with hydrogen carbonate indicator solution. When exposed to light, the encapsulated algae absorb the CO2 from the solution they are stored in during the process of photosynthesis. As a result of the lowered CO2 levels, the pH of the solution will rise. When no light is available, respiration will dominate and the pH will decrease through the release of CO2.  Changes in the levels of carbonic acid can be monitored via the colour changes that occur due to the hydrogen carbonate indicator.

In this practical, students have the opportunity to tangibly observe photosynthesis. They are tasked with conducting a number of small experiments and observing how the colour changes from yellow/orange to purple when the algal balls and solution are placed under a light source as photosynthesis takes up dissolved CO2 and the pH rises. Additionally, students create a light filter to observe its effects. Vials are stored in the dark for a short period to observe the colour change from purple to yellow/orange as respiration occurs and the pH falls. Varying the number of algal balls and light intensity gives students the opportunity to explore the effect on algae. This is a great practical to introduce students to the basic principles of photosynthesis in a fun, simple and interactive way. 


Algal Ball Culture Care

  • Remove the algal ball container from its packaging as soon as it arrives and loosen the lid to allow for airflow. 
  • If you do not plan to use it immediately, store in a cool place away from direct sunlight and areas of high heat.
  • We recommend using the algal balls as early as you can; otherwise, they are best used within 2 weeks.

Experiment Preparation

  • Rinse the vials for your experiment with approximately 1mL of hydrogen carbonate indicator solution. To do this, replace the cap on the vial and shake the container. Next, pour the rinse liquid into the second vial and repeat until you have rinsed all required vials. Discard the rinse liquid from the last vial. 
  • Divide the correct number of algal balls into each vial. 
  • Use a plastic pipette to add 3mL of hydrogen carbonate indicator to each vial.
  • Note colour changes in the indicator solution as the CO2 concentration and the pH change.
  • For Experiment 1, provide students with one vial of twelve balls which has already been exposed to light. The vial should be exposed to light long enough for it to turn purple or near purple on the colour spectrum.


Method 1 : Expose to light

  • Collect your vial of twelve algal balls which has already been exposed to light.
  • After observing the starting colour, place the vial in a dark cupboard or drawer to remove it from the light.
  • Prepare another vial of twelve algal balls and hydrogen carbonate indicator.
  • Position this vial near a light source, but do not expose it to too much heat.
  • Observe any colour changes in both vials. 

Method 2: Coloured Light Filter 

  • Prepare two vials of twelve algal balls and hydrogen carbonate indicator. 
  • Wrap a piece of coloured transparent film around each vial and position near a light source, but do not expose them to too much heat.
  • Experiment with different coloured filter paper/ cellophane and observe the effects on the rate of photosynthesis. We recommend wrapping one vial in red, blue or purple, and one in orange, yellow or green. If you have enough algal balls and cellophane, you can try the whole rainbow!

Method 3: Varying the Number of Algal Balls per Vial 

  • Prepare five vials of varying numbers of algal balls and hydrogen carbonate indicator, eg one with three balls, one with six, one with nine and so on.
  • Place each vial the same distance from a light source and identify whether the number of balls correlates with the rate of change of pH.

 Method 4: Varying the Light Intensity 

  • Prepare five vials of algal balls and hydrogen carbonate indicator. 
  • Place twelve balls in each vial and place the vials at varying measured distances from a light source. 
  • Ensure no vial casts a shadow on any other. Observe how the rate of change of pH is influenced by the light intensity. 


Exposure to light and dark expected results
After exposing the vial to a light source, the colour in the solution should change from yellow/orange to purple as photosynthesis absorbs the CO2 and the pH rises. After placing the vial in the dark, you should observe the colour in the vial change from purple to yellow/orange as respiration occurs and the pH falls. Respiration and photosynthesis occur simultaneously in the algal cells. As more light becomes available, photosynthesis dominates. However, as conditions become darker, photosynthesis is retarded and respiration takes over as the main process.
Coloured light filter expected results
The pH of vials wrapped red, blue or purple cellophane will experience a greater rise than vials wrapped orange, yellow or green. This is due to the way the algae absorb light - we observe them as green because that is the colour that bounces off them out of the spectrum that makes up white light and so that is the colour that is not used in photosynthesis.

Varying the number of algal balls expected results
As the number of algal balls increases, the time taken for the pH to change should be reduced until a critical mass is reached. As the number of balls increases, so too does the use of carbon dioxide and the production of oxygen until the number of balls becomes too high and begins to block the light from each other.

Varying the light intensity
The rate of photosynthesis should increase with proximity to the light source, until it becomes too close and therefore too hot. Whether this happens will depend on your light source and your batch of algal balls. However, there should be a reduction in the time taken for the colour to change for the closer vials compared to the further vials.


Hydrogen carbonate indicator is sensitive enough to change colour as CO2 dissolves in water to form carbonic acid. 
CO2 + H20 ↔ H2CO3
The indicator is a red/orange colour in water that is in equilibrium with the CO2 in the air. The colour shifts through orange to yellow as the concentration of dissolved CO2 rises and the pH falls, and it shifts through red to purple as the concentration of dissolved CO2 falls and the pH rises. A rise in the concentration of CO2 occurs during respiration as the algae breaks down sugars to release energy, CO2 and water. A fall in the concentration of CO2 occurs during photosynthesis as the algae produces carbohydrates by taking up CO2 and water and absorbing light energy.
You can compare the colour of the indicator solution in your experimental vials to a set of standard vials that contain solutions of boric acid / borax buffers coloured with hydrogen carbonate indicator.


If you are providing students with the already prepared 7mL vials with the algal balls and indicator, it may be a good idea to expose them to light for half an hour or so beforehand to start photosynthesis.  


icn-timer.pngTime Requirements
  1. 90 mins  


icn-material.pngMaterial List
  1. Algal Balls
  2. 5 Empty 7ml vials
  3. Hydrogen Carbonate Indicator
  4. 6 Plastic pipettes 


icn-security.png Safety Requirements
  1. Wear appropriate personal protective equipment (PPE).
  2. Know and follow all regulatory guidelines for the disposal of laboratory wastes.  
  3. Wash your hands thoroughly after the activity.


icn-reference.pngReference Kits

  1. Algal Ball Photosynthesis Kit


icn-link.pngHelpful Links

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