The Effect of Fertilizer and Carbon Dioxide on Algae Growth

  Rationale

Students need a number of activities that require them to make measurements, record and analyze data, and utilize the principles of the scientific method. In addition, activities should work to reinforce reading comprehension skills, stimulate higher level thinking skills, and increase the students’ scientific literacy. In addition to addressing these topics, this activity also demonstrates the concept of photosynthesis and the importance of the carbon cycle to life on earth.

Introduction

    Fertilizers come in many types with different specifications. They are important in agriculture because they supply crops with the required nutrients for growth. To build mass and structure, plants also need Carbon Dioxide and water. Even though fertilizer and CO2 are valuable resources for providing animals with food, they can cause problems if their supply is unrestricted. Fertilizer run-off from farmland is a major source of fresh water contamination in our streams, lakes, and ponds. Fuel combustion from automobiles is a major contributor of Carbon Dioxide.

    Algae are typically a unicellular organism with the ability to use light, water, and carbon dioxide and the process of photosynthesis to produce mass. Algae are important in our biosphere because it produces large amounts of oxygen in the photosynthetic process. When the conditions are right, the uncontrolled growth of algae a bloom and the resulting large quantities of algae can present a health hazard. Neither fertilizer run-off nor carbon dioxide alone can produce an algal bloom. But, taken together, they can generate the conditions needed for uncontrolled alga growth.

    Alga growth is fairly easy to measure. Since the organism is unicellular, growth means that the organism is producing an increasing number of cells per unit of time. Each cell is capable of producing a set amount of oxygen per minute regardless of its environment. The amount of oxygen produced per unit of time by an isolated group of alga cells is therefore directly proportional to the number of cells in the group. This means that if, during the growth phase, bottle A produces twice the volume of oxygen as bottle B, then bottle A has twice the number of algae cells.

Materials for the Activity and for Preparing Stock Algae

Table 1

Procedure for Preparing Stock Algae with Source

The stock culture is the source of algae for the other bottles. There are many ways of making a suspension of concentrated algae (chlorophyta). If a source of algae is present, algae such as a fresh water aquarium tank, then the following procedure will yield a sufficient amount of algae in a short time.

1. Add the fertilizer to bucket and fill the bucket with 9.5 L of hot tap water.

2. If possible, let this suspension sit open overnight to allow some of the chlorine to escape.

3. After the water has cooled to room temperature, add the carbonated water to the bucket and mix the contents with a large spoon or spatula.

4. Scrape as much algae as possible from the alga source and transfer it to the bucket.

5. Use the 2-cup Measuring Cup and transfer 500-ml amounts of the suspension into each of the 18 bottles.

6. Loosely cap each bottle and place all bottles in a lighted area.

7. Observe the bottles daily. Depending on the amount of source algae, the bottle sides should show large amounts of algae in 3 to 5 days.

Stock Culture of Algae without an Algae Source                                      In many cases, an aquarium may not be available or the teacher may want to demonstrate the presence of alga spores in the air we breathe. The preparation of an alga culture is in itself a good exercise for demonstrating the importance of simple organisms such as algae in the carbon cycle. The following procedure will help the students understand the relationship between autotrophs, heterotrophs and the carbon cycle.

Materials for Preparing Stock Algae without a Source

Table 2

Procedure for Preparing Stock Algae without a Source

1.   Drill a 1/8^th-inch hole in the caps of 16-20 ounce plastic screw-cap bottles.

2.   Prepare a 9.5 L suspension of Eco Grow (38 grams) and tap water (no CO2).

3.   Stir the suspension and fill each bottle to 500 mL using a 2-cup Measuring Cup.

4.   Cut a 30-cm piece of tubing. Use scissors and cut one end of the tubing so that the     end comes to a diagonal point of about 0.5 cm.

5.   Insert the pointed end of the tubing into the hole through the top of the cap.

6.   Use a pair of pliers and grab the tip of the tubing from the inside of the cap.

7.   Pull the tubing about halfway through the cap. There should be about 15 cm of tubing on both sides of the cap.

8.   Cut-off the pointed end of the tubing and attach a bubble stone to the cut end.

9.   Use the 2-cup Measuring Cup and transfer 500-mL amounts of the suspension into each of the 18 bottles.

10.   Insert the bubble stone/tubing into each bottle and adjust the length so that the bubble stone is about 4 cm from the bottom of the bottle.

11.   Loosely cap each bottle and place all bottles in a lighted area.

Assign each student a straw and have the students work in groups. Everyday or as often as possible, have the students attach their straw to their assigned bottle. Assign 16 of the 18 bottles for blowing. Do not disturb two of the bottles and use the undisturbed bottles for controls. Tell each student to take a deep breath and hold it as long as possible. When they can no longer hold their breath, tell them to very slowly blow through the straw and into the bottle. Explain that this action places Carbon Dioxide and algae spores into the bottle.

Observe the bottles daily. Depending on the student’s air-holding ability and the number of times the bottles are inoculated (blown), the sides of each bottle should be coated with algae in 5 to 10 days.

Note that algae do not appear in the two control bottles. When the bottles contain a sufficient amount of algae, pour out the suspension from each bottle and fill the 18 algae-containing bottles with Eco Grow, distilled water, and carbonated water according to table 3 and the following procedure.

Test Bottles for Algae Growing Exercise

Note-Fill bottles about 75% with distilled water then add the Eco Grow and carbonated water. Fill the bottles to the neck level, just below the tubing, with distilled water.

Procedure for Measuring Oxygen Production

1.   Drill a 1/8^th-inch hole in the caps of 20-20 ounce plastic screw-cap bottles and insert a piece of tubing into all 20 bottles. In this case, the tubing should project no more than 1 cm into the neck of the bottle.

2.   Cut the beveled end of the tubing and cut the other end of the tubing so that the outside piece of tubing projects about 20 cm from the cap.

3.   Remove the plunger from a 20-mL syringe and cut the plastic guard from the syringe’s hub.

4.   Attach the tubing to the syringe’s hub and secure the connection by wrapping a 2-cm piece of floral wiring around the connection area.

5.   Fill the bottles with the appropriate reagents (see table 3) and screw the syringe/cap assemblies securely onto the bottles.

6.   Insert the plungers onto the open end of the syringe and slowly depress the plunger while holding the bottle. The bottle should stiffen while the plunger is depressed. When the plunger is released, it should return to approximately its starting position near the 20-mL mark of the syringe. If this is not the case, then the syringe has a leak and the tubing/syringe connection needs evaluation.

7.   Remove or loosen the caps on each bottle and depress the plunger to the zero mark on the syringe.

8.   When all of the bottles are filled with their appropriate mixtures and each bottle contains a syringe at the zero mark, place all 20 bottles in the window sill or light box.

9.   Check all bottles within the next 72 hours for gas production. Hold the bottle with one hand and pull the plunger up with the other. Depress the plunger again and stop when the bottle feels neither inflated or deflated (its easier than it sounds).

10. Record the data in the data table. Gas production will cease after about 9 days from the beginning of the experiment.