Environmental Effects on Algal Growth
Water quality continues to be a concern for environmental scientists, citizens, and
educators. A number of kits are available that allow the identification and quantification
of chemical components present in a selected water source. Students can supplement
these valuable analyses by performing experiments designed to test the effects of various
concentrations of a given chemical on a representative aquatic life form. Thus, unlike a
natural source of water possessing a highly complex mix of organisms and chemicals,
this laboratory model allows for a more controlled study of the impact of one variable on
algal growth. This activity is quite flexible in that it allows teacher-guided inquiry,
student-centered discovery, or a more standardized cookbook experiment. One
interesting version directs students to select a given variable (NaCl, ammonia, fertilizer,
dish detergent, etc.) and vary its concentration in a set of equivalent sample tubes. In this
way, students can also learn to employ skills in creating proper dilutions and interpreting
dosage effects. Certainly, students could also select variables such as light intensity or
temperature, negating the necessity of generating varying concentrations within the
sample tubes. The procedure described below focuses on exploring a dosage effect.
Materials:
Algal population (Chlorella, Euglena, Chlamydomonas, etc), 10 x 13 mm borosilicate
culture tubes (an inexpensive alternative to spectrophotometer tubes), spring water,
pipets, wax paper or parafilm, spectrophotometer. OPTIONAL: vortex for suspending
algae.
Procedure:
1. Pipet 2 mL of Algae into each tube. (This volume might be changed based on
density of original culture).
2. Pipet 1 mL of spring water into each tube.
3. Add the appropriate volumes of variable stock solution and spring water (total of
2 mL combined) to generate the desired variable concentrations in each tube.
Notice that each tube will contain a total of 5mL
Examples:
A) 2mL Algae + 2 mL spring water + 1 mL of 10% NaCl stock = a final NaCl
concentration of 2%.
B) 2 mL Algae + 1 mL spring water + 2 mL of 0.2M ammonia = a final concentration of
0.08M ammonia
4. Cover the top of each tube with a piece of wax paper or parafilm and mix by
inversion (or vortex gently, if available).
5. Immediately measure the absorbance at 430 nm using a spectrophotometer. This
wavelength is within a maximum absorbance peak for chlorophyll. Thus,
population density is measured indirectly by quantifying the amount of
chlorophyll present in a sample. (Other wavelengths can be used, representing
non-optimal absorbance).
6. Place all tubes in an area of equal lighting and temperature.
7. Measure absorbance at selected time intervals (Monday, Wednesday, and Friday
for 2-3 weeks would suffice). Remember to mix the tubes by inversion or
vortexing prior to spectrophotometer readings.
8. Record all data and present as a population growth graph as shown below. The yaxis (dependent variable) represents absorbance, and the x-axis represents elapsed
time in days (dependent variable).
Algal Population Growth
0.8
0.7
Absorbance 430nm
0.6
0.5
0.4
100uM
10uM
1uM
Control
0.3
0.2
0.1
0
Day 1
Day 3
Day 5
Day 7
Day 9 Day 11 Day 13 Day 15 Day 17 Day 19 Day 21 Day 23
Time Elapsed
Tube 1
Tube 2
Tube 3
Tube 4
Chlamy/Euglena
2 mL
2 mL
2 mL
2 mL
Soil Water
1 mL
1 mL
1 mL
1 mL
(Name and Concentration
of Chemical stock solution
used___10%_______)
__mL
__mL
__mL
__mL
Spring Water
__mL
__mL
__mL
__mL
Total Volume
5 mL
5 mL
5 mL
5 mL
[X] Chemical
Final Concentration of
Chemical in Test Tube
TO CALCULATE DILUTIONS OF CHEMICALS USE THIS FORMULA:
Number of mLs of Stock Solution
Total Volume of Dilution
[Concentration
of Stock Solution]
X
=
Final Concentration of
Chemical in Test Tube
For example:
HOW WOULD YOU PREPARE A 2% NACL SOLUTION FROM A 10% NACL STOCK
SOLUTION?
STEP 1: WE KNOW: Total volume of solution is 5 mL
Concentration of Stock solution is 10%
Final Concentration of Chemical desired is 2%
WE DON’T KNOW: The number of mL of Stock solution we need to prepare
this dilution (let X represent this value).
STEP 2: PLUG IN THE NUMBERS YOU KNOW INTO THE EQUATION AND SOLVE
FOR X:
X
5
10
*
=
2
STEP 3: SOLVE FOR X:
X
=
1
STEP 4: PUT IT ALL TOGETHER:
(remember, the units are in mLs)
To make 5 mL of a 2% solution from a 10% stock solution, mix 1 mL of 10% NaCl stock
solution plus 2 mL of sterile water for a volume of 3mL. When this chemical dilution is
added to the 2 mL of Chlorella solution, it will bring the total volume to 5 mL and further
dilute the NaCl to a final concentration of 2%.
Environmental Effects on Algal Growth Lab
Absorbance
(represents population density)
(measured at 430 nm)
Date of first reading (day 1)_______
Day
1
Chlamyd
omonas
Tube 1
Tube 2
Tube 3
Tube 4
Euglena
Tube 1
Tube 2
Tube 3
Tube 4
Day
Day
Day
Day
Day
Day
Day
Day