This semester I am not teaching environmental science, but for display purposes I have been setting up a few experiments. One which got a lot of attention at the booth I rented at the Pennsylvania Science Teacher Association conference last week was the algae culture I produced by applying different doses of carbon dioxide. Below are the I got after 3 and 8 weeks. The bottles in each picture are placed in order of decreasing doses of carbon dioxide (the last bottle on the right is the control):

3 weeks

8 weeks

Note that in the upper image representing three weeks, that the fastest initial growth is occurring in the bottles with intermediate doses of CO2. This is often the case, probably because the highest dose of CO2 has a slightly inhibitory effect. The difference between the highest dose and intermediate doses is no longer evident in the lower image representing eight weeks.