If the chlorophyll is removed from the bacteria, but the bacteria are otherwise unharmed, they no longer show any tendency to move into the areas lit with the particular shade of red. The bacteria show little tendency to move into areas containing light in colors other than the particular shade of red, even if their chlorophyll can produce some energy from light in those colors. There are species of bacteria that do not contain chlorophyll but do move into areas lit with particular colors when placed in a test tube lit with different colors in different places. The bacteria show no tendency to move into areas lit with blue even when those areas are lit so brightly that the bacteria's chlorophyll produces as much energy in those areas as it does in the red areas.
Biologist: When bacteria of a particular species are placed in a test tube that has different areas lit with different colors of light, the bacteria move only into the areas lit with a particular shade of red. The bacteria contain chlorophyll, a chemical that allows them to produce energy more effectively from this color of light than from any other. This suggests that the bacteria detect this color of light by monitoring how much energy their chlorophyll is producing.
Which one of the following, if true, most weakens the biologist's argument?