Lichens are difficult to classify. They are an interaction between a fungus and a photosynthetic species, usually green or blue green algae. The species form a symbiosis, but there is a debate as to what the association between the two species really is. To answer this question, researchers designed a series of experiments. They took advantage of the photosynthetic abilities of an algae to examine the role of association with fungi in the production of sugar, a product of photosynthesis. The carbon in the sugar molecules comes from carbon dioxide (CO2) in the air. Researchers exposed the algae to CO2 molecules containing radioactive carbon. The following experiments were done and the data is graphed (note: all graphs use the same scale).
Algae were grown in culture in the lab and exposed to radioactive CO2. The growth rates and production of sugar containing radioactive carbon are shown in the graphs below.
Algae were grown in the presence of fungi (therefore, this is lichen grown in the laboratory). The lichen was exposed to radioactive CO2. The growth rate of the algae species and the production of sugar containing radioactive carbon are shown in the graphs below.
Lichen were grown under dehydration conditions while exposed to radioactive CO2. The growth rate of the algae and the production of sugar containing radioactive carbon are shown in the graphs below.
|A||The fungal species causes a decrease in sugar production in the algae.|
|B||The fungal species causes an increase in sugar production in the algae.|
|C||The algae cannot use the radioactive CO2 efficiently to produce sugar.|
|D||The algae grows better in the presence of the fungus.|
The graphs clearly show that, when the fungal species is present, the production of sugar is greatly increased and growth of the algae is decreased. Since sugar is made, the algae has no problem using the radioactive isotope of carbon.