USA Biology Olympiad (USABO) Practice Exam

What is the most likely type of organism that serves as the photobiont in a lichen?

1. Fungi

2. Bacteria

3. Green algae or cyanobacteria

4. Protozoans

The correct answer is: Green algae or cyanobacteria

The correct answer is the type of organism that primarily serves as the photobiont in a lichen is green algae or cyanobacteria. In a lichen, the photobiont is the photosynthetic partner that produces carbohydrates through photosynthesis, providing energy and organic compounds for both itself and the fungal partner. Green algae and cyanobacteria are commonly found as photobionts due to their ability to efficiently harness sunlight to create organic matter. Typically, green algae contribute a significant amount of the carbon needed for the symbiotic relationship, while some lichens with cyanobacteria can also fix atmospheric nitrogen, which benefits both partners by providing essential nutrients. This mutualistic relationship is key to the resilience and adaptability of lichens in varied environments. Other organisms listed, such as fungi, do not serve as photobionts. Fungi mainly provide structure and protect the photobiont while absorbing water and nutrients from the environment. Bacteria and protozoans are not typically involved in the photobiont role within lichens, as they do not perform photosynthesis in the same way that green algae and cyanobacteria do. Thus, the choice of green algae or cyanobacteria accurately reflects the composition and functionality of the symbiotic