You are looking at a sample from the top green layer of a colorful bacterial mat. Using phase contrast and fluorescence microscopy, you see evenly spaced cells in the middle of a filament that are slightly smaller than the others. The smaller cells are not fluorescent, indicating they lack photosystem II, but the rest of the filament is fluorescent. What type of filamentous bacteria are you most likely looking at?
A) purple nonsulfur bacteria
B) green sulfur bacteria
C) Acidobacteria
D) Cyanobacteria

Question

Quizplus · Accepted Answer

The smaller cells that lack photosystem II indicate that the filamentous bacteria are likely a type of cyanobacteria. Cyanobacteria are photosynthetic prokaryotes that are capable of oxygenic photosynthesis and have specialized cells called heterocysts that lack photosystem II. These cells fix atmospheric nitrogen and provide the rest of the filament with a source of nitrogen for growth. The fluorescence observed in the rest of the filament is likely due to the presence of chlorophyll a, which is used in photosynthesis. Purple nonsulfur bacteria and green sulfur bacteria are types of anoxygenic photosynthetic bacteria that do not perform oxygenic photosynthesis and are not filamentous. Acidobacteria are a phylum of bacteria that are known for their diversity in soil environments and are not known to form filamentous structures.

You Are Looking at a Sample from the Top Green