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 is the most likely function of the smaller cells in the filament?
A) survival in the dark
B) nitrogen fixation
C) sulfur storage
D) nitrate storage

Question

Quizplus · Accepted Answer

The presence of evenly spaced cells that lack photosystem II but are smaller than the surrounding cells suggests that they are specialized cells called heterocysts that perform nitrogen fixation. Nitrogen fixation is an energy-intensive process that requires strict anaerobic conditions, which is facilitated by the presence of heterocysts in some filamentous cyanobacteria. The lack of fluorescence in these cells indicates that they do not participate in photosynthesis but rather serve to provide fixed nitrogen to the rest of the filament. The other choices (survival in the dark, sulfur storage, and nitrate storage) are not consistent with the information provided.

You Are Looking at a Sample from the Top Green