Note: Nostoc was replaced by Anabaena. All answers apply to both Nostoc and Anabaena.

What form does this cyanobacterium display?

Nostoc is a filamentous cyanobacteria.

Some cells in the Nostoc sample appear larger than the others. What are these cells and what is their function?

These cells, called heterocysts, are sites where gaseous nitrogen is converted to ammonia or nitrate. Heterocysts are oxygen excluding cells, a requirement for nitrogen fixation as the enzyme used to break up gaseous nitrogen does not function in the presence of oxygen.

Why is this function important to Nostoc and to other neighboring organisms?

Nitrogen gas is very stable and does not readily combine with other elements. As a result, living organisms cannot simply take nitrogen from the air and use it to make proteins and nucleic acids. The incorporation of nitrogen gas into organic compounds by Nostoc, known as nitrogen fixation, "fixes" nitrogen gas into a form that living organisms can use.

Back to Bacteria Index Page