Anabaena is a simple multicellular photosynthetic cyanobacterium. Under specific environmental conditions, newly developing cells along a filament express genes that code for adaptive enzymes and become non-photosynthetic heterocysts. Heterocysts do not carry out photosynthesis but instead, through chemical signals received from nearby cells, specialize to extract a critical substance from the environment. This substance can then be shared with surrounding cells as a building block to help them in their synthesis of polypeptides and nucleic acids. What is the most likely identity of the substance?