These microbes, these nitrogen-fixing microbes, are special because they have an enzyme called nitrogenase. This enzyme catalyzes the break-up of that strong bond holding nitrogen gas together, freeing up the nitrogens to become ingredients in more accessible chemicals like ammonia. There's one catch though: nitrogenase stops working completely when it's exposed to oxygen. So nitrogen-fixing organisms have a few options for how to live. Many of them live in anaerobic conditions, where the lack of oxygen means their vital enzyme will be safe. And other organisms, like the cyanobacteria we've been looking at, create their own internal forms of protection, sequestering the nitrogenase into special cells called heterocysts. You can see the heterocysts standing out like special green beads on this dolichospermum. And they look so different because they need thick walls so that oxygen can't pass through, which allows the nitrogenase to do its thing.