A saprophyte lives off of dead organic matter, breaking it down so that a plant can use it. An example of a saprophyte would be a mushroom. Bacteria (Rhizobia) live in or near the roots of some plants like legumes (peas, soybeans, peanuts) in nodules and fix nitrogen (N2 in the atmosphere is converted to ammonia that the plant can use) that helps the plant grow in a symbiotic manner. A prokaryote is a single celled organism that doesn't contain a nucleus. Many prokaryotes can fix nitrogen. A bacteria is an example of a prokaryote. Bacteria, which are prokaryotes act symbiotically to help fix nitrogen in legumes. So I would say that saprophyte doesn't fit.