These are the simplest of all living organisms. They differ from members of the four other kingdoms because they are single-celled organisms with little organization to their cell structure. Monerans are cells with no membrane-bound structure protecting their genetic material. They reproduce by simply dividing into two separate cells. Blue-green algae and some bacteria make their own food from sunlight, like green plants do.
Some bacteria use nitrogen, sulphur, hydrogen, or iron to create their food.
However most bacteria break down and absorb compounds made by other organisms. Bacteria are necessary organisms in most ecological systems. The cycling of nitrogen, which is essential for plant and animal growth, depends upon bacteria. Dead and dying material is broken down by bacteria and fungi, allowing nutrients to be recycled. Bacteria or algae form the main food source for many other organisms.