Photoheterotrophs require organic molecules for metabolic processes and synthesize ATP using light energy. Most bacteria are chemoheterotrophs, which require organic molecules as a source of both energy and organic carbon. The ability or inability to survive in the presence of oxygen also reflects variation in metabolism. Several groups of bacteria metabolize nitrogen compounds unavailable to other organisms. By doing so, these prokaryotes play critical roles in the cycling of nitrogen in the environment.
Protists exhibit the most diverse spectrum of structure and life cycles of all known organisms.
Most protists are aerobic in metabolism. Mixotrophs combine photosynthesis and eating for nutrition. Protists are eukaryotes. If saprotrophs are the same things as saprobes then saprobic fungi absorb nutrients from nonliving organic material, such as fallen logs, animal corpses, or the wastes of live organisms and, in the process, the fungi decompose this material. Fungi secrete digestive enzymes.