When it comes to cellular respiration, it is pretty clear that the theme of unity and diversity is relevant. Despite the variety of life forms we have on Earth, one thing remains the same among them, and that is glycolysis. Glycolysis unites diverse organisms, as it happens in almost every group of organisms alive today.
The other parts of cellular respiration, the citric acid cycle and oxidative phosphorylation, are shared by all eukaryotes and many prokaryotes. These organisms are further united by their energy metabolism.
All cells need energy, no matter if they are the entire organism or one cell among millions. The fact that all eukaryotes carry out cellular respiration is pretty impressive. Humans, seagulls and mushrooms are all doing the same thing when it comes to breaking down glucose. To add to the excitement, metabolic processes can break down a diverse range of carbohydrates, proteins, and fats by converting them to glucose before cellular respiration.
Vibrio chlorae, the bacterium that causes cholera, uses anaerobic respiration as its metabolism.
Only eukaryotes have mitochondria, though, so how the heck do prokaryotes get energy? They don't have the fancy citric acid cycle pinball game in their cells. Prokaryotes are diverse in their metabolisms.
Some prokaryotes are photosynthetic, and also use an electron transport chain to make ATP. Cellular respiration may actually have evolved from modifying photosynthetic processes to extract energy from food.
Other prokaryotes are diverse in their metabolisms; some need oxygen, others can live without it. Organisms that do not use oxygen in their metabolism are called anaerobes. Anaerobes are a large, diverse group. Some anaerobes live by fermentation and others use different electron acceptors, such as nitrate or sulfate ions, at the end of their electron transport chains. Some prokaryotes fix nitrogen (N2) from the atmosphere and make ammonium from it. Others use a metabolic process called nitrification, oxidizing ammonia to nitrite. Another possible metabolism is denitrification, which reduces nitrate to nitrite. Although these are diverse forms of metabolism, they are united by redox reactions that allow energy transfers to be made.
Some bacteria that live in the ocean use oil for their energy—usually, they metabolize naturally-occurring oil but they can also be used to clean up oil spills. Other prokaryotes reduce sulfate, and some oxidize hydrogen gas to make methane.