ATP synthase is used by cells to synthesise ATP from ADP using energy from a proton gradient. Depending on which sites I look at, this is either a universal feature of living cells, or it's "almost universal", used by the vast majority of organisms.
As Superbeast points out in a comment, ATP can also be synthesised on the substrate level via the Krebs cycle. However, I would like to know if there are any organisms that only synthesise ATP this way. In other words, are there any known organisms that entirely lack the ability to produce ATP synthase, and produce ATP entirely through the Krebs cycle, or through some other mechanism not related to ATP synthase?
If there are, I would like some information about them. Are there many such species or just a few? Are they all prokaryotes or all eucaryotes, or some of each? What kind of environments do they live in?