Positronic Synthesis

A complex but theoretical biological process that an organism could theoretically use to synthesise antimatter safely. 

The process is highly reliant on a type of theoretical enzyme that can contain antimatter in walls of Hardlight. Each enzyme only stores a small quantity of antimatter, which is used by the enzymes to power themselves and maintain the hardlight walls as hardlight decays. This releases heat, so organisms using this process would need to vent excess heat.

Because of the sheer strength of the hardlight barrier, it is very difficult to cause it to break and release the contained antimatter, which is only released under very specific conditions - it is thought that only the internal systems of an organism can achieve this but the organisms exploit this so that they can utilise the antimatter when desired. There is also a very small quantity of antimatter inside each individual enzyme, so breakage should only release a small quantity of heat. This makes this type of energy storage extremely efficient and allowing a creature to survive for years without food, as long as their other needs are met (including having sufficient nutrients, as opposed to the energy which antimatter provides). 

Any organisms which use this process would have to be explicitly designed to use this process from the start, or perhaps evolved form another organism which does. The organism would need to fabricate its own antimatter containment enzymes and would need to rely mainly on that enzyme through their entire digestive process else they risk allowing antimatter to escape and setting the creature in question on fire. However, as the enzyme is self powering, it should remain safe even if excreted from the creatures body, which might happen if they have too much energy and not enough nutrients to repair their body with. The enzymes would have to be very large and complex to perform all these functions, so an ecosystem utilising this might have to be engineered. If this phenomenon is seen within an ecosystem, the entire ecosystem is likely to depend on it because this becomes any creature's primary power source. They would need a source to consume antimatter from, sparking the necessity that all creatures would need it, including producers. This has been simulated in virtual universes, and is shown to work as long as producers have sufficient energy to create antimatter from. Usually this comes in the form of gamma rays from very hot stars and sometimes the planet having a weak atmosphere.

Because of the immense power density of antimatter, it can be used as an ATP to store a huge amount of energy in a much less mass and volume-dense method than storing regular fat. 

It is also possible that a sentient species could make use of the process, and it's highly likely they would only need to eat very rarely but they would probably have to gather nutrients in another way. They might literally eat dirt to process the nutrients present, but as they are in a highly entropic form this would be extremely energy-negative, which is likely far less of a problem for beings powered by antimatter. Their digestive systems might be complex to handle this, but they would still have enough energy to sustain sentience.