autochemotrophs are a thing. They mostly live in the deep ocean in hot water vents that are filled with reduced sulfur compounds, but in an irradiated environment, a strong scouring UV source that breaks atomic bonds down (and thus produces energetic free radicals that can then be used for metabolic purposes) could enable a plethora of inorganic substances to be used as chemical energy sources. It would mean that our biology would be very high-energy resistant, and thus would need pretty strongly chemically active materials to use as food. (Strong acids, highly reactive alkaline earth metals, corrosive gasses like chlorine, etc) that form very strong bonds, and have strong bond energies. (Otherwise the solar radiation would degrade our organism as well!)
I am thinking this:
Silicon based life that uses chlorine instead of oxygen. Perhaps with silicon tetrachloride + water reaction to produce silicon oxide and hydrochloric acid as waste product. The water should be the result of internal chemistry from the decomposition of minerals, via UV radiation, in a process similar to photosynthesis. (in that a high energy radiation source is harnessed as an energy source to produce substances inside a lifeform. In this case, it is the harsh high energy particles and UV light from our star.) This would allow us to produce a silicondioxide exoskeleton, (or a complex of compound silicates to exploit chirality and crystal structures to create something both flexible and highly resistant to radiation, external sources of water, and mechanical attack)
This kind of metabolism would enable us to have a ready supply of "Highly deadly" waste we could employ against ordinary lifeforms.
Since chlorine is so reactive, suggest we consume it in the form of rare-earth salts, and to have evolved to consume such salts in crystallized form, as well as adaptations (evolutionary) to consume strongly saline water (despite the toxicity of the water.)
Flavor description: Exoskeleton replete with microscopic ridges and structures between secreted silicate mineral crystal type boundaries, which capture and absorb high energy photons, and convert that energy directly into internal electrical potentials. (Photovoltaics, and the like.) (see also,
this article about how adding these to silicon based photo cells improves their performance.) This would make our appearance very "Prismatic", similar to the "Structural colors" found in blue insects, or on peacock feathers.
Basically-- We eat salt, and rocks. We exhale/urinate concentrated hydrochloric acid solution as a metabolic biproduct. We utilize the rare-earth minerals and silicon compounds in the surrounding regolith, along with the ever-present high energy radiation from our star to perform this chemistry. Our internals are liquid solutions suspended in silicon chloride solution. Water is theoretically toxic to us, but our exoskeletons prevent exposure from outside sources being harmful to us, and our digestive systems produce water in trace amounts as a biproduct, necessitating their resistance to it. We do not respire in the normally conventional way; We expel a vapor/mist as a waste product, but do not inhale. We metabolize some of the water our primary energy consumption mechanism produces to create silicon rubbers, and other flexible materials for our internal body structures. Our exoskeletons are thick, and ornate in appearance, with a shimmering prismatic quality, due to their use as both environmental protection and as an energy harvesting organ. Nanostructures in the exoskeleton force lower energy photons to be re-emitted as higher energy ones, (shorter wavelengths) by acting as naturally occuring nanoscopic waveguides. To humans, our appearance is black and iridescent. Our optical systems are adapted to use higher frequencies of light than humans. We utilize light in the ultraviolet and Xray spectra. We appear translucent to other members of our species.
Downside: We *NEED* the high energy environment to survive. Being naked in space poses little trouble to us aside from temperature and internal pressure regulation; we do not breathe, and the harsh, unfiltered radiation of a star is our food. Requirement of chloride minerals limits our ability to move to naked planetoids. (Without an atmosphere to help recycle those compounds via reaction with a local regolith, we would quickly run out of metabolic material and starve to death, despite being surrounded by otherwise edible rocks. HCl is a gas, and would escape into space without a suitably strong gravity well, and atmosphere.) As such, we still need "Biospheres", and space suits.