How would an ecosystem based around chemotrophs produce oxygen?
Outline
A planet which is heated tidally by its Brown Dwarf parent and permanently dark needs to have native alien life which has [somewhat similar biochemistry to Earth] and also be [permanently habitable] by Humans without [extensive life support] and/or [extensively genetically engineered] bodies.
This planet has very active tectonics and volcanism, a result of this tidal heating, a combination of this volcanism and the biochemical nature of this planet's ecosystem should be be capable of producing an atmosphere composed of 22% oxygen, 50-70% nitrogen, 0.02-5% CO2, ~7% water vapor, + [leftover] + trace.
If this atmosphere is not achievable, the atmosphere needs to be otherwise [permanently habitable] to humans, these humans could have physiological adaptations to adapt to this atmosphere through non-[extensive genetic engineering].
Volcanism on this planet produces similar gases and particulates in similar concentrations to those on Earth (just use Earth numbers);
The principal components of volcanic gases are water vapor (H2O), carbon dioxide (CO2), sulfur either as sulfur dioxide (SO2) (high-temperature volcanic gases) or hydrogen sulfide (H2S) (low-temperature volcanic gases), nitrogen, argon, helium, neon, methane, carbon monoxide and hydrogen. Other compounds detected in volcanic gases are oxygen (meteoric), hydrogen chloride, hydrogen fluoride, hydrogen bromide, nitrogen oxide (NOx), sulfur hexafluoride, carbonyl sulfide, and organic compounds. Exotic trace compounds include mercury, halocarbons (including CFCs), and halogen oxide radicals. 258
(From wikipedia: volcanic gas)
Most members of Trophic level 0 (producers) on this planet produce energy through the chemical equation:
12H$2$S + 6CO$_2$ → C$_6$H${12}$O$_6$ + 6H$_2$O + 12S
However other methods do exist; such as those relating to elemental Sulfur, ferrous Iron, etc. (see Chemotroph) Photosynthesis is not a viable way to produce energy on this planet.
Through some [reasonable] system, (either other processes which these trophic level 0 lifeforms engage in or through related related lifeforms, etc.) this combination of geology and ecosystem needs to produce the atmosphere outlined above, or at least an atmosphere habitable by Humans.
Note that really, the main goal of this is to create an ecosystem which can produce oxygen through some chemical process involving life, so that this planet can have Earth-like quantities of oxygen in its atmosphere
Note that CO2 can have a lot of variance in its percentage of the atmosphere, greenhouse heating can be countered simply by moving the planet farther away from the brown dwarf, though this only works while the planet is still being heated tidally (CO2 likely wouldn't be produced as much from volcanic gases because less tidal heating means less active volcanoes); this is why I've let the range of allowed CO2 concentrations be between 0.02% and 5%)
Definitions
somewhat similar biochemistry to Earth:
Humans can gain energy through carbohydrates from alien flora and fauna, and alien life lacks substances which are particularly dangerous to Humans. Note that this does not necessitate things like having nutrients, proteins, and lipids which are necessary for Human life.
permanently habitable:
Humans can live on this world for an excess of 15 thousand years
extensive life support:
Life support apparatuses which either contain
entirely different atmospheres inside the apparatus than outside, or
are built to keep hazardous gases and particulates from contacting all parts of the Human body
(Eg. this does not include things like gas-masks)
extensively genetically engineered:
Different from Earth-humans to an extent that
radically changes the human appearance to an extent where we would not recognise them as human,
extensively changes the physiology of humans (adding organs or systems, changing the basic function of organs or systems, significantly changing the human skeletal structure)
Note that this does not include things like adding some kind of filter to the Human respiratory system which gets rid of dangerous gases or particulates, adapting human skin to be resistant to corrosive atmospheric gases or particulates, etc.
Reasonable
The system needs to have some reason to exist; for example, water electrolysis is an obvious way to produce oxygen from the products of H$_2$S chemosynthesis, however the lifeforms on this planet need to have a reason (and way) to engage in water electrolysis. 279