Your tree strips water from H2O and hydrogenates aromatic carbon to alkanes. Oxygen is the waste product.

WHat is available in a comet?

https://sites.google.com/site/cosmopier/palaeometeorstream/comet-s-chemical-composition

According to analysis of the spectra of Tempel 1 on July 4, 2005 reveals the IR signatures of a host of amorphous and crystalline inorganic powders. Included are minerals such as magnesium-rich forsterite and iron-rich fayalite (both in the olivine family); ferrosilite, an iron-rich pyroxene; and nontronite, a smectite clay containing iron, aluminum, and sodium. The spectra also feature telltale signs of other minerals, amorphous carbon, water ice, sulfides, and polyaromatic hydrocarbons.

The reduction of benzene (aromatic carbon) is exothermic is exothermic but does not yield as much energy as splitting the water requires, so the tree will have to use exogenous energy inputs like solar energy to make that happen. The alkanes produced are extremely long chains and are used to make the body of the tree: polyethylene plastics.

You will also need some regular plants because your astronauts immoderately exhale CO2 which will accumulate. The Dyson tree does not deal with oxidized carbon.

Your tree strips hydrogen from H2O and hydrogenates aromatic carbon to alkanes. Oxygen is the waste product.

WHat is available in a comet?

https://sites.google.com/site/cosmopier/palaeometeorstream/comet-s-chemical-composition

According to analysis of the spectra of Tempel 1 on July 4, 2005 reveals the IR signatures of a host of amorphous and crystalline inorganic powders. Included are minerals such as magnesium-rich forsterite and iron-rich fayalite (both in the olivine family); ferrosilite, an iron-rich pyroxene; and nontronite, a smectite clay containing iron, aluminum, and sodium. The spectra also feature telltale signs of other minerals, amorphous carbon, water ice, sulfides, and polyaromatic hydrocarbons.

The reduction of benzene (aromatic carbon) is exothermic is exothermic but does not yield as much energy as splitting the water requires, so the tree will have to use exogenous energy inputs like solar energy to make that happen. The alkanes produced are extremely long chains and are used to make the body of the tree: polyethylene plastics.

You will also need some regular plants because your astronauts immoderately exhale CO2 which will accumulate. The Dyson tree does not deal with oxidized carbon.