Silent Running (1972)

Silent Running is a 1972 American environmental-themed science fiction film. It is the directorial debut of Douglas Trumbull, and stars Bruce Dern, Cliff Potts, Ron Rifkin, and Jesse Vint.
In the future, all forests on Earth have become extinct from careless environmental exploitation. As many specimens as possible have been preserved in a series of enormous greenhouse-like geodesic domes attached to large cargo spaceships, forming part of a fleet of eight "American Airlines Space Freighters", stationed outside the orbit of Saturn.

Freeman Lowell, one of four crewmen, is the resident botanist and ecologist on one of these ships, the Valley Forge. He carefully maintains a variety of plants for their eventual return to Earth and the reforestation of the planet. He spends most of his time in the domes, cultivating the crops and attending to the animal life.

The crew of each ship receives orders to jettison and destroy their domes and return the freighters to commercial service. After four of the six Valley Forge domes are jettisoned and destroyed with nuclear charges, Lowell rebels and opts to save his ship's plants and animals.