Chemoautotroph. Definition. noun, plural: chemoautotrophs. An organism (typically a bacterium or a protozoan) that obtains energy through chemical process, which is by the oxidation of electron donating molecules from the environment, rather than by photosynthesis. Supplement.n organism (typically a bacterium or a protozoan) that obtains energy through chemical process, which is by the oxidation of electron donating molecules from the environment, rather than by photosynthesis. Supplement.