The word "decomposes" refers to the process of breaking down organic matter into simpler substances. Antonyms for this word could include terms such as "builds up," "constructs," or "synthesizes." These words signify the opposite of decomposition by implying the creation or development of new structures or substances. Other antonyms for "decomposes" might include "preserves," "maintains," or "conserves," as these words suggest the protection or stabilization of existing matter. Overall, antonyms for "decomposes" are words that convey growth or stability, rather than decay or degradation.