before vowels chem-, word-forming element denoting "relation to chemical action or chemicals," from combining form of chemical (adj.), used to form scientific compound words from c. 1900. In 19c., chemico- was used.
1610s, "deductive reasoning," from Latin synthesis "collection, set, suit of clothes, composition (of a medication)," from Greek synthesis "composition, a putting together," from syntithenai "put together, combine," from syn- "together" (see syn-) + tithenai "to put, to place," from reduplicated form of PIE root *dhe- "to set, put." From 1733 as "a combination of parts into a whole." Earlier borrowed in Middle English as sintecis (mid-15c.). Plural syntheses.
<a href="https://www.etymonline.com/word/chemosynthesis">Etymology of chemosynthesis by etymonline</a>
Harper, D. (n.d.). Etymology of chemosynthesis. Online Etymology Dictionary. Retrieved $(datetime), from https://www.etymonline.com/word/chemosynthesis