symbiont

noun

1. An organism that lives in a symbiotic relationship.

   “The close analogies between DNA-containing eukaryotic cell organelles and microbial symbionts require revision of classic cell theory, wrote Scwemmler and Schenk (1980) on introducing the field of endocytobiology.”

   “It had been known that sponges somehow took in dissolved organic matter, but it remained unclear whether they could do it on their own or needed help from their bacterial symbionts.”