algal

noun

1. Any of a diverse group of chiefly photosynthetic and aquatic plantlike organisms that range from unicellular to large multicellular forms, are typically classified as protists, and include the green, yellow-green, brown, golden-brown, and red algae in the eukaryotes and especially formerly the cyanobacteria in the prokaryotes

   "… the divers were witnessing the collapse of the delicate symbiotic relationship between the coral animals and the single-celled photosynthetic algae, called zooxanthellae, that normally flourish within them."