Another tale set in Slonczewski's distinctive far-future universe, the action here taking place some two centuries after that of Daughter of Etysium (1993). Planet Prokaryon's native biochemistry is poisonous to humans, so those who wish to live there must be genetically modified; infants adapt rapidly, but adults take years to adjust. The life-reverent Spirit Callers, led by the human Brother Rod and a couple of Sentients (artificial intelligences who have ""woken up"" and earned their freedom) named Geode and the Reverend Mother Artemis, have started a colony comprising orphan children rescued from the plague-stricken planet L'li. Oddly, though no intelligent life-forms have been found, Prokaryon's flora grow in orderly ranks, it rains only at night, and if forest fires break out, rainstorms at once extinguish them. The colonists eke out a living from farming and trading in gemstones--until the immortal Elysian, Nibur, gains control of the planet and orders them to evacuate (Nibur proposes to sterilize the continent and lease it for development). Rod, Artemis, and Sarai, a biology-whiz Sharer who's doing research, must prove that Prokaryon's life-forms are sentient in order to thwart Nibur's plans. Problem is, these intelligences appear to be the size of bacteria, and none of the visiting officials and bigwigs will believe them--until they attempt to leave Prokaryon, only to discover that they've been infected with the brainy microbes! Beautifully constructed and absorbingly related--but many will find Slonczewski's thinking-bacteria notion (cf. Greg Bear or Michael Kanaly) very hard to swallow.