Konstantin Sergivich Merezhkovsky (1855-1921) (also transliterated Konstantin Sergeevich Merezhkovsky, Constantin Sergeevič Mérejkovski, Constantin Sergejewicz Mereschcowsky, Konstantin Sergejewicz Mereschkovsky and Konstantin Sergejewicz Mereschkowsky) was a prominent Russian biologist and botanist active mainly around Kazan, whose research on lichens led him to propose the theory of symbiogenesis - that larger, more complex cells evolved from the symbiotic relationship between less complex ones. He presented this theory in the 1926 book Symbiogenesis and the Origin of Species. However, he had used the term as early as 1909, and the fundamentals of the idea had appeared in his 1905 work, The nature and Origins of Chromatophores in the Plant Kingdom.
He was inspired by his work as a leading lichenologist - lichens were of major interest at the time as it had recently been shown that they exhibit a symbiotic relationship between fungi and algae. Around the turn of the century he collected a sizeable lichen herbarium, containing over 2000 specimens from lands in Russia, Austria and around the Mediterranean. The collection is currently in the possession of Kazan University. He also studied hydras.
Merezhkovsky rejected Darwinian evolutionary theory. He did not believe that natural selection could explain biological novelty, rather the acquisition and inheritance of microbes. He was criticised by another Russian lichenologist, Alexandr Alexandrovich Elenkin .