Gloeomargarita lithophora, a cyanobacterium, is the proposed closest present day relative of all chloroplasts[1] (except for the independently evolved in the amoeboid Paulinella chromatophora).

The ancient relative of Gloeomargarita's was engulfed by a eukaryotic host in a singule endosymbiotic event around 1900-1400 million years ago.

[2][3] The origin of plastids by endosymbiosis signifies the beginning of photosynthesis in eukaryotes,[4] and as such their evolutionary relationship to Gloeomargarita lithophora, as the sister group,[3] is of high importance to the evolutionary history of endosymbiotic organelles and photosynthesis.

G. lithophora are gram-negative, unicellular rods with oxygenic photoautotrophic metabolism and gliding motility.

[5] Some evidence suggests that Gloeomargarita lithophora could serve as a biological buffer to treat water contaminated with strontium, barium, or radioactive pollutants such as radium.