are often seen as eukaryote-eukaryote hybrids, contributing to the confusion in the genetic analyses. A sister of Gloeomargarita lithophora has been engulfed by an ancestor of the Archaeplastida, leading to the plastids which are living in permanent endosymbiosis in most of the descendent lineages. Because both Gloeomargarita and related cyanobacteria, in addition to the most primitive archaeplastids, all live in freshwater, it seems the Archaeplastida originated in freshwater, and only colonized the oceans in the late Proterozoic. Morphology All archaeplastidans have plastids (chloroplasts) that carry out photosynthesis and are believed to be derived from endosymbiotic cyanobacteria. In glaucophytes, perhaps the most primitive members of the group, the chloroplast is called a cyanelle and shares several features with cyanobacteria, including a peptidoglycan cell wall, that are not retained in other members of the group. The resemblance of cyanelles to cyanobacteria supports the endosymbiotic theory. The cells of most archaeplastidans have walls, commonly but not always made of cellulose. The Archaeplastida vary widely in the degree of their cell organization, from isolated cells to filaments to colonies to multi-celled organisms. The earliest were unicellular, and many groups remain so today. Multicellularity evolved separately in several groups, including red algae, ulvophyte green algae, and in the green algae that gave rise to stoneworts and land plants. Endosymbiosis Because the ancestral archaeplastidan is hypothesized to have acquired its chloroplasts directly by engulfing cyanobacteria, the event is known as a primary endosymbiosis (as reflected in the name chosen for the group 'Archaeplastida' i.e. 'ancient plastid'). In 2013 it was discovered that one species of green algae, Cymbomonas tetramitiformis in the order Pyramimonadales, is a mixotroph and able to support itself through both phagotrophy and phototrophy. It is not yet known if this is a primitive trait and therefore defines the last common ancestor of Archaeplastida, which could explain how it obtained its chloroplasts, or if it is a trait regained by horizontal gene transfer. Since then more species of mixotrophic green algae, such as Pyramimonas tychotreta and Mantoniella antarctica, has been found. Evidence for primary endosymbiosis includes the presence of a double membrane around the chloroplasts; one membrane belonged to the bacterium, and the other to the eukaryote that captured it. Over time, many genes from the chloroplast have been transferred to the nucleus of the host cell. The presence of such genes in the nuclei of eukaryotes without chloroplasts suggests this transfer happened early