The primary endosymbiotic hypothesis of algae
Different early eukaryotes also engulfed photosynthetic bacteria (i.e. cyanobacteria). Descendants of this engulfment event are known as red and green algae. Cyanobacteria are gram-negative bacteria capable of undergoing oxygenic photosynthesis. Gram negative bacteria have two cell membranes (possibly from another engulfment event). In an event known as the primary endosymbiosis of the chloroplast, an early eukaryotic organism engulfed the double-membraned (Gram-negative) cyanobacterium, but did not consume it. The cyanobacteria produced glucose via photosynthesis and the eukaryotic host provided protection. The resulting proto-alga had three membranes: two bacterial, one eukaryotic. This hypothesis is supported by the fact that the plastid DNA of the red and green algae closely resembles that of cyanobacteria. Somehow the remnant eukaryotic cell membrane was lost (nobody knows how) Modern day green and red algae have chloroplasts made of two membranes, both which resemble cyanobacteria.