Chloroplasts are considered endosymbiotic Cyanobacteria. Cyanobacteria are sometimes called blue-green algae even though they are prokaryotes. They are a diverse phylum of gram-negative bacteria capable of carrying out photosynthesis. Cyanobacteria also contain a peptidoglycan cell wall, which is thicker than in other gram-negative bacteria, and which is located between their two cell membranes. Like chloroplasts, they have thylakoids within them. On the thylakoid membranes are photosynthetic pigments, including chlorophyll a. Phycobilins are also common cyanobacterial pigments, usually organized into hemispherical phycobilisomes attached to the outside of the thylakoid membranes (phycobilins are not shared with all chloroplasts though).