The blue-green algae, or cyanobacteria, are classified within the phylum Cyanophyta. This diverse group includes unicellular, colonial, and filamentous forms, and is common in both marine environments and freshwaters. Like other bacteria, the cells are prokaryotic and therefore lack nuclei, chloroplasts, mitochondria, and flagella, but are autotrophic. Cyanobacteria are the only bacteria that produce oxygen as a photosynthetic byproduct like other algal groups and land plants. The cells contain chlorophyll a and phycobilin and carotenoid pigments, and have membranous thylakoids. Many genera have specialized cells or structures, including heterocysts involved in nitrogen fixation and resting cells called akinetes. Cyanobacteria are notorious for forming blooms that can cause a variety of problems, some of which are toxic.