4.3 Nonproteobacteria gram-negative bacteria and phototrophic  (Page 4/7)

The green nonsulfur bacteria are similar to green sulfur bacteria but they use substrates other than sulfides for oxidation. Chloroflexus is an example of a green nonsulfur bacterium. It often has an orange color when it grows in the dark, but it becomes green when it grows in sunlight. It stores bacteriochlorophyll in chlorosomes, similar to Chlorobium , and performs anoxygenic photosynthesis , using organic sulfites (low concentrations) or molecular hydrogen as electron donors, so it can survive in the dark if oxygen is available. Chloroflexus does not have flagella but can glide, like Cytophaga . It grows at a wide range of temperatures, from 35 °C to 70 °C, thus can be thermophilic.

Another large, diverse group of phototrophic bacteria compose the phylum Cyanobacteria ; they get their blue-green color from the chlorophyll contained in their cells ( [link] ). Species of this group perform oxygenic photosynthesis, producing megatons of gaseous oxygen. Scientists hypothesize that cyanobacteria played a critical role in the change of our planet’s anoxic atmosphere 1–2 billion years ago to the oxygen-rich environment we have today. A. De los Rios et al. “Ultrastructural and Genetic Characteristics of Endolithic Cyanobacterial Biofilms Colonizing Antarctic Granite Rocks.” FEMS Microbiology Ecology 59 no. 2 (2007):386–395.

Cyanobacteria have other remarkable properties. Amazingly adaptable, they thrive in many habitats, including marine and freshwater environments, soil, and even rocks. They can live at a wide range of temperatures, even in the extreme temperatures of the Antarctic. They can live as unicellular organisms or in colonies, and they can be filamentous, forming sheaths or biofilms. Many of them fix nitrogen, converting molecular nitrogen into nitrites and nitrates that other bacteria, plants, and animals can use. The reactions of nitrogen fixation occur in specialized cells called heterocysts .

Photosynthesis in Cyanobacteria is oxygenic, using the same type of chlorophyll a found in plants and algae as the primary photosynthetic pigment. Cyanobacteria also use phycocyanin and cyanophycin , two secondary photosynthetic pigments that give them their characteristic blue color. They are located in special organelles called phycobilisomes and in folds of the cellular membrane called thylakoids , which are remarkably similar to the photosynthetic apparatus of plants. Scientists hypothesize that plants originated from endosymbiosis of ancestral eukaryotic cells and ancestral photosynthetic bacteria. T. Cavalier-Smith. “Membrane Heredity and Early Chloroplast Evolution.” Trends in Plant Science 5 no. 4 (2000):174–182. Cyanobacteria are also an interesting object of research in biochemistry, S. Zhang, D.A. Bryant. “The Tricarboxylic Acid Cycle in Cyanobacteria.” Science 334 no. 6062 (2011):1551–1553. with studies investigating their potential as biosorbents A. Cain et al. “Cyanobacteria as a Biosorbent for Mercuric Ion.” Bioresource Technology 99 no. 14 (2008):6578–6586. and products of human nutrition. C.S. Ku et al. “Edible Blue-Green Algae Reduce the Production of Pro-Inflammatory Cytokines by Inhibiting NF-κB Pathway in Macrophages and Splenocytes.” Biochimica et Biophysica Acta 1830 no. 4 (2013):2981–2988.

Unfortunately, cyanobacteria can sometimes have a negative impact on human health. Genera such as Microcystis can form harmful cyanobacterial blooms , forming dense mats on bodies of water and producing large quantities of toxins that can harm wildlife and humans. These toxins have been implicated in tumors of the liver and diseases of the nervous system in animals and humans. I. Stewart et al. Cyanobacterial Poisoning in Livestock, Wild Mammals and Birds – an Overview. Advances in Experimental Medicine and Biology 619 (2008):613–637.

[link] summarizes the characteristics of important phototrophic bacteria.

Key concepts and summary

• Gram-negative nonproteobacteria include the taxa spirochetes ; the Cytophaga , Fusobacterium , Bacteroides group; Planctomycetes; and many representatives of phototrophic bacteria.
• Spirochetes are motile, spiral bacteria with a long, narrow body; they are difficult or impossible to culture.
• Several genera of spirochetes contain human pathogens that cause such diseases as syphilis and Lyme disease.
• Cytophaga , Fusobacterium , and Bacteroides are classified together as a phylum called the CFB group . They are rod-shaped anaerobic organoheterotrophs and avid fermenters. Cytophaga are aquatic bacteria with the gliding motility. Fusobacteria inhabit the human mouth and may cause severe infectious diseases. Bacteroides are present in vast numbers in the human gut, most of them being mutualistic but some are pathogenic.
• Planctomycetes are aquatic bacteria that reproduce by budding; they may form large colonies, and develop a holdfast.
• Phototrophic bacteria are not a taxon but, rather, a group categorized by their ability to use the energy of sunlight. They include Proteobacteria and nonproteobacteria, as well as sulfur and nonsulfur bacteria colored purple or green.
• Sulfur bacteria perform anoxygenic photosynthesis, using sulfur compounds as donors of electrons, whereas nonsulfur bacteria use organic compounds (succinate, malate) as donors of electrons.
• Some phototrophic bacteria are able to fix nitrogen, providing the usable forms of nitrogen to other organisms.
• Cyanobacteria are oxygen-producing bacteria thought to have played a critical role in the forming of the earth’s atmosphere.

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