Presentation on theme: "Purple Bacteria Gram negative; about 30 species; bacteriochlorophylls a or b in cytoplasmic membrane invaginations, along with antenna pigments and ETS."— Presentation transcript:
Purple Bacteria Gram negative; about 30 species; bacteriochlorophylls a or b in cytoplasmic membrane invaginations, along with antenna pigments and ETS. These membrane invaginations may be vesicular, lamellar or tubular. Two alternative systems to generate reducing power: 1. Hydrogen is the electron donor and the first stage involves quinone reduction. The quinone reduces pyridine nucleotides (NAD(P)) directly. 2. Reduced sulphur (H 2 S, or sometimes S, thiosulphate or sulphite) is the electron donor and the first stage involves quinone reduction. Electrons subsequently pass through reverse electron transport to generate a protonmotive force for reduction of (NAD(P)). Purple Sulphur Bacteria CO 2 is fixed by the Calvin-Benson cycle; reverse electron transport generates reductant; cyclic photophosphorylation generates ATP. Most can grow photoautotrophically with H 2 as the electron donor, in anaerobic conditions, using inorganic C or photoassimilating acetate as a C source. Some can grow chemoautotrophically, under low oxygen conditions, using reduced sulphur. Some need exogenous vitamin B 12. Some require small amounts of H 2 S as a sulphur source. Some species are motile by means of flagella, and some possess gas vacuoles. Most deposit sulphur as intracellular granules, but Ectothiorhodospira deposit sulphur extracellularly. Genera: Thiospirillum (M), Ectothiorhodospira (M), Chromatium (M), Thiocystis (M), Thiocapsa, Lamprocystis (M, GV), Thiodictyon (GV), Thiopedia (GV) and Amoebobacter (GV). Purple Non-sulphur Bacteria Photoheterotrophs; most can grow photoautotrophically with H 2 and some with H 2 S, in low concentrations, or thiosulphate; many can grow aerobically in the dark by chemoautotrophism using H 2, e.g. Rhodobacter capsulatus; some can grow anaerobically in the dark; can photoassimilate fatty acids, other organic acids, primary and secondary alcohols, carbohydrates and aromatic compounds by photoheterotrophism. The same range of organic substrates photoassimilated may be respired aerobically in the dark (except benzoate which can not be respired). Prefer sulphide-poor freshwater habitats. Some, e.g. Rhodopseudomonas palustris and Rhodobacter sphaeroides are capable of denitrification, using organic compounds as energy sources. Rhodobacter sphaeroides can grow on nitrate as the sole N source by denitrification and nitrogen fixation. Most require the vitamins biotin, niacin and thiamin, and Rhodocyclus purpureus needs vitamin B12. Most will also grow better if amino acids are supplied. Genera: Rhodospirillum (M), Rhodopseudomonas (M), Rhodomicrobium (M, prosthecate, exospores), Rhodopila (M), Rhodocyclus (M or I), Rhodobacter (M or I). Rhodopseudomonas reproduces by budding from the cell pole, Rhodomicrobium by budding from the hyphal tip, the rest reproduce by binary fission.
Green Bacteria Bacteriochlorophylls a, c, d, e in stalked vesicles attached to the CM, the stalk or baseplate contains the bacteriochlorophyll a antenna pigment and the chlorosome bacteriochlorophyll c, d or e, and the CM contains the reaction centre and ETS. Have PSI only. Green Sulphur Bacteria Small, usually immotile bacteria; 5 genera. Strictly anaerobic photoautotrophs that use H 2 S, H 2 or other reduced sulphur compounds as an electron donor; the sulphur produced is deposited extracellularly prior to oxidation to sulphate. Require sulphide as a source of sulphur and some require vitamin B 12. Many can fix nitrogen. Occur, along with purple sulphur bacteria, in sulphide-rich anaerobic illuminated aquatic environments. Can photoassimilate acetate if CO 2 and H 2 S are also supplied. Fix CO 2 by the reductive (reverse) TCA cycle. Genera: Chlorobium, Prosthecochloris (prosthecate), Pelodictyon (GV, forms nets), Ancalochloris (GV, prosthecae) and Chloroherpeton (gliding filamentous). Green Non-sulphur Bacteria (Chloroflexus Group) Chloroflexus is a filamentous gliding bacterium, with filaments up to 300 m long and are thermophiles, thriving in neutral or alkaline springs at 45-70 o C. It is a photoheterotroph and facultative photoautotroph or facultative chemoheterotroph. Form orange to dull-green mats several mm thick. Produce bacteriochlorophylls under anaerobic conditions and can not grow at all anaerobically in the dark. Can grow aerobically, in complex media, in the light or dark, but have very low bacteriochlorophyll content, and so appear orange due to carotenoids. Obtains organic matter from cyanobacteria, such as Synechococcus, with which it can be grown in a mineral medium. Heliothrix is also a gliding filamentous organism. Genera: Chloroflexus (gliding filamentous), Chloronema (GV, gliding filamentous), Heliothrix (gliding filamentous) and Oscillochloris (trichomes, gliding motility, GV). Heliobacterium Heliobacterium chlorum are gliding rod-shaped cells; can grow as anaerobic photoheterotrophs requiring biotin and fixing N 2. Contain bacteriochlorophyll g. No chlorosomes, pigment probably in the CM.
Cyanobacteria Nitrogen Fixation Cyanobacteria are the only organisms able to perform both oxygenic photosynthesis and nitrogen fixation. Low concentrations of oxygen rapidly and irreversibly inactivate the nitrogenase enzymes. Most of these cyanobacteria are filamentaous and produce specialised nitrogen-fixing cells, called heterocysts. Heterocyst and nitrogenase synthesis is repressed when combined nitrogen is already present. Lack of combined nitrogen stimulates heterocyst and nitrogenase production, but if N 2 is also absent, then development arrests at an intermediate stage, called the proheterocyst. About 5-10% of the cells develop into heterocysts in a 30 hour period. Heterocyst. The heterocysts have thick outer wall layers and the thylakoids become concentrated near the cell poles and special polar connections form where the heterocyst is attached to vegetative cells. Chlorophyll a is present, but phycobiliproteins are absent. PS II is inactive and rubisco is also lacking, so they can not fix CO 2 nor produce O 2 in the light. Respiration uses H 2 generated during nitrogen fixation (nitrogenase produces one H 2 for every N 2 fixed). The heterocyst depends upon the vegetative cells to supply reductant. Heterocysts are formed at regular intervals in long filaments. Adjacent cells in the filament are joined by minute channels called microplasmodesmata that exchange metabolites between cells. Non-heterocystous nitrogen-fixing cyanobacteria are facultative anoxygenic photosynthesisers and fix nitrogen under anaerobic growth conditions. Anoxygenic Photosynthesis Oscillatoria limnetica, found in hypersaline lakes, is capable of sulphide-dependent CO 2 photoassimilation. Sulphide inhibits PS II and induces enzymes that allow sulphide to donate electrons to PS I. The oxidised product is elemental sulphur, which accumulates as extracellular granules. In the dark, stored polyglucose can be respired anaerobically, using sulphur as the electron acceptor, or else the cells can undergo homolactic fermentation. Many strains are facultatively chemotrophic in the dark, but these maintain constituitve photosynthetic apparatus and can photosynthesise immediately when light is introduced. Many phycoerythrin-producing strains exhibit complementary chromatic adaptation: when grown in green light they have a high phycoerythrin to phycocyanin ratio, but when grown in red light they have very little phycoerythrin. This response appears to be mediated by a phytochrome-like light-sensitive pigment.
Nostoc colonies Below left: a cyanobacterial surface crust covering an arid soil surface in Utah. These colonisers add nitrogen and biomass. They rapidly hydrate and resume growth when moisture is present, producing the nodules shown below right. Left: green snow in springtime glacial regions contains cyanobacteria. Cyanobacteria also occur inside rocks (endolithic) in Arctic and Antarctic deserts and inside limestone and inside coral rubble and coral sand. Others deposit limestone in reefs and hot springs.
Left: stromatolites (stromatoliths) are large columnar deposits of calcium carbonate built-up over immense periods of time by cyanobacteria. The oldest fossil stromatolites are 2.7 billion years old. Symbiosis Hornworts (Anthocerophyta) have internal mucilage-filled chambers that contain endophytic nitrogen-fixing cyanobacteria. Hornworts are primary colonisers of wet exposed areas.
Phototrophic Archaebacteria Halobacterium Flagellated, phototactic rods. In aerobic conditions the cell membrane is red, due to carotenoids and the cells grow as chemoheterotrophs. However, in anaerobic conditions, purple membrane is laid down in discrete patches (about 50% of total membrane area) 75% of which is bacteriorhodopsin, the remainder being lipid. Bacteriorhodopsin spans the membrane 7 times and is in contact with both the external and internal media and has a retinal carotenoid chromophore linked via a Schiff base to a lysine residue. Reaction with (orange) light causes a conformational change in the protein and deprotonation of the Schiff base. The Schiff base is reprotonated with protons from inside the cell and deprotonation releases these protons to the outside. Thus, bacteriorhodopsin is a light-driven proton pump that generates a proton gradient for ATP synthesis. This process maintains the cell under anaerobic conditions, but does not allow it to grow, since O 2 is needed for retinal synthesis from beta-carotene. Photoheterotrophic growth occurs when O 2 levels are high enough to synthesise bacteriorhodopsin, but not too high to induce chemoheterotrophic growth. The solubility of O 2 is low in the brine in which these halophiles grow. Halobacteria give many salt lakes and salt fields their pink colour: