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• Phototrophy – Conversion of radiant energy from the sun into ATP and NADPH • Autotrophy involves carbon fixation – Conversion of inorganic carbon into organic molecules • Photoautotrophy – Involves light rx (energy step) and dark rx (carbon fixing step) – Photophosphorylation (light rx) provides ATP and reducing power (NADPH) to power the Calvin-Benson Cycle (dark rx) • 3 types of phototrophy – Oxygenic phototrophy – Anoxygenic phototrophy – Rhodopsin based phototrophy • Pathways of oxygenic light reaction – Pair of chlorophyll based photosystems embedded in membrane • Chloroplast or plasma membrane Cyclic photophosphorylation produces only ATP Non-cyclic photophosphorylation produces ATP, NADPH and O2 • Pathways of anoxygenic light reaction – – – – – Single bacteriochlorophyll based photosystem Limited to cyclic photophosphorylation Use different methods to generate reducing power Molecules other than water are used as electron donor O2 is not produced • Archaea have no chlorophyll based photosystems • They utilize a membrane protein called bacteriorhodopsin to capture radiant energy • In oxygen poor environments the pigment functions as a light-driven proton pump Dark rx uses ATP and NADPH to fix carbon Chemolithotrophy • Inorganic compounds serve as electron donors and energy source • Common electron donors include – H, reduced N, S or Fe • Photolithrotrophs require additional energy from sun – Purple bacteria • Low energy yield so they consume high quantities of inorganic molecules – Significant ecological impact • Iron bacteria – oxidize ferrous iron (Fe2+) into ferric iron (Fe3+) – Ferrobacillus ferrooxidans • Nitrifying bacteria – oxidize ammonia (NH3) to nitrate (NO3) – Nitrosomonas and Nitrobacter • Hydrogen bacteria – oxidize hydrogen gas (H2) to water (H2O) – Alcoligenes eutrophus • Sulfur Oxidizing Bacteria – oxidize sulfides, sulfur and thiosulfate to sulfuric acid (H2SO4) – Thiobacillus thiooxidans • Many chemolithotrophs are autotrophic using CO2 as carbon source – Use reverse electron flow to reduce NAD Reverse electron flow is necessary for chemolithoautotrophs to generate reducing power NADH reduction by sulfide and nitrite • Chemolithoautotrophy is very inefficient – much of the energy is expended on generating reducing power rather than ATP – Many will grow as heterotrophs if supplied with organic carbon sources • Many can grow either aerobically or anaerobically by varying the final electron acceptor