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Chemistry of the Cycles Sulfur Cycle • Sulfur is one of the constituents of many proteins, vitamins, and hormones. It recycles like other biogeochemical cycles • The essential steps of the sulfur cycle are: • Mineralization of organic sulfur to the inorganic form, hydrogen sulfide: (H2S). • Oxidation of sulfide (s2-) and elemental sulfur (S) and related compounds to sulfate, (SO42-). • Conversion of sulfate (SO42-) to sulfide (s2-). • Microbial immobilization of the sulfur compounds and subsequent incorporation into the organic form of sulfur Sulfur Cycle • The Chemistry: • Sulfate (SO42-) is reduced to organic sulfhydryl groups (R-SH) by plants, fungi, and various prokaryotes. • Organic molecules containing sulfur can be desulfurated, producing hydrogen sulfide gas (H2S) • Hydrogen sulfide, (H2S) is broken down producing elemental sulfur (So), This reaction is done by the photosynthetic green and purple sulfur bacteria and some chemolithotrophs (organisms using inorganic compounds for ATP production). • Further oxidation of elemental sulfur by sulfur oxidizers produces sulfate, (SO42-) • Elemental sulfur can be converted to hydrogen sulfide, (H2S) • Sulfate reducers generate hydrogen sulfide, (H2S) from sulfate, (SO42-) Phosphorus Cycle • The phosphorus cycle process is as follows: • The phosphorus enters the soil and water as phosphate ions , (PO4)3- through the weathering of rocks. • Plants take in these phosphate ions from the soil. • The phosphates, (PO4)3- are then transferred from plants to herbivores animals. • These herbivores animals are eaten by carnivores. • The phosphates that are absorbed by the animals are returned to the soil through excretion and from decomposition of plants and dead material by microbes. • The dead plant materials and other waste products are decayed through the action of bacteria. • The phosphate is released out to the environment by this process. • The phosphate in the soil is leached out or eroded into water. • The water is again utilized by algae and plants as nutrients. Phosphorus cycle • The phosphorus cycle differs from the other major biogeochemical cycles in that it does not include a gas phase; although small amounts of phosphoric acid (H3PO4) may make their way into the atmosphere, contributing—in some cases—to acid rain. Nitrogen Cycle • Fixation by Lightning The energy from lightning causes nitrogen (N2) and water (H2O) to combine to form ammonia (NH3) and nitrates (NO3). Precipitation carries the ammonia and nitrates to the ground, where they can be assimilated by plants. • Biological Fixation About 90% of nitrogen fixation is done by bacteria. Cyanobacteria convert nitrogen into ammonia and ammonium. N2 + 3 H2 → 2 NH3 • Ammonia (NH3) can be used by plants directly. Ammonia and ammonium may be further reacted in the nitrification process. Nitrification • Nitrification occurs by the following reactions: • 2 NH3 + 3 O2 → 2 NO2 + 2 H+ + 2 H2O 2 NO2- + O2 → 2 NO3- • Aerobic bacteria use oxygen to convert ammonia (NH3) and ammonium (NH4)1+ • Nitrosomonas bacteria convert nitrogen (N2) into nitrite (NO2-) and then nitrobacteria convert nitrite (NO2-) to nitrate (NO3-). • Some bacteria exist in a symbiotic relationship with plants (legumes and some root-nodule species). Plants utilize the nitrate as a nutrient. Animals obtain nitrogen by eating plants or planteating animals. Ammonification • When plants and animals die, bacteria convert nitrogen nutrients back into ammonium salts and ammonia. This conversion process is called ammonification. Anaerobic bacteria can convert ammonia into nitrogen gas through the process of denitrification: • NO3- + CH2O + H+ → ½ N2O + CO2 + 1½ H2O • Denitrification returns nitrogen to the atmosphere, completing the cycle. Nitrogen Cycle Carbon Cycle • Key Concepts • Carbon is constantly being used and reused by living things • The original energy source is the sun • Chlorophyll is green plants absorbs energy from the sun and converts it to chemical energy through photosynthesis • Photosynthesis converts atmospheric CO2 into glucose, C6H12O6 • In the cells of living things energy is released during respiration • Respiration converts glucose C6H12O6 into CO2 • Carbonates CO32- is leached out of sedimentary rocks into ground and groundwater then converted into CO2 Chemistry of Carbon Cycle • Photosynthesis: • 6CO2 + 6 H2O C6H12O6 + 6O2 • Animals eat plants or other animals that have eaten plants and incorporate the C atoms into their cells. • Plant and animals respire converting glucose into CO2 • C6H12O6 + 6 O2 CO2 + 6H2O • During decomposition carbon containing molecules are converted into elemental C • Carbonate ion contained in sedimentary rock is leached out and converted to CO2 by the following process: Chemistry of Carbon Cycle • Humans burn fossil fuels converting elemental C into CO2 • C + O2 CO2 • CH4 + O2 CO2 + 2H2O