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Carbon in anaerobic aquatic environments Carbon in the form of CO2, HCO3- and CO3-2, are oxidized forms of C, and tend to be the only forms present where O2 is plentiful. In anoxic environments methanogens (Archaea) convert organic C and CO2 into methane (CH4). Methane is a gas and can bubble out of the water or it can be oxidized to CO2 by methylotrophic bacteria. Methanogens are not true bacteria, they belong to the Archaea Most methanogens can grow on CO2 and H2 as their sole energy source: Chemoautotrophs —chemical bond energy is their energy source they utilize CO2 as their C source http://faculty.plattsburgh.edu/jose.deondarza/images/Organisms/methanogen.jpg C-transformations in aerobic and anaerobic environments Oxidation state Where do we find methanogens? -4 0 +4 •Under anaerobic conditions organic molecules break down to methane instead of CO2—This process is facilitated by methanogens (Archaea), which are chemoautotrophic bacteria. •They utilize the energy released from 2H2 + Organic C (CH2O)→CH4 + H20 to build their biomass. How to assign Oxidation numbers We keep track of the e- transfer using Oxidation numbers (Ox#) For each e- transferred the Ox# changes by 1 2H2 + O2 0 0 2H2O +1 -2 Some rules for Oxidation numbers 1. In free elements Ox# =0 2. For ions with one atom Ox# = charge. eg H+ Ox# of H+ = 1 3. Ox# of O in most compounds is -2, 4. Ox# of H in most compounds is +1, 5. For a complex ion like SO4-2 , the net Ox# = charge (Thus S=+6) Chemical equation for the reduction of CO2 by H2 CO2 H 2 CH 4 H 2O E(energy) CO 2 4 H 2 CH 4 2 H 2O ( 4) (0) (-4) - ( 1) 8 e are accepted per C - 1 e is donated per H Nutrition and Metabolic Diversity Four nutritional categories Carbon Source Energy Source CO2 Organic Light Photoautotroph Photoheterotroph Chemical Chemoautotroph Chemoheterotroph Solar Saltern Plant: a typical habitat of Halobacterium salinarium Functional classification? •found in water saturated or nearly saturated with salt (Halophiles) and rich in organic matter. •Large blooms appear reddish, from the pigment bacteriorhodopsin. This pigment is used to absorb light, which provides energy to create ATP. •The process is unrelated to other forms of photosynthesis involving electron transport, however, and halobacteria are incapable of fixing carbon from carbon dioxide. http://www.biochem.uni-luebeck.de/public/groups/metalloproteins/hubmacher/Hubmacher_Abb2.jpg The Cycling of Nitrogen N is an important nutrient that frequently limits primary productivity in aquatic ecosystems It is rare in the earth’s crust, but makes up 79% of the atmosphere (N2) (oxidation state =0) Most algae and plants require NO3¯(+5) (NO2 ¯) (+3) or NH3 (NH4+) (-3)to synthesize amino acids to make proteins N-fixing microorganisms can take up N2 and convert it to NH3 N2 + 3H2 → 2NH3 Many plants have N-fixing mutualists (eg Azolla) Denitrifying bacteria can convert NO3¯ back to N2 Azolla, an aquatic fern used in rice culture •The leaves of this aquatic fern have cavities that harbour filamentous cyanobacteria Anabaena azollae •The large cells (heterocysts) are specialized for N-fixation •Traditional rice farming in many countries involve planting Azolla to build up N concentrations in rice paddy. The Nitrogen cycle involves many different oxidation states, and the redox processes are facilitated by plants and wide variety of bacteria Chemoheterotrophs (CH) -3 CH PA 0 +1 +3 Nitrite CH +5 Photoautotrophs (PA) Chemoautotrophs(CA) Chemoheterotrophs (CH) -2 0 +4 +6 CH PA Photoautotrophs (PA) Chemoautotrophs(CA) Streams draining mine tailings are extremely acidic—the effect of Thiobacillus oxidizing pyrites and iron. Thiobacillus ferrooxidans oxidizes both the iron, Fe(+2) to Fe(+3) and the sulphur in the pyrites, S(-1) to S(+6) using molecular oxygen. This reaction splits water to produce a great deal of acid. How do you suggest that mine tailings should be stored? 2FeS 2 7.5O2 4H 2O 4SO4 2 Fe2O3 8H Energy Desulfovibrio : Sulfate reducing bacteria commonly found in anaerobic aquatic environments with high levels of organic material, such as mud in lakes and ponds. •have metal reductases which can precipitate metal sulfides from the water— •bioremediation potentials for toxic radionuclides such as uranium by a reductive bioaccumulation process. Sulfate reduction can absorb H+ and counteract acid rain They also contribute to methylation of Mercury Chemical equation for the oxidation of acetate by sulfate CH 3COO SO 4 2 H S H 2O CO2 E 2 CH 3COO SO 4 H S 2 H 2O 2CO 2 ( 6) (0) (-2) ( 4) 8 e - are accepted per S - 4 e are donated per C Where do sulphate reducing bacteria fit in the functional classification?