Lecture 25: The Carbon Cycle PDF

Lecture 25 The Carbon Cycle Overview of the carbon cycle Microbial involvement Applications Waste water treatment, composting, etc. Anoxic environments Methanogens and Syntrophs How many of you have heard of the “failed” experiment of Biosphere 2?...

Lecture 25 The Carbon Cycle Overview of the carbon cycle Microbial involvement Applications Waste water treatment, composting, etc. Anoxic environments Methanogens and Syntrophs How many of you have heard of the “failed” experiment of Biosphere 2? What went wrong?  Dr. Dan Buckley studies soil nutrient movement through the lens of bacteria https://www.pnas.org/doi/ 10.1073/pnas.2115292118  Carbon cycle – crude overview CO2 CO2 O2 Used up as ETC terminal electron n Carbon fixation tio acceptor i ra sp re biomass biomass decomposition Plants, animals, fermentation bacteria Aerobic respiration (uses up oxygen) Anaerobic respiration  Aerobic Calvin Cycle Decomposition CO2 Fixation Reductive Chemolithotrophy Acetyl-CoA Cycle Reverse Citric Acid Cycle Oxidative Reductive  Carbon fixation pathways 1. Calvin cycle (plants and cyanobacteria, purple bacteria, lithotrophs) Dark reactions in photosynthesis RubisCo CO2 Sugar (Ribulose-1,5-bisphosphate carboxylase/oxygenase) Glucose (gluconeogenesis)  Carbon fixation pathways 2. Reverse TCA cycle (green sulfur bacteria etc.) CO2 in, glucose out Glucose in, CO2 out + others! ournal of Biological Chemistry Volume 285 Issue 46 Pages 35848-35854 (November 2010)  Aerobic Calvin Cycle Decomposition CO2 Fixation Reductive Chemolithotrophy Acetyl-CoA Cycle Reverse Citric Acid Cycle Oxidative Reductive  Decomposition Breakdown of dead biomass (i.e. organic carbon, (CH2O)n) Organic C is oxidized to CO2 Releases nutrients (N, P, S, etc.) Mediated by chemo-organo- heterotrophs through aerobic respiration, anaerobic respiration, and fermentation. Like us! We are secondary decomposers, breaking down things for the gut micobes to carry out primary decomposition. Human activity (anthropogenic effects) has disturbed the natural carbon cycle https:// Microbial metabolism and the Carbon cycle: Applicatio omposting: erate the plant matter to promote decomposition IWWTP! Wastewater Treatment: oals: eliminate pathogens from water reduce biological oxygen demand (BOD), remove nutrients and contaminants (e.g. N, P, metals) Stages: Primary Treatment (Physical): -screens, sedimentation, Secondary Treatment (Biological) : -activated sludge (aerobic) -anaerobic digestion (degrade organic matter further – biogas!) Tertiary Treatment (Chemical) : -e.g. chlorination, UV, chemical flocculation Wastewater Treatment: aerate the waste to promote decomposition  Different kinds of wastewater treatments Low-tech/nature-based Septic tanks = Settling and anaerobic digestion processes reduce solids and organics Constructed wetlands = Engineered systems that use the natural functions of vegetation, soil, and organisms to provide secondary treatment to wastewater. Waste stabilization ponds = reduce the organic content and remove pathogens from wastewater.  Different kinds of wastewater treatments Low-tech/nature-based Septic tanks = Settling and anaerobic digestion processes reduce solids and organics Constructed wetlands = Engineered systems that use the natural functions of vegetation, soil, and organisms to provide secondary treatment to wastewater. Waste stabilization ponds = reduce the organic content and remove pathogens from wastewater.  Different kinds of wastewater treatments Low-tech/nature-based Septic tanks = Settling and anaerobic digestion processes reduce solids and organics Constructed wetlands = Engineered systems that use the natural functions of vegetation, soil, and organisms to provide secondary treatment to wastewater. Waste stabilization ponds = reduce the organic content and remove pathogens from wastewater.  How exactly does aeration of wastewater reduce “biological oxygen demand” 1. Water stores oxygen, so it won’t be available for microbes to use. 2. Aeration enables anaerobic fermentation of organic compounds, turning them into something other bacteria can use later 3. Aeration enables aerobic respiration, which removes organic compounds from the wastewater by oxidizing them 4. Aeration selects for autotrophs that fix CO2 through the Calvin Cycle  How exactly does aeration of wastewater reduce “biological oxygen demand” 1. Water stores oxygen, so it won’t be available for microbes to use. 2. Aeration enables anaerobic fermentation of organic compounds, turning them into something other bacteria can use later 3. Aeration enables aerobic respiration, which removes organic compounds from the wastewater by oxidizing them 4. Aeration selects for autotrophs that fix CO2 through the Calvin Cycle  Anoxic Habitats (e.g., anaerobic digestions in wastewater treatment) Where do we find anoxic environments (and anaerobic organisms)? Anywhere there is stagnant water and microbial O 2 consumption! Examples: -anaerobic digestion -gut communities -wet soil -wetlands and sediments Even a layer of water 100 µm deep is enough to result in an anoxic environment if O2 consumption is high. Anaerobic DigestionMethane is the main component of biogas Anaerobic respiration Fermentation Denitrification* Methanogenesis Aeration (activated sludge) Aerobic respiration Nitrification* *we’ll get to that on Monday! micro bes Methanogens (Archaea) 4 H2 + CO2  CH4 + 2 H2O Acetate  CH4 + CO2 ∆G0 ‘ = 32 kj/mol (more prominent pathway) Chemo-litho-autotrophs or Chemo- organo-heterotrophs, obligate anaerobes Electron donor: H2 and/or Acetate Carbon source: CO2 and/or Acetate Habitats: wetlands, sediments, gut Methanogens (and bacteria) often form ”syntrophic” relationships with other prokaryotes Syntrophs (“eating together”, i.e. cross-feeding; chemo-organo-heterotrophs, obligate anaerobes) Volatile fatty acids  H2 + CO2 or Volatile fatty acids  acetate Carry out fermentation reactions that are ENERGETICALLY UNFAVORABLE under standard conditions! Reactions are made favorable by the consumption of H or acetate by Ziels et al, mSystems 2019 It takes a village: Anaerobic breakdown of organic matter and syntrophic foodwebs Complex polymers (cellulose, proteins, lipids, nucleic acids) Fermentation & Respiration Monomers (sugars, amino acids, fatty acids) Fermentation Fermentation Volatile Fatty Acids (propionate, butyrate, succinate) Syntrophs H2, CO2 Acetate Methanoge Acetogens Methanoge ns ns CH4 CH4, CO2