Lecture 13.pdf

BIOL371: Microbiology Lecture 13 – Nutrient cycles 1 Topics of today 1. 2. 3. 4. The carbon cycle Syntrophy and methanogenesis The nitrogen cycle Human and nutrient cycles Materials covered:  Chapters 21.1-21.3, 21.8-21.9  Figures 21.1, 21.2, 21.4, 21.6, 21.7, 21.9, 21.22-21.25 2 Carbon r...

BIOL371: Microbiology Lecture 13 – Nutrient cycles 1 Topics of today 1. 2. 3. 4. The carbon cycle Syntrophy and methanogenesis The nitrogen cycle Human and nutrient cycles Materials covered:  Chapters 21.1-21.3, 21.8-21.9  Figures 21.1, 21.2, 21.4, 21.6, 21.7, 21.9, 21.22-21.25 2 Carbon reservoirs  Carbon is cycled through all of Earth’s major carbon reservoirs  Includes atmosphere, land, oceans, freshwater, sediments, rocks, and biomass  All nutrient cycles are linked to the carbon cycle, but the nitrogen (N) cycle links particularly strongly because, other than water, carbon and nitrogen make up the bulk of living organisms  Reservoir size and turnover time are important parameters in understanding the cycling of elements 3 The carbon cycle – removal of carbon dioxide  While the sediments and rocks in the Earth’s crust are the largest reservoirs, CO2 in the atmosphere is the most rapidly transferred carbon reservoir  Carbon dioxide is removed from the atmosphere by photosynthetic land plants and marine microbes, so a large amount of carbon is found there  More carbon is found in humus (dead organic material) than is found in living organisms in a region 4 Carbon turnover  CO2 is returned to the atmosphere by  Respiration and decomposition  Microbial decomposition is the largest source of CO2 release to the atmosphere  Since the Industrial Revolution, human (anthropogenic) activities have increased atmospheric carbon by 40%  This rise in carbon dioxide has led to steadily increasing temperatures worldwide (global warming) because carbon dioxide is a greenhouse gas 5 Photosynthesis and respiration  Phototrophic organisms produce organic or fixed carbon and reduce the level of carbon dioxide in the atmosphere  Oxygenic phototrophic organisms can be divided into two groups: plants and microorganisms  Plants dominate terrestrial environments  Microorganisms dominate aquatic environments  Photosynthesis and respiration are part of redox cycle  Photosynthesis: reduces inorganic carbon dioxide to organic carbohydrates CH2O  CO2 + H2O (CH2O) + O2  Respiration: oxidizes organic carbohydrates to inorganic carbon dioxide  (CH2O) + O2 CO2 + H2O 6 End products of decomposition  The two major products of decomposition are:  Methane (CH4) – a potent greenhouse gas and is produced in anoxic (oxygenfree) environments  Carbon dioxide (CO2) – most methane is converted to carbon dioxide by methanotrophs  Some CO2 enters the atmosphere 7 Methane hydrates  Methane hydrates: form when high levels of methane are under high pressure and low temperature  Huge amounts of methane are trapped underground as methane hydrates; e.g., beneath the permafrost in the Arctic and in marine sediments  Methane hydrates can absorb and release methane  Methane hydrates fuel deep-sea ecosystems called cold seeps Methane hydrates in shallow coastal sediments (350-400 metres deep) are sensitive to seasonal changes in bottom water temperature. Flares of methane bubbles are observed when water temperatures warm by as little as 1-2°C 8 Coupled cycles  All nutrient cycles are interconnected and feed back upon one another  Major changes in one cycle affect the functioning of other cycles  The carbon cycle and the nitrogen cycle are closely coupled  Example: the rate of carbon fixation and plant growth is often limited by the available nitrogen; adding nitrogen to farm fields increases yield High Denitrification decrease primary production and nitrification increase NO3– and primary production. Low NH4+ decreases primary production. High organic carbon will increase N2 fixation while low organic carbon will decrease it 9 Syntrophy and methanogensis  Methanogenesis is central to carbon cycling in anoxic environments  Most methanogens use carbon dioxide as a terminal electron acceptor, reducing CO2 to CH4 with H2 as an electron donor  Some can reduce other substrates (e.g., acetate) to form CH4  Syntrophy: methanogens teaming up with partners that supply them with necessary substrates In anoxic decomposition, various groups of fermentative anaerobes cooperate in the conversion of complex organic materials to CH4 and CO2. This pattern holds for environments such as freshwater lake sediments, sewage sludge, bioreactors, and the rumen. 10 Acetogenesis  Acetogenesis: a H2-consuming process that compete with methanogenesis in some environments  Occurs in termite hindgut and rumen with the methanogens and protists  Methanogenesis is energetically more favorable than acetogenesis  Acetogens can ferment glucose and methoxylated (methyl group bound to oxygen) aromatic compounds (like lignin, found in woody biomass), whereas methanogens cannot. This expands the diet of termites 11 Nitrogen  N2 is the most stable form of nitrogen and is a major reservoir  N2 comprises ~70% of the Earth’s air  Most of the nitrogen recycled on Earth is already fixed; that is, in combination with other elements such as ammonia (NH3) or nitrate (NO3–) 12 The nitrogen cycle – nitrogen fixation, denitrification, ammonification  Major nitrogen transformation by microorganisms  Nitrogen fixation: N2 + 8H 2 NH3 +H2  Carried out by few free-living bacteria and archaea as well as symbiotic bacteria  Denitrification: reduction of nitrate (NO3–) to gaseous nitrogen (N2, NO (nitric oxide))  Ammonification: decomposition of organic nitrogen compounds (such as amino acids and nucleotides) to ammonia  Dissimilative nitrate reduction to ammonia: pathway used by nitrate-reducing bacteria under anoxic, nutrient-rich environments (e.g., human gastrointestinal tract) 13 The nitrogen cycle – nitrification, anammox  Nitrification: oxidation ammonia (NH3) to nitrate (NO3–)  Two-step process in many cases  Oxidation of ammonia (NH3) to nitrite (NO2–) performed by many bacteria and archaea  Oxidation of nitrite (NO2–) to nitrate (NO3–) by some other bacteria  Comammox (complete ammonia oxidxation) bacteria oxidize ammonia completely to nitrate  Anammox (anaerobic ammonia oxidation): ammonia is oxidized anaerobically with nitrite (NO2–) as the electron acceptor, forming N2 as the final product  Beneficial in wastewater treatment by removing fixed nitrogen that could trigger algal and other microbial blooms 14 Humans and nutrient cycling 1. Mercury transformations 2. Human impacts on the carbon and nitrogen cycles 15 Mercury and the environment  Mercury is not a microbial nutrient, but an ingredient in many pesticides, a pollutant from the chemical and mining industries, and a contaminant of aquatic systems and wetlands  Elemental mercury (Hg0) is the major form in the atmosphere, which is volatile and oxidized to mercuric ion (Hg2+) photochemically  Most mercury enters aquatic environments as Hg2+ 16 Mercury transformations  Mercury has a tendency to accumulate in living tissues and is highly toxic  Hg2+ readily adsorbs to particulate matter where it can be modified by microorganisms  Primarily by sulfate-reducing and iron-reducing bacteria and methanogenic archaea  Specific enzymes are involved in the methylation of mercury to yield methylmercury, CH3Hg+  Methylmercury is extremely toxic and accumulates in the muscle tissues of fish  In addition to neurotoxicity, methylmercury can cause liver and kidney damage in humans  Other microbial transformation of mercury include:  H2S + Hg2+ HgS by sulfate-reducing bacteria  CH3Hg+ Ch4 + Hg0 by methanogens 17 Mercury resistance  Some bacteria can convert the toxic forms of mercury to nontoxic or less toxic forms  The enzyme organomercury lyase cleaves CH3Hg+ to Ch4 and Hg2+, which is reduced by mercuric reductase to Hg0  Plasmid- or transposon-borne operon of mercury-resistant genes  MerP in the periplasm binds Hg2+ and transfers it to MerT  MerT interacts with the mercuric reductase MerA to reduce Hg2 to Hg0  Hg0 is volatile and nonpolar, exits through the cytoplasmic membrane 18 Human impact on the carbon cycle  Since the start of the Industrial Revolution, CO2 levels have increased by >40%  CO2 is a greenhouse gas that traps long-wave (heat waves) from the Earth’s surface, in effect making the entire planet a large greenhouse. This phenomenon is called radiative forcing  Dissolved carbon dioxide decreases the pH of the ocean. This acidification endangers coral reefs, which release calcium carbonate as they die  Air and ocean water temperatures are increasing, which increases the oxygen minimum zones in the ocean 19 Major sources of methane emissions  Increases in atmospheric methane account for ~20% of the increase in radiative forcing 20 Human impact on the nitrogen cycle  Humans produce large amounts of nitrogenous fertilizers  Ecological effects of fertilizers are unknown, but the alteration of nitrogen cycles will also change iron availability and the carbon cycle  Nutrient cycles are coupled: change in either nitrogen or carbon cycles will affect other cycles Overview of greenhouse gas emissions 21