Unit 2–Lecture 11 (Nutrient Cycles) Flashcards
The Carbon Cycle (1)
Carbon is cycled through all of earth’s major carbon reservoirs
—-atmosphere, land, oceans, sediments, rocks, and biomass
All nutrient cycles are linked to the carbon cycle, but nitrogen is very closely linked because nitrogen and carbon are macronutrients
The Carbon Cycle (2)
The sediments and rocks in the Earth’s crust are the largest carbon reservoir
CO2 in the atmosphere is the most rapidly transferred carbon reservoir
CO2 is removed from the atmosphere by photosynthetic land plants and marine microbes (a large amount of carbon is found there)
More carbon is found in humus, or dead, organic material, than living organisms
CO2 is returned to the atmosphere by respiration and decomposition as well as by human-related (anthropogenic) activities
The Carbon Cycle (3)
Photosynthesis and respiration are part of the redox cycle
Photosynthesis
—-reduces inorganic carbon dioxide to organic carbohydrates
CO2 + H2O –> (CH2O) + O2
Respiration
—-oxidizes organic carbohydrates to inorganic carbon dioxide
(CH2O) + O2 –> CO2 + H2O
The two major end products of decomposition are methane (CH4) and carbon dioxide (CO2)
Most methane is converted to carbon dioxide by methanotrophs; however, some enters the atmosphere
Methane Hydrates
Methane Hydrates
—-form when high levels of methane are under high pressure and low temperature
—-methane hydrates fuel deep-sea ecosystems called cold seeps
Syntrophy and Methanogenesis
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 (acetate) to form CH4
Methanogens team up with parters (syntrophs) that supply them with necessary substrates
The Nitrogen Cycle
Nitrogen:
Key constituent of cells
Exists in a number of oxidation states
Four major nitrogen transformations
—-nitrification
—-denitrification
—-anammox
—-nitrogen fixation
Anammox
The anaerobic oxidation of ammonia to N2 gas
Denitrification and anammox result in losses of organic nitrogen from the biosphere
Nitrogen Fixation
Synthesis of amino groups
Fully reduced nitrogen
—-necessary fore minor’s acid synthesis
—-fixed nitrogen often limiting for cell growth
Haber process, dependent on natural gas
Energy-intensive process
—-40 ATPs consumed for each N2 fixed
Enzyme production strictly regulated
—-only made when O2, NH4 + levels low
—-anaerobic heterocyst make NH3
Natural Nitrogen Fixation
Enzyme nitrogenase
—-has atom of Molybdenum
—-transfers electrons
Must stay anaerobic
Nitrogen Assimilation
Incorporation of NH4+ into amino acids
Transamination: Glutamine donates NH3 to make other amino acids
Nitrification
NH4+ –> NO2- –> NO3-
Oxidation of NH4+ provides electrons, energy
In soil, one species oxidizes NH4+ TO NO2-
—-Nitrosomas
2nd species oxidizes NO2- to NO3-
—-Nitrobacter
Excessive fertilizer use causes nitrate runoff
—-eutrophication of streams
—-danger to water supplies
Denitrification
NO3- –> NO2- –> NO –> N2O –> N2
Dissimilatory nitrate reduction
—-nitrate is anaerobic electron acceptor
The Sulfur Cycle (1)
Sulfur transformation by microorganisms are complex
The bulk of sulfur on Earth occurs in sediments and rocks as sulfate and sulfide minerals
The oceans represent the most significant reservoir of sulfur (as sulfate) in the biosphere
The Sulfur Cycle (2)
Hydrogen sulfide is a major volatile sulfur gas that is produced by bacteria via sulfate reduction or emitted from geochemical sources
Sulfide is toxic to many plants and animals and reacts with numerous metals
Organic sulfur compounds can also be metabolized by microorganisms
The most abundant organic sulfur compound in nature is dimethyl sulfide (DMS)
The Iron and Manganese Cycle
Iron is one of the most abundant elements in Earth’s crust but often a limiting nutrient for microbial growth
On Earth’s surface, iron exists naturally in two oxidation states
—-ferrous (Fe2+)
—-ferric (Fe3+)
