chapter 21 - nutrient cycles Flashcards
What is the carbon cycle?
cycling carbon through all earths major carbon reservoirs
Reservoirs
rocks+sediments, oceans, methane hydrates, fossil fuels
How does the carbon-removal cycle work?
CO2 in atmosphere is transferred carbon reservoir, with the help of photosynthetic land plants and marine microbes
Carbonturnover
CO2 is returned through respiration and decomposition
What is the largest source of CO2?
microbial decomposition
What has increased atmospheric carbon by 40%
humans
What are some oxygenic phototropic organisms?
- plants (terrestrial)
- microorganisms ( aquatic)
Photosynthesis
- reduces inorganic CO2 to organic CH2O
CO2 + H2O = CH2O + O2
Respiration
- oxidizes organic CH2O to inorganic CO2
CH2O + O2 = CO2 + H2O
What are the end products of decomposition
- methane (CH4): greenhouse gas, produced in oxygen free environments
- carbon dioxide (CO2): methane converted to CO2 by methanotrophs
- methane hydrates: when high levels of methane are under high pressure + low temperature
Example of a coupled cycle
carbon cycle and nitrogen cycle are closely coupled
Methanogenesis
central to carbon cycling in anoxic environments.
Methanogenesis mechanism
methanogens use CO2 as terminal electron acceptor, reducing CO2 to CH4 with H2 as electron donor
Acetate is always being formed from monomers.
Acetogenesis
H2 - consuming process that competes with methanogenesis, but is less favourable
Nitrogen fixation
reduces N2 + 8H = 2NH3 + H2
Denitrification
reduces nitrate to gaseous nitrogen
- NO3- = N2/NO
Ammonification
no redox change
- decomposition of organic nitrogen compounds to NH3
Dissimulative nitrate reduction to ammonia (DNRA)
reduction
- pathway used by nitrate- reducing bacteria ( NO3- or NO2- to NH3
Nitrification
oxidation
- oxidize NH3 to NO2-
- then oxidizes NO2- to NO3-
Comammox
oxidation
- oxidize NH3 to NO3-
Anammox
oxidation and reduction
- NH3 is oxidized with NO2- anaerobically to N2
Assmiliation
- Reduction : NO3- to NH2 groups of proteins
- No redox change: NH3 to NH2
How is mercury in the enviornment?
Hgº is the elementary mercury that gets oxidized to Hg2+ then enters aquatic environments.
How is Hg2+ modified by microorganisms?
- sulfate reducing bacteria (HgS)
- specific enzymes are involved in methylation of mercury
- methylmercury is toxic (CH3Hg+)
Organomercury lyase
cleaves CH3HG+ to CH4 and Hg2+
then reduced by mercutic reductase to Hgº
What is the mercury resistance mechanism
MerP in periplasm binds Hg2+ and transfers it to MerT then interacts with mercuric reductase MerA to reduce Hg2+ to Hgº
Hgº is volatile and nonpolar and so it exits through cytoplasmic membrane.
Radiative forcing
entire planet feels like greenhouse
What are the major sources of methane emissions
atmospheric methane account for 20% of increased radioactive forcing
- fossil fuels
- wetlands
- plants
- biomass burning
Human impact of nitrogen cycle
humans produce lots of nitrogenous fertilizers like CO4, CH4, N2O (greenhouse gas emissions
