Ricklefs ch23

Ricklefs Chapter 23
Pathways of Elements
NItrogen (N2) main source is amosphere - dissolves somewhat into water, absent form native rock.
Lightning converts some N2 to forms plants can use (ammonia), but most enters Nitrogen Cycle thru microorganisms (Nitrogen Fixation: N2 -> NH3)

Ammonification-
Plants obtain N2 from soil as ammonia or nitrate.
Then must reduce to organic form via Ammonification: breaking down proteins into component amino acids by hydrolysis/oxidation of C in those amino acids
—> results in Ammonia (NH3). (N2 energy does not change in this process despite C oxidization)

Nitrification-

the oxidation of ammonia to nitrite (NH3 -> NO2-) via Nitrosomonas (on land) and Nitrosococcus (marine) bacterias, then to nitrate (NO2- -> NO3-) via Nirobacter (land) and Nitrococcus (marine) bacterias

process is 2 step:
1.N atoms stripped of 6 electrons
2.N atoms stripped of 2 electrons
—> results in release of chemical potential energy

this can only occur in presence of Oxidizing agent (ie O).

Denitrification
In anaerobic conditions, NO2- adn NO3- can act as oxidizers to nitric oxide, under following reaction:
NO3- -> NO2- -> NO by Pseudomonas denitrificans bacteria.
This results in loss of nitrogen through gas conversion to Nitric oxide and N2
this is likely cause of low Nitrogen levels in marine environments

NItrogen Fixation
loss of nitrogen due to denitrification offset by nitrogen fixation by Azotobacter, Rhizobium, cyanobacteria.
process: reduce nitrogen N2 -> NH3 by oxidizing sugars, organic compounds.

Sulfur-
required by plants and animals by virtue of bein part of 2 amino acids
Most oxidized form: sulfate (SO4^2-)

Aerobic conditions
Most reduced form: hydrogen sulfide (H2S) and organic forms
reduction (SO4^2- -> organic S) aka (sulfate -> organic S) balanced by oxidation (organic S -> SO3^2- -> SO4^2-) aka (organic S -> sulfite -> sulfate)

This occurs under excessive animal excretory conditions & microorganisms decompose plant/animal detritus

Anaerobic conditions
Desulfovibrio & desulfomonas use sulfate to to oxidise carbon
reduced sulfur used by photosynthetic bacteria to assimilate carbon via same photosynthetic pathway but using Sulfur (S) in place of O atom as electron donor. This leads to S accumulation until exposure to O and then oxidized to sulfite/sulfate
reduced sulfur commonly escapes as hydrogen sulfide (H2S) gas (smells like rotten eggs).
or combined with reduced ferrous iron as FeS, associated with coal/oil deposits. When exposed to armosphere or burned for energy, S oxidises to sulfate, combined with water to form sulfuric acid (H2SO4): aka acid rain.

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