lec 15 Flashcards
which has a larger effect on bodies of water: N, or P?
both are really important. ultimately P has more of an effect, but in the short term (weeks to months) N has a large effect too
what is the Redfield ratio? what is its significance?
redfield ratio is 106:16:1 for C:N:P
it is generally what stable bodies of water stay at
we can look at N:P ratio to see which is more limiting
if N:P > 16, there is more N –> P is limiting
if N:P < 16, there is less N –> N is limiting
what are heterocysts?
things used in N fixation by cyanobacteria - good indicators for cyanobacteria biomass
what is the main source of N?
atmospheric N (makes up like 78% of the atmosphere)
what are our many forms of nitrogen’s redox states?
NH4+ is oxidized, NO3- is reduced, and theres also gaseous N2, NO, and N2O
what is one problem assoc w fixing N?
very costly, since N has triple bonds which are difficult to break
since this pathway is costly, N fixation does not occur if NO3 or NH4 are available
what is the point of nitrification?
redox reaction where electron gained from oxidation of NH4+ to NO2- provides energy to reduce CO2 to organic carbon
what is ammoniafication? what are the two processes by which it can occur in oxic/anoxic conditions?
ammonia is created as a byproduct of many animals, due to organic N being consumed
in oxic conditions, ammoniafication is excretion
in anoxic conditions, ammoniafication is fermentation
what is assimilation? how can N be assimilated, and what form is preferred? are other forms taken up?
the active uptake of a chemical - in this case, inorganic N
this occurs in the form of ammonia, nitrate, or nitrite, where ammonia is the form that organisms use. nitrates and nitrites are still taken up, but they must first be converted to ammonia before use.
what can denitrification be framed as? why does it occur?
denitrification can be though as the exit point from the N cycle.
when N is not taken up, it moves to nitrate/nitrite
in anoxic conditions, NO3- becomes NO –> N2O –> N2
problem! transition from N2O to N2 req carbon, so if carbon isn’t readily available itll diffuse out the water column as N2O, which is a really big greenhouse gas
what is the solubility of oxygen compared to that of nitrogen gas? why?
nitrogen is less soluble bc it has more bonds that need to be broken (triple vs double)
what are the enzymes involved in turning unusable nitrate to usable ammonia? how do nitrates compare to ammonia in terms of energy?
nitrate reductase converts nitrate into nitrite, and nitrite reductase converts nitrite into ammonia
this process is expensive! ammonium has higher potential energy, so turning nitrate into ammonia works against the energy gradient (doesn’t naturally occur)
how can some bacteria and archaea fix nitrogen? what are the conditions for this to occur?
nitrogen can be fixed using the nitrogenase enzyme
nitrogenase is inactivated by the presence of oxygen, so organisms either need to be in anoxic environments or prevent oxygen from reaching the enzyme (ex heterocysts prevent oxygen from reaching the enzyme)
what happens when DO is high in conc and there is plenty of ammonia?
ammonia has higher potential energy than nitrites/nitrates, which bacteria can exploit to free up this energy and use it for themselves
aka we get nitrification!
if denitrification produces N2O or N2, why is necessary at times?
when N is too high in a lake, it needs to get removed
also useful for anoxic lakes that need to respire, since nitrates can be used in unideal situations
