N,S cycling Flashcards
assimilative reactions - give example
nutrients are incorporated into the biomass of an organism - assimilatory NO3 reduction –> NH4, requires assimilatory nitrate reductase
T/F - nitrate reductase is only single enzyme
false - nitrate reductaste can be assimilatory or dissimilatory
dissimilative reactions
when compounds are reduces as terminal e acceptors in energy metabolism - dissimilatory NO3 reduction —>N2
requires dissimlatory nitrate reductase
denitrification
dissimilatory reaction, oxidized N compounds is e acceptor in energy metabolism
what is oxygen requirements of denitrifiers
most are facultative aerobes - prefer to use o2
where do most denitrfication reactions happen
anoxic environments
is e. coli a denitrify?
no - it can do the first step (nitrate to nitrite), but can’t do the rest
what are all the steps of denitrificaitions
no3 –> no2 –> NO –> N2O –> N2
last 3 are gasses
what are the practical imlications of N cycle rxns
n2 fixation ammonifixation nitrification denitrification agriculture practices
n2 fixation
converts unusable n2 to combined N (eg. ammonia)
ammonification pros and cons
releases NH4+ for subsequent use in new biomass [good]
NH4+ is most efficient form of inorganic N for incorporation
NH4 is cationic, thus held in soil (clay) [good]
some NH3 loss from soilt o atmosphere [bad]
NH4+ positives
hangs out in soil surface, and when it rains, it won’t get washed down below root zone
empirical formula for biomass
C106H263O110N16P
true/false - atmosphere is a major reservoir for nitrogen
true, 80% nitrogen but this isn’t usable by most organisms
n2 fixation
conducted by relatively few prokaryotes that use nitrogenase complex (enzyme)
vital to life
makes nitrogen more readily usable
N2 –> NH3 + H2
what is the oxidation state of organic N, ammonia, nitrogen gas, and nitrate?
-3, -3, 0, +5
what are the major steps in the biotic nitrogen cycle?
N2 fixation mineralization anammox assimilation nitrification denitrification
PICTURE
picture answer
nitrification
NH4+ + 2H2O –> NO2- + 8H+ + 6e-
NO2- + 2H2O –> NO3- + 2H+ + 2e-
nitrosomonas
nitrobacter
denitrification
NO3- –> N2
bacillus, parcoccus, pseudomonas
types of nitrogen redox reactions listen by # of prokaryotes who perform the reaction (most to least)
n2 fixation > denitrification > nitrification > ammonification
nitrogenase complex function and feature
n2 fixation
o2 sensitive and subject to strict regulatory control by combined N
N2 + 8H+ + 8e- –> 2NH3 + H2
what is the site of reduction for dihydrogenase
FeMo-co
examples of nitrogen-fixing microorganisms
symbiotic (mutualisms)
free-living aerobes
free-living anaerobes
examples of symbiotic nitrogen-fixing microorganisms
legume-rhizobia symbioses - soybeans, clover, alfalfa with rhizobial bacterium, and actinorhizal plants like alder trees
examples of free-living nitrogen-fixing aerobes
chemoorganotrophs including Azotobacter, klebsiella, methylcoccus, and other phototrophs: many cyanobacteria, chemolithotrophs: alcaligenes, some thiobacillus and others
examples of free-living nitrogen-fixing anaerobes
chemoorganotrophs inlcuding clostridium, desulfovibrio and phototrophs including chromacium, thiocapsa, chlorobium, rhodospirillium, and chemolithotrophs - methanosarcina, methanococcus and other methanogens
heterocyst
modified cells found in certain filamentous cyanobacteria
lack photostem II (and thus do not produce O2) but possess nitrogenase and can conduct N2
symbiotic n2 fixation - steps in the formation of a root nodule in a legume infected by rhizobium
- bacterium attaches to root hair
- root hair curls around bacterium forming shepherd’s crook
- infection thread forms and bacteria replicates
- infection thread extends into root cortex as root cells divide
- infection thread branches, bacteria enter cell walls, lose their cell walls, and become n2-fixing bacteria
- nodule consisting of plant cells is stimulated to grow by infecting bacteroids
flavanoid
class of plant and fungus secondary metabolites
how is root curling induced by rhizobium in n2 fixation
plant excretes flavanoids, attracting rhizobia, incuding nod factor synthesis
bacterial signalling molecules (nod factors) control nodule formation (eg root cell formation)
what is the major carbon compound metabolized by bacteria as they fix nitrogen?
C4 dicarboxylic acids (succinate, fumarate, malate)
bacteroids
rhizobial cells transformed into swollen, misshapen branched shapes within the plant cell, possess nitrogenase activity, incapable of cell division
symbiosome
single or small groups of bacteroids surrounded by portions of plant cell membrane
n2 fixation begins after symbiosome formation
leghemoglobin
o2-binding protien
supplies o2 to bacteroid and protects nigtrogenase from o2 inactivation
synthesis requires input from the plant and the bacterium
Lb delivers bound O2 directly to bacterial ETS, protects nitrogenase complex from O2 damage
how does the exchange of C and N compounds between bacteria and plants work?
C4-dicarbozylic acids are taken up by bacteroids in plant cytoplasm to enter into TCA cycle to make ATP, then the ATP and electrons that are generated are used by nitrogenase to convert nitrogen gas into ammonia (nitrogen fixation), ammonia is then used by bacteroids to make amino acids
excess ammonia used by plants to make ammonium
ammonification
RNH2 –> NH3 (NH4+)
production of ammonia during the decomposition of organic nitrogen compounds
conducted by wide range of aerobes, anaerobes
what is ammonification like in oxic conditions?
some NH3 volatilization to atmosphere (favoured by dark, alkaline soil)
assimilation by plants, microorganisms, production of new biomass
NH4 subjected to nitrification
what is ammonification like in anoxic conditions?
NH4+ tends to be stable and persist (exception anammox - anoxic ammonia oxidation; occurs in some ammonia rich habitats such as sewage)
what environment conditions are good for nitrification?
well-drained, neutral-pH soils, aquatic environment
what type of metabolism should a microbe have for nitrification?
aerobic, chemolithotrophic
