CMMB 343 - Lecture 14 The Nitrogen Cycle and Symbiosis

methanogenesis

last resort, only performed by archaea, highly specialized

CO2 as electron acceptor

oxidation states

more +ve means more oxidized, N atoms accept electrons from H, C, and donate electrons to O

NO₃^- has +5 → good to donate e-

lithotrophy

oxidation of reduced inorganic compounds (NH3, H2S, Fe2+) for energy

includes nitrification (NH4+ → NO3-) and comammox

anaerobic respiration

reduction of oxidized inorganic compounds (NO3-, SO4^2-, Fe3+) as terminal electron acceptors

includes denitrification (NO3- → N2) - nitrate as e- acceptor

also includes DNRA (dissimilatory nitrate reduction to ammonia)

nutrient acquisition

reductions, N2 fixation, ammonification, anammox for assimilation

done via symbiosis with plants

anammox

combines NO2- reduction to N2 and NH3 oxidation to N2

involves both chemolithotrophy and anaerobic respiration (nitrite as terminal e- acceptor, ammonia as e-donor/energy source)

annamoxosome

internal membrane structure to house anammox, highly reactive, contained

assimilatory reduction

of inorganic compounds (NO3-, N2, SO4^2-, Fe3+, etc) is done to use the reduced compounds as nutrients in biosynthesis

dissimilatory reduction

of inorganic compounds is their use in anaerobic respiration

symbiosis

prolonged and intimate relationship between organisms

can be characterized based on their effect on the host (either another microorganism or a macroorganism) - parasitic, pathogenic, commensal, mutualistic

parasitic

microbe benefits at some expense to the host

pathogenic

the microbe causes a disease in the host

commensal

the microbe has no discernable impact on the host

mutualistic

the microbe is beneficial to the host

legume root nodule symbiosis

plants can’t fix nitrogen, bacteria can

mutualistic relationship between leguminous plants and nitrogen-fixing rhizobia (a few proteobacteria)

legumes include soybeans, clover, alfalfa, beans, peas, etc.

used in crop rotation to improve soil fertility

flavonoids

from plant, activate NodD in bacteria

NodD

a positive regulator of other nodABC genes in bacteria

nodABC genes

encode proteins that produce oligosaccharides called nod factors

nod factors

induce root hair curling, trigger plant cell division, responsible for specificity

symbiosomes

group of a few bacteroids (symbiotic rhizobia) surrounded by a plant cell membrane

needs some O₂ for N2 fixation but not enough to destroy nitrogenase (N2 → NH3, oxygen sensitive) - microaerophile

nitrogen as NH3 sent to plant and converted to glutamine or asparagine for easier vascular transport

leghemoglobin

controls O2 levels in the nodule

an O2 binding protein (red colour)

cross inoculation groups

symbiosis is specific between certain plant species and certain rhizobial species (flavonoids can cause varying responses, same with nod factors)

bacteroids

rhizobia multiply in plant cells and become swollen, misshapen, and branched cells

surrounded by plant membranes

imports organic acids from plant (succinate, malate, fumarate, pyruvate) for ATP production and to provide reducing power for nitrogenase