Lecture 8, 2005
Physiology, cont…
Some Archaea (Extreme Halophiles) are capable of
light mediated synthesis of ATP that does NOT involve
chlorophyll.
i.e. a light driven proton pump……
Some unusual physiologies only found in
the Archaea:
> Methanogenesis
> Rhodopsin driven phototrophy
Methanogens
• produce methane (CH4) as part of energy
metabolism.
• Occupy diverse habitats:
> Marshes and swamps
> Animal digestive tracts
> Hydrothermal vents
> Waste Management facilities
Methanogens are strictly anaerobic.
The basic chemical reaction most methanogens use:
4H2 + CO2
CH4 + 2H2O
A type of anaerobic respiration H2 is the e- donor and CO2
the e- acceptor. Only favorable energetically under
extremely reducing conditions (when no other e- acceptors
are available…..)
Other substrates may also be used such as
formate
Methyl substrates, e.g. methanol (CH3OH)
Acetrotrophic substrates, e.g. acetate (CH3COOH
Another physiology only done by prokaryotes =
In any of those cases, the production of
methane is an exergonic reaction, thus the
energy can be used to synthesize ATP.
Most also use CO2 as a carbon source, and
are therefore chemoautotrophs…...
Nitrogen Fixation
The incorporation of atmospheric N2 into organic compounds….
!Only found in some Bacteria and Archaea…
Bacteria, e.g. Azotobacter (free living aerobic),
Rhizobium, some cyanobacteria and
many anaerobic bacteria
Archaea, e.g. Methanosarcina
History
N-fixation discovered
in the late 19th century by
Beijerinck. Then Winogradsky
showed that nodules on legumes
had symbiotic bacteria
(Rhizobium) fixing N in them…
Nitrogen Fixation - Anabolic incorporation of nitrogen
N2 + 8H+ + 16ATP
2NH3 + H2
!Energetically Expensive
!Inhibited by O2
!Nitrogenase = multi-protein enzyme complex,
see next slide…….
Nitrogen fixation evolved when the earth had an
anaerobic atmosphere…….
Fig 30.1. Nodules on legume root use leghemoglobin
to bind free oxygen….
Even today the process is inhibited by oxygen… so
how can aerobic organisms carry out this
processes?
Fig. 11.9. Heterocyst of Anabaena, site of nitrogen
fixation. Only cyclic photophosphorylation takes
place in heterocysts…
Answer: in protective structures where O2 is
controlled or excluded (see next 2 slides…….
Expanding our understanding of physiologies to a
global level -- the Nitrogen and other
biogeochemcial cycles are giant
oxidation/reduction reactions……
Figure 30.11. The nitrogen cycle… N-fixation is the
primary way that nitrogen is fixed from the
atmosphere…..
Let’s go back and put some of the other physiologies in a global context:
Examples (see Figure 30.11) :
1) Nitrifying bacteria are chemoautotrophs that use reduced
N compounds as their e- donors
2) Denitrifying bacteria are anaerobic chemoheterotrophs
that use NO3- as their e- acceptor
We can also expand carbon physiologies to a
global level….. See Figure 30.9……….