Transcript Lecture 21-23, Redox processes

Carbon in anaerobic aquatic environments
Carbon in the form of
CO2, HCO3- and CO3-2, are
oxidized forms of C, and tend
to be the only forms present
where O2 is plentiful.
In anoxic environments
methanogens (Archaea) convert
organic C and CO2 into
methane (CH4).
Methane is a gas and can
bubble out of the water or it can
be oxidized to CO2 by
methylotrophic bacteria.
Methanogens are not true bacteria, they belong to the Archaea
Most methanogens can grow on CO2 and H2 as their sole energy source:
Chemoautotrophs —chemical bond energy is their energy source
they utilize CO2 as their C source
http://faculty.plattsburgh.edu/jose.deondarza/images/Organisms/methanogen.jpg
C-transformations in aerobic and anaerobic environments
Oxidation
state
Where do we find methanogens?
-4
0
+4
•Under anaerobic conditions organic molecules break down to methane instead of CO2—This
process is facilitated by methanogens (Archaea), which are chemoautotrophic bacteria.
•They utilize the energy released from 2H2 + Organic C (CH2O)→CH4 + H20 to build their
biomass.
How to assign Oxidation numbers
We keep track of the e- transfer using Oxidation numbers (Ox#)
For each e- transferred the Ox# changes by 1
2H2 + O2
0
0
2H2O
+1 -2
Some rules for Oxidation numbers
1. In free elements Ox# =0
2. For ions with one atom Ox# = charge. eg H+ Ox# of H+ = 1
3. Ox# of O in most compounds is -2,
4. Ox# of H in most compounds is +1,
5. For a complex ion like SO4-2 , the net Ox# = charge (Thus S=+6)
Chemical equation for the reduction of CO2 by H2
CO2  H 2  CH 4  H 2O  E(energy)
CO 2  4 H 2  CH 4  2 H 2O
( 4)
(0)
(-4)
-
( 1)
8 e are accepted per C
-
1 e is donated per H
Nutrition and Metabolic Diversity
Four nutritional categories
Carbon Source
Energy Source
CO2
Organic
Light
Photoautotroph
Photoheterotroph
Chemical
Chemoautotroph
Chemoheterotroph
Solar Saltern Plant: a typical habitat of Halobacterium salinarium
Functional classification?
•found in water saturated or nearly saturated with salt (Halophiles) and rich in organic
matter.
•Large blooms appear reddish, from the pigment bacteriorhodopsin. This pigment is used
to absorb light, which provides energy to create ATP.
•The process is unrelated to other forms of photosynthesis involving electron transport,
however, and halobacteria are incapable of fixing carbon from carbon dioxide.
http://www.biochem.uni-luebeck.de/public/groups/metalloproteins/hubmacher/Hubmacher_Abb2.jpg
The Cycling of Nitrogen
N is an important nutrient that frequently limits primary productivity in
aquatic ecosystems
It is rare in the earth’s crust, but makes up 79% of the atmosphere (N2)
(oxidation state =0)
Most algae and plants require
NO3¯(+5) (NO2 ¯) (+3) or NH3 (NH4+) (-3)to synthesize amino acids to
make proteins
N-fixing microorganisms can take up N2 and convert it to NH3
N2 + 3H2 → 2NH3
Many plants have N-fixing mutualists (eg Azolla)
Denitrifying bacteria can
convert NO3¯ back to N2
Azolla, an aquatic fern used in rice culture
•The leaves of this aquatic fern
have cavities that harbour
filamentous cyanobacteria
Anabaena azollae
•The large cells (heterocysts) are
specialized for N-fixation
•Traditional rice farming in many
countries involve planting Azolla to
build up N concentrations in rice
paddy.
The Nitrogen cycle involves many different oxidation states, and the
redox processes are facilitated by plants and wide variety of bacteria
Chemoheterotrophs (CH)
-3
CH
PA
0
+1
+3
Nitrite
CH
+5
Photoautotrophs (PA)
Chemoautotrophs(CA)
Chemoheterotrophs (CH)
-2
0
+4
+6
CH
PA
Photoautotrophs (PA) Chemoautotrophs(CA)
Streams draining mine tailings are extremely acidic—the effect
of Thiobacillus oxidizing pyrites and iron.
Thiobacillus ferrooxidans
oxidizes both the iron,
Fe(+2) to Fe(+3)
and the sulphur in the pyrites,
S(-1) to S(+6) using
molecular oxygen.
This reaction splits water to
produce a great deal of acid.
How do you suggest that mine
tailings should be stored?
2FeS 2  7.5O2  4H 2O  4SO4 2  Fe2O3  8H   Energy
Desulfovibrio : Sulfate reducing bacteria
commonly found in anaerobic aquatic environments with high levels of organic material, such
as mud in lakes and ponds.
•have metal reductases which can precipitate metal sulfides from the water—
•bioremediation potentials for toxic radionuclides such as uranium by a reductive bioaccumulation process.
Sulfate reduction can absorb H+ and
counteract acid rain
They also contribute to methylation of
Mercury
Chemical equation for the oxidation of acetate by sulfate
CH 3COO   SO 4 2  H   S   H 2O  CO2  E

2


CH 3COO  SO 4  H  S  2 H 2O  2CO 2
( 6)
(0)
(-2)
( 4)
8 e - are accepted per S
-
4 e are donated per C
Where do sulphate reducing bacteria fit in the functional classification?