Week 10 - Chemolithotrophy
Sunday, 26 March 2023 10:53
LO1: Understand the concept of chemolithotrophy, and the inorganic electron
sources exploited by chemolithotrophs
LO2: Outline the various strategies for chemolithotrophy with reference to
nitrogen, sulfur, hydrogen and iron metabolism, and anammox
Chemolithotrophy
- Chemolithotrophy is a mode of metabolism that uses the oxidation of inorganic
compounds to provide energy to synthesise ATP via an ETC, and (usually)
reducing power (electrons) for anabolic metabolism
- ORGANIC compounds always have a carbon atom, most inorganic
compounds DON’T have a carbon atom in them
- These are the two main things it uses electrons for, by extracting the
electrons from the inorganic compounds that they find in the
environment
- Most chemolithotrophs are autotrophs (i.e. self feeders that don't depend on
other organisms' carbon compounds, instead they fix CO2 to make their own
carbon compounds (e.g. calvin cycle (photosynthesis, different from Krebs cycle)
does this), it obviously needs electrons for that, so they use the electrons took
from the inorganic compounds to run the calvin cycle) from the atmosphere to
produce their organic molecules via biosynthesis)
- Can assimilate CO2 into organic molecules for biosynthesis
- A few are mixotrophs
- Get energy from inorganic compounds via redox reaction and require
complex organic compounds as carbon source
- SO they don't fix carbon like autotrophs do
- Remember electrons naturally want to go from a donor to an acceptor because
it is an energetically favourable reaction, to HARNESS that energy, the ETC is
used
- By pumping protons across the membrane and out of the cell and building up a
gradient, it makes it energetically favourable for them to re-enter, so by
coupling this to ATP-synthesis, energy is conserved
- It's important to emphasize that it's the movement of protons back through ATP
synthase that drives ATP synthesis
History of chemolithotrophy
, coupling this to ATP-synthesis, energy is conserved
- It's important to emphasize that it's the movement of protons back through ATP
synthase that drives ATP synthesis
History of chemolithotrophy
- Concept proposed by Sergei Winogradskyin 1885
- Studied (H2S) Sulfide-rich streams where he noticed strange growths which
turned out to be sulfur oxidising bacteria
- He noticed they were dependent on sulfide for growth, i.e. more sulfide in the
stream = more of these bacteria growth found
- Colourless sulfur bacteria Beggiatoa and Thiothrix highly abundant, declined at
lower H2S
- Winogradsky also made similar observations with nitrification (extracting
electrons from ammonia or nitrite)
- Amount of organic matter proportional to quantity of ammonia or nitrite
oxidised
- This is how he came up with chemolithotrophy, a new concept that bacteria
were getting their electrons from inorganic compounds they scavenged from
the environment, then using the reducing power of the electrons to fix CO2 from
the atmosphere
- Chemolithotrophs don't get their electrons from other organic
compounds (chemoorganotrophs) nor from light (photolithotrophs),
they get it from inorganic compounds
- Contradicted dogma of time that synthesis of organic matter only occurs in
chlorophyll-containing plants
- Birth of chemolithotrophy
Winogradsky is also famous for his columns
- Image shows some of Ian Sanders' Winogradsky columns (after 6 weeks'
growth)
- They're microbial microcosms, shows different microorganisms growing at
different levels of the column according to the different [oxygen]
- Mud and sludge at the bottom and water at the top, making a gradient of
, - Image shows some of Ian Sanders' Winogradsky columns (after 6 weeks'
growth)
- They're microbial microcosms, shows different microorganisms growing at
different levels of the column according to the different [oxygen]
- Mud and sludge at the bottom and water at the top, making a gradient of
[oxygen], top = anaerobic, bottom = anoxic
Chemolithotrophy
- Reduced inorganic donors transfer electrons to acceptors to generate proton-
motive force (PMF) means ETC are ALWAYS involved they just vary in what
complexes are part of the ETC and the end product
- Examples of electon8 donors: H2S, NH3 (or NH4+), Fe2+, H2, NO2-...things
other than NADH can be oxidised and feed electrons into the redox tower
Reduced inorganic compounds – where do they come from?
- A common source for reduced inorganic compounds in the environment is from
the breakdown of organic matters of vegetation etc, or any kind of biomass,
when biomass is broken down completely, it releases some reduced inorganic
compounds
- All organic matter contains the macronutrients C, N, P and S
- 106C:16N:1P (and some S)
- Formula for complete oxidation of any biomass:
- (CH2O)106(NH3)16H3PO4 + 106 O2 = 106CO2 + 16NH3 + H3PO4 + 106H2O
- (CH2O)106 (NH3)16 H3PO4 + 53SO42- = 106CO2 + 53S2- + 16NH3 + H3PO4 +
106H2O
○ I.e. Reduced inorganic compounds can come from oxidation of
organic compounds, when biomass is broken down
- What lecturer is trying to get at, is look at the complete oxidation of any
biomass equation and notice how ammonia is there and even sulfide is there!
- Therefore reduced inorganic compounds come from broken down organic
matter
- In addition, reduced inorganic compounds come from other sources like mineral
sourced, that release copious amounts of reduced inorganic compounds and
electrons
