Chemolithoautotrophy

Question 1

What are chemolithoautotrophs?

Answer 1

rock eaters 
fix CO2 as carbon source
use inorganic chemicals for energy
require 2 electron donors for both ATP generation and CO2 fixation
oxidise inorganic compounds

Question 2

What are the types of chemolithoautotrophs - in order of decreasing energy yield?

Answer 2

• hydrogen oxidisers
• methanogens (homoacetogens)
• methyltrophs
• sulfur oxidisers
• iron oxidisers
• manganese oxidisers
• nitrogen oxidisers

Question 3

Describe the metabolism of hydrogen oxidisers?

Answer 3

• are the most widespread chemolithoautotrophs
• can be aerobic (TEA = O2) or anaerobic (TEA = S or CO2)
• can use many different TEAs
• can be facultative anaerobes ( PED = NO3- Mn(IV), Fe(III)
• can be obligate anaerobes (PED = SO42- or CO2)

Question 4

Why is chemoheterotrophy more favourable than chemolithoautotrophy?

Answer 4

• difference in electrode potential is greater (between the PED and TEA)
• electron donors are required for both ATP generation and CO2 fixation in chemolithoautotrophy.
• biomass yield per substrate oxidised is lower for chemolithoautotrophy

Question 5

Describe the metabolism for Methanogens

Answer 5

• Methanogens produce methane (methanogenesis)
• can be chemolithoautotrophs or chemoheterotrophs
  (can use inorganic or organic forms of carbon)
  chemoheterotrophic methanogenesis: CH3COOH –> CH4 + CO2
  Chemolithoautotrophic methanogenesis: CO2 + 4H2 –> CH4 + 2H2O
• methanogenesis –> do not use ETC but use specialised enzymes

Question 6

Describe the metabolism of Methanotrophs

Answer 6

• methanotrophs consume (oxidise) methane

- widespread where there is abundant methane

Question 7

Describe the metabolism of Sulfur Oxidisers

Answer 7

2 varieties:
Gradient organisms:
- grow at neutral pH, microaerophillic conditions
- found in environments with reduced sulfur and low oxygen concs, e.g., sulfur springs, microbial mats, sewage polluted freshwater
- abiotic S oxidation at low O2 is slow –> bacteria catalyse it
Acidophiles:
- grow in acid rich environs
- TEA = O2
- can use (NO3-, NO2-, Fe3+)
- PED = H2S
- S has a lot of oxidation states. Different species of S oxidisers can work on different oxidation states at each stage
H2S –> S –> S2O3^2- –> SO3^2- –> SO4^2-
-2. 0. +2. +4. +6
- when acidic, the reduced sulfur can stay in solution without being ppt as a sulfate mineral, so organisms can take advantage of this and speed up reaction

Question 8

Describe the metabolism of Iron Oxidisers

Answer 8

2 Varieties: Neutrophiles or acidophiles 
Neutrophiles
- neutral environs, with low O2
- require some oxygen as TEA
- microaerophillic
- circumneutral Fe-oxidisers (can oxidised Fe if necessary)
- e.g. Gallionella ferruginea 
- may form enctrustments of Fe sheaths 

Acidophiles

• live in acid rich environments
• mine waste
• may form entrustments and make Fe sheaths
• e.g., Acidothiobaccillus

Question 9

Describe the metabolism of Manganese Oxidisers

Answer 9

• in oxic conditions at neutral pH, abiotic oxidation of Mn(II) is slow
• catalysed by manganese oxidising bacteria
• Mn-oxides encrust cells or form sheaths
• not all Mn-oxidising bacteria derive energy from it

Question 10

Describe the metabolism of nitrogen oxidisers

Answer 10

• NH4+ = PED
• O2 = TEA
• nitrifying bacteria: NH4+ –> NO2- –> NO3-
  2 step reaction:
  NH4+ –> NO2- : nitrosomonas, nitrosopira (ammonium –>nitrogen dioxide)
  NO2- –> NO3 - : nitrobacter, nitrococcus (nitrogen dioxide –> nitrate)
• nitrate is negatively charged and bioavalible for the plants to consume (won’t be incorporated into clay structure)

Question 11

Describe the metabolism of Anammox bacteria

Answer 11

Anammox bateria = Anaerobic Ammonia Oxidisers
NH4+ + NO2- –> N2 + H2O
- releases N2 gas into the atmosphere
- ammonia + nitrogen dioxide –> nitrogen gas + water