12. Chemolithoautotrophy

Question 1

Give 7 possible electron donors in chemolithotrophic reactions

Answer 1

Phosphite, hydrogen, sulfide, sulfur, ammonium, nitrite and Ferrous (+2) iron

Question 2

How many oxidation states of sulphur are there?

Answer 2

8 states: -2 –> +6

Question 3

What is sulphide and sulphate present in?

Answer 3

Sulphide present in minerals

Sulphate present in sediments and rocks (oceans contain highest [S] concentration in biosphere)

Question 4

What is the chemical formula for gypsum and pyrite?

Answer 4

Gypsum: CaSO4
Pyrite: FeS2

Question 5

What is the equation for H2S oxidation? What energy is released? Where does this type of oxidation occur?

Answer 5

H2S + 2O2 –> SO4 2- + 2H+

798.2kJ released (exothermic)

Occurs where the anoxic meets the oxic zone by anoxygenic photosynthetic bacteria

Question 6

What is the equation for the oxidation of elementary sulphur?

Answer 6

S + H2O + 0.5O2 –> SO4 2- + 2H+

587.1 kJ/reaction

Question 7

What is the equation for the oxidation of S2O3 2-? How much energy is released?

Answer 7

S2O3 2- + H2O + 2O2 –> 2SO4 2- + 2H+

Question 8

How many different cytochrome oxidases are there in prokaryotes and eukaryotes?

Answer 8

Prokaryotes have more than one type whereas eukaryotes only have one type (aa3)

Question 9

What is the role of the cytochrome oxidase?

Answer 9

complex IV - Water is generated from this

Question 10

What is generated if you reverse the electron flow in the ETC?

Answer 10

NADPH/NADH (used for CO2 fixation)

Question 11

What are the three common oxidation states of iron?

Answer 11

0, +2 (ferrous), +3 (ferric)

Question 12

How may ferrous iron be oxidised? Under which conditions does iron not oxidise as readily?

Answer 12

Chemically or bacterially (usually by oxygen)

Acidic conditions

Question 13

What form of iron is rust?

Answer 13

Ferric - +3

Question 14

What is formed by ferric iron when it is oxidised (at above pH7)?

Answer 14

Forms iron hydroxide (FeOH3)

Question 15

What is the most abundant element in the earth?

Answer 15

Iron

Question 16

How may ferric iron be reduced?

Answer 16

Chemically or bacterially

Question 17

At which pHs is Ferrous iron soluble?

Answer 17

pH4 and below

Question 18

What type of iron may be used as an electron donor in respiration? What is the equation? How much energy does this released

Answer 18

Ferrous (+2)

Fe(II) + H+ + 0.25O2 –> Fe(III) + 0.5H2O

deltaG0 = -32.9 kJ/reaction

Question 19

What fossil fuel may a form of iron be associated with?

Answer 19

Pyrite (FeS2) may be associated with coal

Question 20

What happens if you add pyrite/arsenopyrite to water? What is the name of this process (if it is caused by a mine)

Answer 20

The sulphur in the pyrite dissolves to make sulphuric acid. This lowers the pH of the water (to ~1), attracting acidophilus which oxidise Fe(II) to Fe(III). At low pHs the Fe(III) will remain in solution but as the pH increases to ~7 (over time) the iron will precipitate out. This is known as acid mine drainage (AMD).

Question 21

What is the name of the acidophile responsible (partly… safe to say most of the blame can go to the mine-owning egocentric bastards) for acid mine drainage?

Answer 21

Acidithiobacillus ferrooxidans

Question 22

Describe the electron transport chain of acidithiobacillus ferrooxidans? How is NADH generated? and ATP?

Answer 22

Fe(II) is oxidised to Fe(III) by Cyc2 on the outer membrane. The electron from this is passed through the haem group (of Cyc2) to rusticyanin and then to Cyc1. From here a decision must be made…

Either the electron is passed to aa3 cytochrome oxidase causing reduction of O2 to H2O and the pumping of a proton, eventually leading to ATP generation (from ATP synthase)

OR the electron is passed to bc1, then reduces ubiquinone to ubiquinol. NDH then oxidises ubiquinol causing the generation of a molecule of NADH from NAD+

Question 23

What is rusticyanin?

Answer 23

A copper containing protein which functions as an electron carrier

Question 24

How does rusticyanin act as an electron carrier?

Answer 24

When Fe(II) is oxidised to Fe(III) it causes the reduction of Cu(II) to Cu(I) in rusticyanin’s centre

Question 25

What does the rus operon encode?

Answer 25

Cyc2, Cyc1, Rusticyanin and cytochrome oxidase

Question 26

How is the rus operon regulated?

Answer 26

Positively regulated at transcriptional level by Fe(II)

So as the levels of Fe(II) increase, more proteins are provided to br-br-break it down (and produce NADH and ATP)

Question 27

What evidence is there that the rus operon is regulated by Fe(II)?

Answer 27

Expression of rus operon is higher in Fe(II) than Sulphur medium grown cells. If you switch from Fe(II) to Sulphur medium there will be a rapid increase in the proteins encoded by the rus operon (and vice versa)

Question 28

What happens to the rus operon in media containing Fe(II) and sulphur? Why is this unexpected?

Answer 28

The rus operon proteins are encoded only until the Fe is completely oxidised at which metabolism switches to sulphur.

Fe(II) is the favoured electron donor when sulphur oxidation is an energetically more favourable reaction (more exergonic)

Question 29

What is microbial leaching? What metals are mined this way? What microbes are used?

Answer 29

Use of microbes to make ores accessible for extraction. If low grade ore than microbial leaching is more efficient.

Cu/Au/U - 25% of copper is mine this way

Ferrooxidans are used - the oxidation of sulphur aids solubilisation of the metal.

Question 30

What are the equations for the leaching of uranium and gold?

Answer 30

Uranium: U4+ + Fe3+ –> U6+ + Fe2+

Gold: FeAsS[Au] + O2 –> Fe2+/3+ + S+6 + As+3 + [Au]