Lecture 18 - Iron-Sulfur Cycling

Question 1

How is sulfur oxide emitted into the atmosphere?

Answer 1

microbial processes | fossil fuel burning | volcanoes and host springs

Question 2

What is the largest reservoir of sulfur on the planet? What form is sulfur in within this reservoir? Is it actively cycled?

Answer 2

ocean | sulfate | yes

Question 3

Which sulfur reservoir is not actively cycled?

Answer 3

Earth’s crust

Question 4

What form is sulfur in within the atmospheric sulfur reservoir?

Answer 4

sulfur oxide and hydrogen sulfide

Question 5

What are the 4 processes of the sulfur cycle?

Answer 5

sulfur assimilation | sulfur mineralization | sulfur oxidation | sulfur reduction

Question 6

What is sulfur assimilation?

Answer 6

taking sulfate (SO4) and incorporating it into biological molecules | requires 2 ATP

Question 7

What is the predominant form of sulfur in the environment?

Answer 7

sulfate (SO4)

Question 8

What kind of reaction (reduction or oxidation) does sulfur assimilation occur within the cell?

Answer 8

reduction reaction

Question 9

What does “-SH” stand for?

Answer 9

organic sulfur

Question 10

What is sulfur mineralization?

Answer 10

a metabolic reaction, opposite of assimilation process | releasing sulfur from organic molecules | anaerobic and aerobic environment

Question 11

What do the process of sulfur assimilation and mineralization cycle sulfur between, since they are opposites?

Answer 11

organic and inorganic forms of sulfur

Question 12

What is H2S?

Answer 12

hydrogen sulfide = toxic gas

Question 13

What is sulfur oxidation?

Answer 13

H2S&raquo_space;> SO4 via oxidation reactions

Question 14

What are the 2 ways sulfur is oxidized in sulfur oxidation? (aerobically)

Answer 14

H2S&raquo_space;> So (elemental sulfur) | So&raquo_space;> SO4 (results in acid)

Question 15

Which step in sulfur oxidation results in acid mine drainage and why?

Answer 15

So&raquo_space;> SO4 because it’s really So + H2O&raquo_space;> SO4 + 2H+ and that mixed together results in sulfuric acid

Question 16

How does the So&raquo_space;> SO4 reaction cause acid mine drainage? What is acid mine drainage useful in?

Answer 16

in low pH = causes metals to precipitate out | useful in metal recovery

Question 17

What is a way that sulfur is oxidized anaerobically?

Answer 17

use NO3 as e- acceptor = end up producing calcium sulfate (gypsum)

Question 18

What is photoautotrophic sulfur oxidation?

Answer 18

photoautotrophic organisms that can oxidize sulfur by using light energy

Question 19

Which organisms are capable of photoautotrophic sulfur oxidation?

Answer 19

green and purple sulfur bacteria

Question 20

Why are the green and purple sulfur bacteria an important group of organisms?

Answer 20

they remove sulfide from the environment and oxidize it to sulfur | H2S&raquo_space;> So

Question 21

What is sulfur reduction?

Answer 21

inhibited by oxygen | makes H2S = can cause metal corrosion

Question 22

What are the 3 ways sulfur is reduced?

Answer 22

sulfur assimilation | sulfur respiration | dissimilation

Question 23

What is sulfur respiration?

Answer 23

So&raquo_space;> H2S | uses acetate as one of the reactants

Question 24

What is sulfur dissimilation?

Answer 24

SO4&raquo_space;> H2S | uses methanol as one of the reactants

Question 25

In sulfur reduction, where do methanol and acetate come from? What does it mean?

Answer 25

carbon cycle from fermenter microbes | an anaerobic interaction between the sulfur reducing bacteria and the carbon-fermenting bacteria and methanogens

Question 26

What are 5 things sulfur is required for?

Answer 26

cysteine | methionine | vitamins | hormones | co-enzymes

Question 27

What can the sulfur cycle form in the environment?

Answer 27

acid rain | acid mine drainage | metal corrosion

Question 28

Even though iron is highly abundant in the Earth's crust, why is it not readily used?

Answer 28

bioavailability is limited = not in the form cells can use it

Question 29

Why is iron important? What is it used for?

Answer 29

all cells need iron - used as co-factors in respiration and photosynthesis

Question 30

If all cells need iron, how do microbes attain it when the bioavailability of iron is limited?

Answer 30

cells have developed different ways of attaining iron

Question 31

Does iron only rely on cells to change form? Why or why not?

Answer 31

no it can change form chemically

Question 32

What are the two main processes in iron cycling?

Answer 32

iron oxidation | iron reduction

Question 33

What is iron oxidation?

Answer 33

Fe2 (ferrous iron) >>> Fe3 (ferric iron) | generates low amounts of energy | aerobic, commonly seen under acidic conditions = leads to acid mine drainage

Question 34

What is the result of iron oxidation occuring in neutral conditions?

Answer 34

results in metal corrosion

Question 35

How do the green and purple phototrophic sulfur bacteria play a role in the iron cycle?

Answer 35

they are capable of using ferrous iron (Fe2) as an electron donor instead of oxygen to perform iron-based photosynthesis

Question 36

What hypothesis does iron-based photosynthesis help theorize?

Answer 36

planet wasn't always oxygenated = anaerobic = iron-based photosynthesis may represent a type of photosynthesis that was present on the planet before transitioning to aerobic conditions

Question 37

What are the 2 main purposes/different reactions of iron reduction?

Answer 37

assimilation | energy generation

Question 38

What is iron assimilation?

Answer 38

taking iron and reducing it to be usable within a cell

Question 39

What complex does iron assimilation occur with?

Answer 39

siderophores

Question 40

What are siderophores?

Answer 40

compound that binds to iron secreted by microbes to transport iron

Question 41

How do siderophores help bacteria intake iron?

Answer 41

siderophore recognizes a surface receptor on cell = result in the reduction and transport of iron into cell

Question 42

What form of iron is commonly used by cells?

Answer 42

Fe2

Question 43

What is the energy generation part of iron reduction?

Answer 43

using ferric iron (Fe3) through anaerobic respiration

Question 44

What are the two ways cells generate energy using ferric iron (Fe3)?

Answer 44

direct attachment/contact | electron-shuttle

Question 45

What is iron reductase?

Answer 45

membrane-bound enzyme allowing direct access

Question 46

What is the direct attachment method bacteria use to reduce Fe3?

Answer 46

bacteria directly contacts with iron oxide surface = iron reductase reduces Fe3 to Fe2 = Fe2 used to make energy needed to take organic carbon and fix it into CO2 and water

Question 47

What is the electron shuttle method bacteria use to reduce Fe3?

Answer 47

molecule or protein that shuttles the electrons from the iron oxide surface (post iron reduction) to the cell

Question 48

What is a common electron shuttle, especially in soil environments?

Answer 48

humic acids

Question 49

What are the important things from the iron cycle?

Answer 49

green and purple sulfur bacteria | iron reduction for both assimilation and energy generation