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EART21102 Environmental Microbiology > Microbial Fe, Mn, As cycles > Flashcards

Microbial Fe, Mn, As cycles Flashcards

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1
Q

What is the oxidised form of iron?

A

Fe(III), Fe 3+, ferric

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2
Q

What is the reduced form of iron?

A

Fe(II), Fe 2+, ferrous

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3
Q

What conditions are required for ferrous iron oxidation?

A

Aerobic conditions

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4
Q

What conditions are required for ferric iron reduction?

A

Anaerobic conditions

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5
Q

What do isotopic tests on BIFs show?

A

Half of Fe atoms originated in shallow oceans after being processed by microbe 2.5 Gya

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6
Q

When is Fe(II) stable?

A

Low pH aerobic conditions

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7
Q

What type of organism can use Fe(II) as an electron donor for aerobic respiration?

A

Acidophilic chemolithotroph

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8
Q

Describe the process of using Fe(II) as an e- donor for aerobic respiration

A

FeS2 reacts with oxygen to give Fe(II) (initiator reaction, either bacteria or chemical-driven), O2 is an e- acceptor, producing Fe(III)

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9
Q

How can using Fe(II) as an e- donor for aerobic respiration lead to acid mine drainage?

A

If pyrite is present, Fe(III) can react to give H2SO4

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10
Q

How is autotrophic bioleaching carried out?

A

By acidophilic S- or Fe- oxidising bacteria
e.g. oxidation of pyrite giving sulfuric acid (acid mine drainage)

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11
Q

What is bioleaching used for?

A

To extract low grade ores

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12
Q

Describe water affected by AMD

A

High acidity and elevated Fe(III) and other toxic metals, e.g. Cu, Zn, Ni, As
Toxic to almost all forms of aquatic life

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13
Q

How is AMD treated?

A

Sulfate-reducing bacteria

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14
Q

Describe how SRBs treat AMD

A

Reduces sulfate to sulfide, raises pH, forms H2S, precipitates toxic metals as sulfides

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15
Q

What was tested at Wheal Jane Mine to treat AMD and what was used in the end?

A

Tried active measures and switched to passive measures

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16
Q

Describe the active measures that were tested at the Wheal Jane Mine

A

Three step system that generates lots of sledge
Removes 99% of metals when pH is adjusted to 8.5

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17
Q

Describe the passive measures used at the Wheal Jane Mine

A

Aerobic cells, anaerobic cells, shallow rock filters

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18
Q

Name a common form of aerobic cell to treat AMD and describe how it works

A

Artificial reed beds (commonly used commercially)
Drives the system oxic to precipitate Fe as Fe3 hydroxides that absorb metals
Microbial oxidation around the roots

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19
Q

Describe anaerobic cells to treat AMD

A

Uses compost bioreactors, e- donors include straw and sawdust
pH is raised by metabolic
Only successful when a conditioning phase allows reactors to stabilise

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20
Q

Describe shallow rock filters to treat AMD

A

Algal growth and precipitation of metals via alkalinity, driven by consumption of CO2 during oxygenic photosynthesis
Placed downstream of the anaerobic cells

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21
Q

What are the two methods for anaerobic Fe(II) oxidation?

A

Anoxygenic phototrophic bacteria
Nitrate-reducing bacteria

22
Q

Describe nitrate-reducing bacteria

A

Uses Fe(II) as e- donor for the reduction of nitrate to nitrogen

23
Q

Describe an oxygenic phototrophic bacteria

A

Transfers e- to anything with a redox potential higher than +100mV (Ferric/ferrous iron redox potential in bicarbonate environment is +100mV
e- flow into photosynthetic system (Fe(III)) from Fe(II)

24
Q

What type of organisms can aerobically oxidise Mn(II)?

A

Diverse range of bacteria and some fungi

25
Q

What are the three mechanisms of aerobic Mn(II) oxidation?

A

Oxidise dissolved Mn(II) enzymatically (organism gains energy)
Oxidise Mn(II) sorbed to Mn(IV) oxide (some gain energy, some don’t)
Attack Mn(II) non-enzymatically (high pH environment)

26
Q

What is Fe(III) reduction catalysed by?

A

Dissimilatory iron reducing organisms

27
Q

Where does dissimilatory Fe(III) reduction occur?

A

Outside the cell because Fe(III) is highly insoluble
This is unusual, most e- acceptors are reduced inside the cell

28
Q

What conditions allow for dissimilatory Fe(III) reduction?

A

Anaerobic conditions
The ion is the e- acceptor rather than oxygen

29
Q

What is oxidised in dissimilatory Fe(III) reduction?

A

A carbon substrate
e.g. acetate to CO2

30
Q

What happens to the Fe(II) produced in dissimilatory Fe(III) reduction?

A

Some goes into aqueous environment and some is incorporated into new mineral phases

31
Q

Describe e- donors for Fe(III) reduction

A

Hydrogen, large range of organics (e.g. benzene)

32
Q

Describe e- acceptors for Fe(III) reduction

A

Range from amorphous (bioavailable) to near crystalline
Alternatives include toxic metals and radionuclides

33
Q

How can Fe(III) reduction be stimulated? What is this used for?

A

By adding soluble Fe(III), chelating agent, or e- shuttle
Used for in situ remediation

34
Q

What are the three mechanisms for Fe(III) reduction?

A

Direct contact, chelating agent, electron shuttle

35
Q

What can an e- shuttle do? Where does it come from?

A

Catalyses direct contact
Is either secreted or exogenous

36
Q

Describe the role of Geobacter in Fe(III) reduction

A

Uses e- transfer,
Peripili conducts e- out to iron oxide, increases contact

37
Q

Give three impacts of Fe(III)-reducing bacteria

A

Release of Fe(II), formation of new Fe(II)-bearing minerals, release/capture of trace elements, degradation of organics

38
Q

Give a Fe(II)-bearing mineral and what it can be used for

A

Magnetite (converted by Geobacter)
Has applications in medicine and water treatment (for biotransformation of waste minerals)

39
Q

How can Fe(III)-reducing bacteria be used to generate energy?

A

It can use anodes in fuel cells as en e- acceptor
Also has applications in waste organic treatment and metal recovery (at the cathode)

40
Q

Give four examples of functional bionanominerals

A

Nanocrystals, nanomagnets, quantum dots, nanosilver biocide

41
Q

Give two examples of e- donors for Mn(IV) reduction

A

Hydrogen and large range of organics

42
Q

Give four examples of uses for functional bionanominerals

A

Recording devices, healthcare, photovoltaics, catalysts

43
Q

Give two examples of e- acceptors for Mn(IV) reduction

A

Mn(IV) and Mn(III)

44
Q

What does the reduction of As(V) result in?

A

The mobilisation of As(III), this cannot be reduced further

45
Q

What does selenium (selenate/selenite) reduction result in?

A

The formation of insoluble elemental selenium (immobile), this can be reduced to selenide

46
Q

What can the reduction of As and Se be carried out by?

A

Dissimilatory iron reducing bacteria (DIRB)

47
Q

Give three potential mechanisms for As mobilisation

A

Oxidation of As-rich pyrite
Reduction of As(V)-rich Fe(III) oxyhydroxides
Mobilisation of As(V) by fertiliser phosphate or biogenic carbonate

48
Q

Give three examples of how As poisoning can be prevented

A

Identify at risk aquifers, reoxidation using air/nitrate, stimulate sulfate reduction

49
Q

How does drinking water from wells become contaminated with As?

A

Bringing up water for drinking or irrigate pushes down carbon-rich water, stimulating microbes to reduce As(III)

50
Q

How can arsenic be bioremediated?

A

Bioprecipitation with Fe(III)-(oxyhydr)oxides and sulfides

51
Q

Give two examples of how As can be bioremediated

A

Harnessing natural biogeochemical cycles to remove microbially-mobilised As
Biogenic Fe(III)-(oxyhydr)oxide and sulfides for removal

EART21102 Environmental Microbiology (9 decks)

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