Biogeochem - Chapt 11: Sulphur Cycling

Question 1

What are the main forms of sulphur in the environment?

Answer 1

Sulphate
Sulphur
Sulphide
Organic S
Thiosulphate
DMS

Question 2

What is sulphur a product of

Answer 2

sulphide oxidation

Question 3

What is sulphide a product of

Answer 3

sulphate reduction

Question 4

what is the most oxidised form of sulphur

Answer 4

sulphate

Question 5

what does algae convert sulphate into

Answer 5

H2S

Question 6

What system is H2S only found in?

Answer 6

Anoxic ecosystems as it reacts with O2 to form elemental sulphur

Question 7

how is sulphur dioxide produced?

Answer 7

formed by combustion of organic matter or reduced sulphur compounds (main agent of acid rain).

released to the atmosphere and volcanoes and hydrothermal sites

Question 8

Is sulphate reduction anaerobic or aerobic respiration?

Answer 8

anaerobic respiration

Question 9

What organisms carry out sulphate reduction?

Answer 9

Bacteria and Archaea

Question 10

what conditions are required for sulphate reduction to occur?

Answer 10

When oxygen has been consumed/depleted

Question 11

What determines the depth that sulphate reduction starts?

Answer 11

The amount of organic matter impacts the depth at which sulphate reduction occurs.

If there is more organic matter, the electron acceptors are depleted faster. This results in sulphate reduction starting at shallower depths

Question 12

How does sulphate reduction impact carbon remineralisation?

Answer 12

in productive seas, the sulphate reducing bacteria in the sediment breaks down the organic matter into simpler compounds such as CO2 and CH4.

Question 13

what impact do freshwater systems have on sulphate reduction

Answer 13

Freshwater systems generally have less sulphate in their sediments than in marine systems

this mean that sulphate gets depleted faster because there is less of it

as soon as sulphate is depleted, methanogenesis begins which becomes the dominating carbon degradation pathway

Question 14

Sulphate reduction and carbon remineralisation in abyssal sediments

Answer 14

low organic matter levels means that most organic carbon is broken down using oxygen (aerobically)

sulphate reduction only occurs if oxygen is depleted. this means that sulphate reduction happens at much lower rates compared to shelf and slope sediments which have more organic matter

Question 15

what does the depth of the sulphate-methane transition zone depend on

Answer 15

The organic matter content

The higher the organic C input, the shallower the depth in which sulphate is depleted and methane production starts

Question 16

where does the anaerobic oxidation of methane (AOM) occur?

Answer 16

Methane is produced biologically in very deep sediments and from the degradation of kerogen through the process of catagenesis.

When this methane is produced, it can move up throuh the sediment column into the upper sediment levels until it reaches the sulphate reduction layer.

Question 17

What is the process of AOM

Answer 17

There is a synotrophic consortium (relationship) between methane-oxidising archaea and sulphate reducing bacteria

The methanotrophic archaea convert methane (CH4) to either CO2 + 4H2 or CH3COO- (acetate).

This is then converted by sulphate reducing bacteria which use sulphate to convert it to HCO3, HS and H2O (bicarbonate, hydrogen sulphide and water)

Question 18

How are carbonate mounds and crusts formed?

Answer 18

As AOM produced significant amounts of carbonate, carbonate crusts can form in the sediment column.

Mounds form in the Black Sea where methane seeps into the sulphur filled water column. AOM occurs here and produces carbonate mounds which can be 3m tall.

Question 19

what process is sulphide oxidation?

Answer 19

Lithotrophic

Question 20

What is the product of sulphur oxidation

Answer 20

elemental sulphur

sulphide oxidised with O2 to produce elemental sulphur

Question 21

If metal oxides are present (ie manganese oxides), what is the process of sulphur oxidation?

Answer 21

Sulphide is oxidised abiologically to thiosulphate, which is then oxidised to sulphate by microorganisms

Question 22

What do sulphide oxidising bacteria have to compete with?

Answer 22

chemical oxidation

Question 23

After Step 1 in sulphide oxidation… what happens to the sulphur?

Answer 23

elemental sulphur is not soluble in water and so forms sulphur globules

organisms store it

Question 24

What process produces a lot of acidity?

Answer 24

The oxidation of sulphide to sulphate

Question 25

name an example of a sulphur oxidiser

Answer 25

Beggiatoa

Question 26

What do the sulphide oxidisers, Beggiatoa, produce and where do they occur?

Answer 26

They form dense mats that are white in colour.

They are found at the sulphide-oxygen interface around hydrothermal vents or in forest ditches/ponds

Question 27

Where and why do microorganisms couple sulphide oxidation with nitrate reduction (to either N2 or Ammonium)

Answer 27

This occurs where sulphide and oxygen are spatially separated.

nitrate can be accumulated in a cell while O2 diffuses easily across a membrane

Question 28

What happens in acidic conditions with regards to sulphide oxidation?

Answer 28

microorganisms couple the oxidation of elemental sulphur to iron reduction

Question 29

What are the 3 genera of giant sulphur-oxidising bactera?

Answer 29

Beggiota
Thioploca
Thiomargerita

Question 30

Properties fo Beggiota (sulphur-oxidising bacteria)

Answer 30

• Form mats at the sediment surface
• rely on steep gradients and sulphide being available close to the surface
• position themselves between the O2-Sulphide interface
• cannot survive if sulphide and elemental oxygen are spatially separated

Question 31

Properties of Thioploca (sulphur oxidising bacteria)

Answer 31

• migrate vertically through the sediment
• take up nitrate from the overlying water and accumulate it in their vacuole
• after migrating into the sulphidic layer, nitrate is used as an electron acceptor for sulphide oxidation
• they can use spatially extended gradients
• they are motile

Question 32

Properties of Thiomargerita (sulphur oxidising bacteria)

Answer 32

• they are not motile
• they rely on outbursts of methane gas from underlying gas hydrates
• when the gas load in the sediment is too high, it gets released into the water column, the thiomargerita going with it
• in the water column they take in nitrate and store it in their vacuole (98% of their cell volume)
• when the sediment settles again, they get buried and use the nitrate stored to keep going until the next resuspension event
• this occurs in regions of upwelling.

Question 33

What animal do smaller aerobic sulphur oxidisers associate with?

Answer 33

Nematodes (worms)

Question 34

what is the relationship between Nematodes and Sulphur Reducing bacteria?

Answer 34

the move migrates through the sediment into the sulphur layers where the bacteria can oxidise the sulphur

over time, the worm grazes on the bacteria, the bacteria will regrow on the worm and the cycle continues

Question 35

What’s the next evolutionary step for the nematodes and sulphur oxidising bacteria?

Answer 35

Endosymbiosis where the bacteria are inside the worm, oxidising the sulphur and providing a product that can be utilised by the nematode

Question 36

Describe the endosymbiotic relationship between Riftia pachyptila (hydrothermal tubeworm) and sulphur oxidising bacteria

Answer 36

• tube worm gills are full of blood with haemoglobin which can take up O2 and H2S thats released from hydrothermal vents
• The blood transports the H2S and O2 to the Trophosome in the middle of the worm.
• Trophosome is full of cells which are full of bacteria.
• The bacteria are chemolithoautotrophs so produce organic matter, some of with they use for maintenance and growth, the rest they excrete to the worm

Question 37

Explain how purple sulphur bacteria oxidise sulphur

Answer 37

Purple sulphur bacteria are photoautotrophs and can grow in oxic and anoxic conditions.

If oxygen is present, they will use aerobic respiration

If conditions are anoxic, they will use anoxygenic photosynthesis to oxidise sulphur

Question 38

Why is DMS (dimethylsulphide) seen as an anti-greenhouse gas?

Answer 38

When algal blooms collapse, huge amounts of DMSP are released which is degraded by bacteria in the water column to DMS.

DMS is volatile, meaning significant amounts of it get into the atmosphere.

This gets oxidised into into methyl sulphonic acid MSA and into SO2. This gets oxidised into SO3 which reacts with water to form sulphate.

Sulphate particles acts as cloud condensation nuclei (CCN) which increases cloud cover, leading to a higher albedo.