MBIO 2230 - Topic 6 (Sulfur) 1

Different states of sulfur 

1.) In solution
2.) Solid
3.) Gaseous

Where is most of the active sulfur on Earth?

1.) Seawater
2.) Sedimentary rocks, such as evaporites and shales 

Significance of where is most of the active sulfur on Earth?

It is all connected to seawater, such that it is either the sea itself or used to be in the sea

Evaporites

Sedimentary rocks that form over time from the evaporation of the sea, leaving behind the solutes

Shales

Sedimentary rocks formed over time from the sediments at the bottom of the ocean

Why do living things need sulfur?

For assimilatory purposes

Where is sulfur used for assimilatory purposes

1.) Co-factors (lipoic acid)
2.) Iron-sulfur clusters in enzyme active sites
3.) Amino acids (cysteine and methionine)

Importance of sulfur in cysteine 

It is related to protein stability, as it helps for disulfide bridges

Two different types of sulfate reduction 

1.) Assimilatory sulfate reduction
2.) Dissimilatory sulfate reduction

Assimilatory sulfate reduction

The reduction of inorganic sulfate into an organic sulfur product 

Significance of assimilatory sulfate reduction

It is not directly associated with energetics

Dissimilatory sulfate reduction 

The use of sulfate as an electron acceptor, where the sulfur is NOT incoporated into an organic compound, but is instead released as an H2S by-product

Significance of dissimilatory sulfate reduction

It is used as an anaerobic respiration pathway, where the sulfate is used as a terminal electron acceptor to generate energy 

Why do biogeochemists care more about dissimilatory sulfate reduction, and less about assimilatory sulfate reduction

Because dissimilatory sulfate reduction processes a lot more sulfur, resulting in larger impacts on the availability of sulfur

Energy yield depending on the terminal electron acceptor

The use of different terminal electron acceptors leads to different energy yields

ETC arrangement when undergoing dissimilatory sulfate reduction

It is arranged in a way that prevents the machinary from being exposed to the H2S, which is a toxic waste product

Significance of H2S

A gaseous compound that is toxic at sufficiently high concentrations

Sequential use of terminal electron acceptors

Refers to microorganisms using different terminal electron acceptors in a specific order, based on…
1.) Their change in concentration over time
2.) Their spatial difference over time

Sequential use of terminal electron acceptors based on changes in concnetration over time ex.) landfills

Water in landfills contain a lot of different dissolved compounds, whose concentrations will change over time, therefore the terminal electron acceptor used depends on which one is available

Sequential use of terminal electron acceptors based on spatial differenfe over time ex.)

The availability of the terminal electron acceptors changes between the edge (the youngest) and the core (the oldest)

Sulfate/sulfur ________ can be paired with the ________ of a variety of energy sources

Sulfate/sulfur reduction can be paired with the oxidation of a variety of energy sources

Sulfate/sulfur reduction can be paired with oxidation of a variety of energy sources such as…

1.) CH2O
2.) CH4
3.) H2

Sulfate reduction paired with a methanotrophic process reaction

Methane + sulfate --> HS- + bicarbonate + water

"
Depending on ones' focus, they may be called… or…"

1.) Anaerobic methane oxidation
2.) Sulfate reduction

Where does assimilatory sulfate reduction occur 

It occurs in oxic sediments, but the product it releases eventually sinks into an anoxic environment

Where does dissimilatory sulfate reduction occur?

It occurs anoxic environments, where there is no oxygen

Some organisms use ______ as terminal electron acceptor, while some organisms use ________ __ as terminal electron acceptor

Some organisms use sulfate as terminal electron acceptor, while some organisms use elemental S as terminal electron acceptor

Different reduction reactions with sulfur

1.) Sulfate reduction (either sulfate to elemental sulfur or sulfate directly to H2S)
2.) Sulfur reduction (elemental sulfur to H2S)

How might the phase in which these forms of S (elemental S and sulfate) are found create constraints for organisms that use these substances as terminal electron acceptors?

Elemental sulfur is in the solid phase, making it harder for organisms to access it for reduction

Waste products formed by reduction of sulfate to elemental S _________ out as a _____

Waste products formed by reduction of sulfate to elemental S precipitate out as a solid

What happens to H2S produced from dissimilatory sulfate reduction

Since it is in the gas phase, it will want to rise to the atmosphere, but it may not have a way to get up there or it may interact with other compounds along the way

H2S can interact with iron to form _______

Pyrite (FeS2)

Pyrite formation can either happen ___________ or _________

Pyrite formation can either happen biologically or chemically

Chemical pyrite formation reaction 

1.) H2S + Fe2+ --> FeS
2.) FeS + H2S --> FeS2 (pyrite) 

FeS

A black insoluble solid

Significance of pyrite 

It is important in the formation of iron-sulfur clusters that make up enzyme active sites

Difference in sulfate concentration in seawater vs. lakes

Seawater = 30 mM (higher)
Lakes = 70 uM (lower)

The abundance of sulfate in sea water has implications in…

1.) The decomposition of organic matter
2.) Iron trap for P 

How does the abundance of sulfate in seawater affect the decomposition of organic matter

It drives the decomposition of organic matter in anoxic environments, as the lack of oxygen causes organisms to use sulfate as a terminal electron acceptor instead

Implication of sulfate reduction in the iron trap for P in the ocean

The iron trap for P in the ocean is less prevalent, because iron has a higher affinity for sulfur then phosphorus, as it forms things like pyrite, allowing P to be released from its trap

Dissimilatory sulfate reduction reaction

Sulfate + CH2O + H⁺ --> H2S + CO2 + H2O

The dissimilatory sulfate reduction reaction is used as an ______ source

Energy

How is sulfate reduction and sulfide reduction able to both be favourable, despite being opposite reactions

Because they are processes that do not occur under the same environmental conditions

In sulfate reduction, what is the role of S?

It acts as the terminal electron acceptor (i.e. it's the oxygen in respiration)

In sulfide oxidation, what is the role of S?

It is the electron donor/energy source (i.e. it acts as the glucose in respiration)

Sulfide oxidation vs. sulfur oxidation

Sulfide oxidation = Start with sulfide, turn it into either sulfate or elemental sulfur
Sulfur oxidation = Start with elemental sulfur, turn it into sulfate

When elemental sulfur forms, it _________ out

Precipitates

Sulfate reduction vs. sulfide oxidation (reactants vs. products)

Sulfate reduction = Its by-products become the reactants for sulfide oxidation
Sulfide oxidation = Its by-products become the reactants for sulfate reduction

Sulfate reduction vs. sulfide oxidation (complementation)

They are opposite reactions but they are complimentary

Variety in oxidation of S

The oxidation of S can be paired with a number of different terminal electron acceptors

Where do sulfides exist?

1.) Waste-products of the respiration of sulfate reducers, such that it makes biogenic sulfides
2.) Emitted from the Earth's interior

Biogenic sulfides

Sulfides made by living things

Where does sulfide oxidation occur?

In oxic environments

"
Explain the decomposition of this seal "

1.) In the anaerobic centre of the carcas are microbes that are reducing sulfate, forming hydrogen sulfide as a result 
2.) At the edge of the carcus, where oxygen is readily available, microbes use the H2S produced from the sulfate reducers as the electron donor and energy source

"
Does the dead seal have a lot of sulfur?"

No, the majority of sulfur comes from the ocean

Would the production of sulfides from the respiration of sulfate reducers occur in lakes?

No, because they do not have enough sulfates

Beggiatoa 

A colourless sulfur bactrium with metabolic flexibility, such that it can both reduce and oxidize sulfur 

Aerobic sulfide oxidation reaction (that forms elemental sulfur)

H2S + O2 --> S + H2O

Sulfur oxidation reaction 

S + H2O + O2 --> Sulfate + H⁺

Why might sulfide oxidation be a challenging combination of reactants (H2S + O2)

Because H2S is made in an environment with no oxygen, but its oxidation requires oxygen

How is the challenge with the combination of the sulfide oxidation reactants (H2S + O2) overcome?

H2S is a gas, therefore it is able to eventually move out of the anoxic environment it was made in

Beggiatoa have ________ that allow for ______ motility

Beggiatoa have filaments that allow for gliding motility

Significance of the gliding motlity of Beggiatoa

It allows them to move in a way that they can situate themselves into the environment they need, especially since environmental conditions are constalty fluctuating

Terminal electron acceptor of Beggiatoa

It can harvest energy from a wide range of conditions, such that it can use more than only one terminal electron acceptor

Because Beggiatoa can use more than one electron ________, it can do both ______ and _________ sulfide oxidation

Because Beggiatoa can use more than one electron acceptor, it can do both aerobic and anaerobic sulfide oxidation

Anaerobic sulfide oxidation reaction

H2S + NO3^- + H⁺ --> S + NH4 + H2O 

Significance of Beggiatoa being able to do anaerobic sulfide oxidation

It means they require additional different proteins in the ETC

NO3^- in Beggiatoa as an electron acceptor

It is imported into the cell, in a way that it accumulates 

Links between nitrogen, carbon, and sulfur metabolism in Beggiatoa 

1.) Nitrate respiration and CO2 fixation in the cytoplasm
2.) Sulfate reduction in the sediments

Beggiatoa is the largest _________ on Earth

Prokaryote

Beggiatoa are facultatively __________trophic

Beggiatoa are facultatively chemoautotrophic

Beggiatoa as a chemoautotroph

1.) Uses sulfides as an electron donor (energy source)
2.) Reduces CO2 to form organic carbon

Energy of carbon fixation in beggiatoa

The transfer of electrons is not energetically favourable, such that it has a net requirement rather than net release

How does Beggiatoa get the energy & reducing power it needs to be able to fix carbon? Especially since it has no pigments to interact with sunlight

The oxidation of reduced sulfur produces energy and reducing power, which are then used in the calvin cycle to fix CO2 into organic carbon, which is why Beggiatoa are chemoautotrophs

Emission of sulfides from the Earth's interior

These types of sulfides are abiotic in origin, such that high amounts of reduced sulfur are released from volcanoes, lava, etc

Sulfide oxidizing autotrophs can perform __________ photosynthesis, with ________

Sulfide oxidizing autotrophs can perform anoxygenic photosynthesis, with pigments

Anoxygenic photosynthesis with sulfide oxidation reaction

H2S + CO2 --> CH2O + S + H2O

Bacteria that can perform anoxygenic photosynthesis with sulfide oxidation

Green sulfur bacteria

What role do the pigments play in anoxygenic photosynthesis with sulfide oxidation

The green colour means they are interacting with sunlight, in order to provide energy to perform photosynthesis

Pigmented autotrophic sulfide oxidizing bacteria examples

1.) Green sulfur bacteria
2.) Purple sulfur bacteria

 Most environments in which sulfides are available do not have bright light available! --> Slide 14???

Different ____________ exist, but they all perform photosynthesis and use _______ to boost the energy state of the electrons

Different photosystems exist, but they all perform photosynthesis and use sunlight to boost the energy state of the electrons

Which terminal electron acceptor is likely to be tied to sulfide oxidation in the symbionts of these tube worms? 

Most likely oxygen, but likely is also able to use nitrate 

Trophosome

Appendage in gutless worms that have lots of sulfur, such that it has bacteria that uses the H2S from sea vents as an energy source

What might the bacteria get out of this trophosome arrangement 

It gets a home that is able to migrate to different environments that has the conditions it needs to perorm its reactions

What might the nematode get out of this trophosome arrangement 

It gets CH2O