MBIO 3282 - Section 7 - Sulfur Cycle

Sulfur Abundance (reservoirs)

- Lithosphere: Sulfate minerals like gypsum (CaSO4), Reduced forms like iron sulfides (FeS2)
- Water (mostly oceans) (SO42-, and organic)
- Atmosphere (SO2, and organic)
- Terrestrial biosphere (organic)
- Aquatic biosphere (organic)

Sulfur flux between reservoirs

- Sulfur in lithosphere turned over slowly
- Ocean sulfate slowly cycle
- Sulfur enters atmosphere as biogenic H2S or organic form, or S2 from fossil fuels, leaves as sulfurous acid
- Biosphere requires sulfur for protein and coenzymes (Rarely a limiting nutrient, Lots of recycling within biosphere)

Sulfur cycle - forms and transformations

- Assimilation: SO42- to organo-sulfur compounds (cysteine)
- Sulfur reduction: S° to H2S
- Sulfate reduction: SO42- to H2S
- Sulfide oxidation: H2S to S° or SO42-
- Sulfur oxidation: S° to SO2-

Sulfate reduction

- SO42- to H2S
- Sulfate reducing bacteria use sulfate as electron acceptor
- Dissimilatory sulfate reduction
- Form of anaerobic respiration
- SO42- + 8H+ + 8 electrons -> H2S + 2H2O + 2OH-
- Widespread, anoxic environments
- Usually limited by supply of electrons (organic substrates for heterotrophs, hydrogen for autotrophs)
- Inhibited if better electron acceptors present (O2, NO3-, Fe(III))
- These would react instead of SO42- -> inhibiting reduction

Microbes that reduce sulfate

- Mesophilic Gram negative 𝛿 -Proteobacteria (Desulfovibrio spp.)
- Mesophilic/thermophilic Gram positive bacteria (Desulfotomaculum)
- Thermophilic/almost hyperthermophilic gram negative bacteria (Thermodesulfobacterium spp.)
- Hyperthermophilic archaean (Archaeoglobus spp.
)
- Many can also use thiosulfate, but NOT S° (elemental sulfur -> elemental sulfur reduction more rare)

Sulfur reduction

- S° to H2S
- Anaerobic respiration
- Mesophilic bacteria like desulfuromonas spp.
- Hyperthermophilic archaea like pyrodictium spp.
- S° + 2H+ = 2 electrons -> H2S
- Source of electrons usually H2, can be organic
- S° is a solid, insoluble substrate

Sulfur oxidation

- S° to SO42- (sulfate)
- Common wherever S° and O2 any available
- Example of chemolithoautotrophy
- S° + 1.5 O2 + H2O -> H2SO4
- Many microbes
- Diverse phylogenetic groups
- Aerobic and anaerobic
- All temperature ranges
- Most oxidize both S° and H2S

Sulfide oxidation

- H2S to S° or SO42-
- H2S + 0.5 O2 -> S° + H2O
- H2S + 2O2 -> H2SO4
- O2 is most common electron acceptor, also NO3-
- H2S spontaneously oxidizes with O2 at neutral pH
- At acidic pH, sulfide is stable in O2
- Many diverse microbes carry out reactions
- Phototrophs/chemotrophs
- Aerobes/anaerobes
- Autotrophs/heterotrophs

Sulfide oxidizing microbes

- Aerobes at neutral pH
- Aerobes, acidic pH
- Anaerobes (phototrophs)
- Cyanobacteria

Aerobes at neutral pH

- Colourless sulfur bacteria like Beggiatoa spp. (intercellular sulfate granules), Thioploca spp. And Thiomargarita spp. (one of the largest bacteria known)
- Filamentous or large
- Chemolithoautotrophs
Live along H2S/O2 gradients
- Thioploca vertical migrations in lake and intertidal sediments

Thioploca vertical migrations in lake and intertidal sediments

- Lives at aerobic/anaerobic interface,
- Glides up into aerobic sediments to accumulate nitrate in vacuoles
- Glides back down into anaerobic zone to oxidize sulfide, using stored nitrate as terminal electron acceptor (anaerobic respiration, denitrification
- Sulfide converted to elemental sulfur and nitrate becomes nitrogen gas

Aerobes, acidic pH

- Chemolithoautotrophs like Thiobacillus spp. (acidithiobacillus spp.)
- T. thioxidans use O2 produce H2SO4, acid tolerant
- T ferrooxidans use O2, produce H2SO4, acid tolerant
- T. denitrificans use NO3-, does not produce acid
- Hyperthermophilic archaea like sulfolobus spp. (>60°C)
- Rio Tinto in Spain: stretch of natural river 50 km, incredibly acidic/toxic pH = 2
- Cueva de villa luz, Mexico: cave lake, enriched sulfate in cave walls and access to air from outside, mixing of sulfate and oxygen, snotties = sulfide oxidising chemolithoautotrophs (related to Thiobacillus) dripping sulfuric acid, bats have become adapted to this environment

Anaerobes (phototrophs)

- Some deposit S° intracellularly, some extracellularly, some can later oxidize S° further
- Use sulfide as an electron source (get energy from light)
- Have different tolerances for sulfide (toxic)
- Purple sulfur bacteria like chromatium spp. And ectothiorhodospira
- Purple non sulfur bacteria like photo pseudomona
- Green sulfur bacteria like chlorobium (high concentrations of sulfide tolerated)
- Green non sulfur bacteria like chloroflexus

Cyanobacteria

- Some like oscillatoria spp. that are normally oxygenic phototrophs can also oxidize sulfide to sulfur under some conditions
- Don't like this tho -> don't grow incredibly well -> granules on outside (extracellular)

Sulfur assimilation

- SO42- to organo-sulfur compounds (cysteine)
- Most microbes use SO42- as a biosynthetic sulfur source
- Internally reduce it to sulfide (REQUIRES ATP) (assimilatory sulfate reduction)
- Reacts with serine to form cysteine (otherwise would bind to cytochromes)
- Rarely a limiting nutrient
- Many organosulfur compounds can be formed (Proteins, Coenzyme A, Vitamins, Odours/flavours -> garlic smell from allicin a sulfurous volatile -> danger response from lysed cells of garlic)

Marine organosulfur compounds

- Marine algae have large amounts of a compatible solute to combat osmotic pressure due to salinity
- Dimethylsulfonium propionate (DMSP)
- Dimethylsulfide (DMS)
- Dimethyl sulfoxide (DMSO)

Dimethylsulfonium propionate (DMSP)

- (CH3)2SCH2CH3COO
- Also called dimethyl propiothetin
- Can be degraded as carbon + energy source releasing: DMS

Dimethylsulfide (DMS)

- (CH3)2S
- Volatile, enters atmosphere
- Can be used as substrate for methanogens releasing G2S, CH4
- Usually used as electron donor, oxidized to: DMSO

Dimethyl sulfoxide (DMSO)

- (CH3)2SO
- Used as electron acceptor reduced back to DMS
- Seed for cloud formation
- Acid rain: dimethylsulfide enters atmosphere -> makes SO2- + SO42- -> cause acid rain
- Much SO42-= much H2SO4 = much acid rain

Issues

- Microbial diversity
- Marine sulfur cycle
- Acidification
- London Smog 1952

Microbial diversity

- Organisms that oxidize sulfide are diverse in phylogeny, morphology, physiology, and habitat

Marine sulfur cycle

- DMS and H2S enter atmosphere
- H2S is toxic, corrosive gas that stinks
- Sulfate and DMSO are seeds for cloud formation
- Sulfurous acid (H2SO3) = acid rain

Acidification

- S° and sulfide (H2S or FeS2) oxidation produces sulfuric acid (acid mine drainages)

London Smog 1952

- 100,000 people ill: 12,000 dead
- The reason: SO2 formed from burning coal