Table 2 Compendium of experimental observations manifesting DIET between cocultures of defined microorganisms where one serves as an electron donor and the other as an electron acceptor in an AD system

G. metallireducens

G. sulfurreducens

• Ethanol and fumarate

• With BC

• Ethanol was metabolized and fumarate was reduced to succinate on day 2

• Cells were attached to BC but did not aggregate

[123]

• Without BC

• Ethanol metabolism started at day 30

• Ethanol and fumarate

• With carbon cloth

• There was a syntrophic metabolism of ethanol and a reduction of fumarate to succinate on day 2

• Higher metabolism when the carbon cloth was doubled

• Acetate did not accumulate

• Cells were dispersed

[192]

• With cotton cloth

• No ethanol oxidation and succinate production due to the very low conductivity of the carbon cloth

[192]

G. metallireducens

None

• With BC

• Ethanol metabolized slowly with an increase in acetate

• BC served as an electron acceptor

[123]

• Without BC

• No ethanol metabolism

M. barkeri

None

• Pure culture

• Not ethanol metabolism

G. metallireducens

M. barkeri

• Ethanol

• Ethanol was converted to methane

• Transient accumulation of acetate

• Microorganisms were attached to BC but did not aggregate

• BC served as an electrical conductor between the two species and not through cell-to-cell electron transfer

[123]

• Without BC

• Not ethanol metabolism

• Ethanol as the sole electron donor

• Ethanol metabolized to methane on day 7

• Transient accumulation of acetate

• Formation of intertwined aggregates (100–200 μm) that shared electrons via DIET

• M. Barkeri was able to participate in DIET

[186]

• Pure culture

• No metabolism of ethanol and no acetate formed

[186]

• No GAC

• Ethanol started to metabolize after 39 days

[186]

G. metallireducens wild-type

G. sulfurreducens is incapable of producing pili

• Carbon cloth

• The succinate produced is comparable to the coculture initiated with wild-type strains

• Cells were tightly attached to carbon cloth at day 10 of incubation

• This indicates that the removal of pili did not inhibit the attachment of cells

[192]

G. metallireducens is incapable of producing pili

G. sulfurreducens wild-type

• Carbon cloth

G. metallireducens wild-type

G. sulfurreducens Omcs deficient

• Carbon cloth

• There was succinate production

[192]

G. sulfurreducens

None

• Ethanol & fumarate

• With carbon

• No ethanol metabolism or fumarate reduction even with carbon cloth

[192]

G. metallireducens

None

Desulfovibrio vulgaris

G. sulfurreducens

• Ethanol

• With carbon cloth

• The cloth did not accelerate metabolism

[192]

G. metallireducens

M. barkeri strain

• Ethanol

• Cocultures without cloth required metabolized ethanol at day 40

• Cocultures with carbon cloth started to metabolize ethanol began at day 10

• Cells were not closely associated with each other

[192]

pilA-deficient or Gmet 18668 gene deficient strain G. Metallireducnes

M. Barkeri

• No GAC

• Did not metabolize ethanol and no methane was produced

[186]

• With GAC

• The amendment of GAC in the coculture allowed the pili-deficient strain G. Metallireducens to transfer an electron to M. Barkeri resulting in the production of methane

• Proof that GAC can serve as a substitute for pili to shuttle electrons

P. carbinolicus

M. barkeri

• Ethanol

• There was growth in the coculture

• A steady accumulation of acetate was observed

• No multispecies aggregates formed illustrating that DIET requires cell-to-cell for electron transfer

• M. Barkeri, using H₂, metabolized a little of the acetate produced by P. Carbinolicus

• M. Barkeri is the first methanogen known to use both H₂ and or electrons from DIET to reduce CO₂

[186]

P. carbinolicus

G. sulfurreducens

–

• No aggregate formed, suggesting that close physical contact was not necessary for interspecies H₂ transfer

[186]