Anaerobic Food Web

Question 1

What is fermentation?

Answer 1

“Anaerobic catabolism of an organic compound in which the compound serves as both an electron donor and an electron acceptor and in which ATP is produced by substrate-level phosphorylation,” (Brock, 1994)

Question 2

What is industrial fermentation?

Answer 2

Microorganisms are used for a chemical transformation.

Question 3

List the essential elements of fermentation.

Answer 3

• Balanced overall equation, redox balance
• Both oxidation and reduction of substrate
• Substrate is not mineralized; not all substrate carbon is converted to CO2
• Some mechanism yields biologically useful energy (ATP or PMF)

Question 4

List the frequent but non-essential elements of fermentation.

Answer 4

• Often an organic disproportionation that yields both oxidized and reduced products
• Oxidative processes often form high-energy compounds that subsequently
  support substrate level phosphorylation.
• Reductive processes often function to balance oxidative processes; NADH/NAD+ is often cycled between oxidative and reductive parts of the pathway, with the reductive part regenerating NAD+

Question 5

Describe substrate level phosphorylation.

Answer 5

Mechanism of energy conservation, energy coupling.

Question 6

Describe energy yield of SLP vs respiration

Answer 6

SLP yields less energy, less free energy change, a lot of times, products contain energy that isn’t conserved.

Question 7

List characteristics of environments where fermentation occurs.

Answer 7

• Anaerobic, oxygen demand exceeds supply
• Often eutrophic, abundant organic matter, depleted respiratory TEAs
• Syntrophy (nutritional mutualism) common
• Typical environments: sediments, mouth, gut, urogenital tract, skin, waste digesters, spoiled food, decomposing vegetation

Question 8

Describe glycolysis.

Answer 8

Fermentative pathway, usually is the core of other pathways
Preparatory pathway: activation, phosphorylation by kinases using ATP, energy invested via energy coupling aldolase splits fructose-1,6-bisP (6-C) to two 3-C units, glyceraldehyde-3-P

Question 9

Describe the first stage of glycolysis.

Answer 9

Preparatory pathway: activation, phosphorylation by kinases using ATP,
energy invested via energy coupling,
aldolase splits fructose-1,6-bisP (6-C) to two 3-C units, glyceraldehyde-3-P

Question 10

Describe the second stage of glycolysis.

Answer 10

Oxidation: making ATP, energy conservation
oxidation of G3P coupled to reduction of NAD+ by dehydrogenase, so NADH produced,
oxidation forms 1,3-bisP glycerate, a high-energy compound,
hydrolysis to 3-P-glycerate coupled to ADP phosphorylation (SLP),
later, another high-energy compound, PEP, is hydrolyzed, yielding more ATP via SLP, so a net of 2 ATP/glucose synthesized by EMP pathway – relatively little
energy conservation compared to respiration

Question 11

Describe the third stage of glycolysis.

Answer 11

Reduction: not really part of glycolysis, additional process,
completes fermentative process by achieving redox balance,
yields fermentation products,
regenerates NAD+ consumed in the oxidation stage (during glycolysis)
3 possibilities shown (ie, 3 different fermentations), 2 regenerate NAD+:
a) if ethanol produced, then alcohol fermentation
b) if lactate produced, then lactic acid fermentation
c) if acetate + formate produced, then mixed acid fermentation (no NAD+ regeneration, pathway more complex than shown)

Question 12

Describe the ecology of ethanol fermentation.

Answer 12

Used by yeast (fungi), they don’t need O2 for catabolism, but need it for anabolism.
Habitats: rich in sugar, become anaerobic with dense growth, transient, rich resources.
Competition: alcohol tolerance (ethanol)

Question 13

List some applications of ethanol fermentation.

Answer 13

Baking, alcoholic drinks, fuel ethanol.
Feedstocks are required for industrial fermentation, corn is used, but isn’t ideal because it requires a lot of energy and fertilizer.

Question 14

What is the equation of ethanol fermentation?

Answer 14

C6H12O6 → 2CH3CH2OH + 2CO2

Question 15

What are the key enzymes of ethanol fermentation?

Answer 15

Pyruvate decarboxylase (7), forms acetaldehyde 
Alcohol dehydrogenase (8), regenerates NAD+ (reduction step, III)

Question 16

What two high energy compounds support SLP in ethanol fermentation?

Answer 16

1,3-bisphosphoglycerate

2-phosphoenol pyruvate (PEP)

Question 17

Describe lactic acid bacteria (LAB)

Answer 17

Lactic acid bacteria: from order Lactobacillales, from the phylum Firmicutes. These are low-GC, gram-positive bacteria.
Specialists – lack ETC, so not capable of respiration; obligately fermentative; narrow
substrate range (most require sugars); limited biosynthesis (often require vitamins,
amino acids, purines & pirimidines from environment)
Aerotolerant – many have superoxide dismutase but not catalase
Very acid tolerant – their fermentation product, lactic acid, makes their environment
too acidic for many competitors; competitive strategy

Question 18

Describe the ecology of homolactic fermentation.

Answer 18

Habitats – rich, often transient resources: milk, plants (living & decomposing), mouth (dental caries), intestine, vagina, respiratory tract; most are commensal or mutualistic with host; a few are pathogenic
Ecological succession – commonly the final population because of acid tolerance.

Question 19

List some applications of homolactic fermentation.

Answer 19

Food preservation, many uses: dairy (yogurt, cheese, butter), kimchee, sauerkraut, silage (animal feed). Preserves food by removing sugar and lowering pH.
Commonly used probiotic organism.

Question 20

What is the equation for homolactic fermentation?

Answer 20

C6H12O6 -> 2C3H6O3

Question 21

What is the key enzyme for homolactic fermentation?

Answer 21

lactate dehydrogenase (2)

Question 22

Which fermentation pathway involves organic disproportionation?

Answer 22

Only the ethanol fermentation pathway.

Question 23

Describe lactic acid as an uncoupler.

Answer 23

Lactic acid is a weak acid, with a pKa of 3.9. Lower pH (extracellular): the protonated form is dominant. This form can diffuse across cytoplasmic membranes (CM). In CM (more basic): lactic acid/lactate will deprotonate.
Lactic acid is an uncoupler that “de-energizes” the cell, uncoupling proton translocation from the processes driven by the PMF, particularly ATP synthesis. The LAB must somehow tolerate this mechanism of toxicity.

Question 24

Describe lactate/H+ symport.

Answer 24

A symporter couples outward proton (+) translocation
(against the PMF) with outward lactate (-) translocation. Thus, the lactate concentration gradient drives proton translocation, increasing the PMF. Energy conservation. In many environments lactate will be consumed by other organisms or will diffuse away from the producer. In a closed system, including food fermentation, lactate accumulation will eventually be toxic even to LAB.

Question 25

Describe the ecology of mixed acid fermentation.

Answer 25

Habitats: gut environments, plants, water, and soil. Many mixed acid fermenters are facultative aerobes, adapted to transitioning between aerobic and anaerobic environments (eg. from water to gut). This group mainly uses sugar substrates. Enteric bacteria are among those with mixed acid fermentations. As facultative aerobes, the enteric bacteria may play an important role in removing O2 from gut environments. Provides vitamin K.

Question 26

Describe the pathway of mixed acid fermentation.

Answer 26

Overall, glucose or other sugars are converted to a mixture of acids (acetic, lactic, succinic, formic) as well as ethanol, CO2 and H2. The mixed acid fermentation has a higher ATP yield than alcohol or lactate fermentations, because production of acetate allows an extra SLP step. The exact ATP yield is variable, depending on product stoichiometry.

Question 27

Describe the significance of anaerobic food webs.

Answer 27

Fermentations often do not occur in isolation and often are part of a more complex anaerobic food web. They occur in anaerobic digesters, which are engineered
environments, gut, and sediments. All of the above are generally eutrophic environments, which are depleted
of respiratory electron acceptors.

Question 28

Describe syntrophy.

Answer 28

The term syntrophy means “feeding-together” and is nutritional mutualism. In aerobic environments, a single organism will often completely degrade (mineralize)
complex organic substrates. By contrast, complete degradation in anaerobic environments often yields CO2 + CH4, the latter a very reduced form of C. Although CH4 is an organic compound, degradation to CO2 + CH4 is commonly referred to as “anaerobic mineralization”.

Question 29

Describe the hydrolysis (a) guild of anaerobic environments.

Answer 29

catalyzed by certain CAZymes; typically extracellular enzymes; no energy directly conserved; macromolecules (polymers) from plants and other organisms hydrolyzed to small molecules; same process as in aerobic environments.

Question 30

Describe the primary fermentation (b) guild of anaerobic environments.

Answer 30

same organisms as hydrolyzers plus others that are not
hydrolyzers (see 5.6.4); includes fermenters we have previously examined; products include volatile fatty acids (VFAs), alcohols, H2.

Question 31

Describe the acetogenesis from H2 (homoacetogenesis) (c) guild of anaerobic environments.

Answer 31

note that acetogenesis is a broader term, which is not the same as homoacetogenesis

Question 32

Describe the secondary, syntrophic fermentation (obligate H2 producers, obligate syntrophs) (d) guild of anaerobic environments.

Answer 32

require interspecies H2 transfer, an important type of syntrophy; critically important to anaerobic systems, this step is what makes it a food web, rather than a food chain; interconnects guilds

Question 33

Describe the methanogenesis (e) guild of anaerobic environments.

Answer 33

from acetate and from H2 + CO2, the latter may compete or co-exist with homoacetogenesis from H2 + CO2.

Question 34

Describe cheaters.

Answer 34

In food webs dependent upon complex polymers, some primary fermenters, cheaters, do not produce hydrolytic enzymes to degrade the complex polymers, but they do use the hydrolysis products (monomers and oligomers) that hydrolyzers produce. By avoiding the energetic cost of producing polymer hydrolysis enzymes, cheaters have a competitive advantage over non-cheaters.
Mutualism: The anaerobic food web cannot function if cheaters completely displace non-cheaters. In gut systems, it is hypothesized that group selection maintains mutualism and selects against cheating. Group selection is proposed to allow selection at the level of the host, the entire gut microbial community, rather than individual microbial populations. With selection at the level of the host, both the host and its gut microbial community are more likely to survive and reproduce, if that community is mutualistic and provides a fitness advantage to the host.

Question 35

Describe the significance of secondary fermenters / obligate syntrophs.

Answer 35

If this guild is inhibited, the whole anaerobic food web will stop functioning. Substrates for secondary fermentation include VFAs, ethanol and benzoate. All but the latter occur as products of primary fermentations. The carbon in these compounds is relatively reduced. Thus, these compounds would make very thermodynamically favourable electron donors for respiration, but they are unfavourable substrates for fermentation.

Question 36

Describe Methanobacillus omelianskii.

Answer 36

This organism was “isolated” in 1967, growing anaerobically on ethanol, producing acetate + methane as products. It was subsequently found to grow on H2 + CO2, but then it was found that these cultures lost the ability to grow on ethanol. It turned out that the
original “isolate” was a mixture of two highly interdependent syntrophic organisms. The
first is a secondary fermenter (bacterium) and the second, a methanogen (archaeon). Overall, the metabolism of the two organisms is exergonic. However, the secondary
fermentation is endergonic under standard conditions and can only occur when coupled to methanogenesis or some other hydrogen-consuming process.

Question 37

What is the equation of the reactions performed by Methanobacillus omelianskii?

Answer 37

CH3CH2OH + H2O -> CH3COOH + 2H2   
delta G°'=  +9.6 kJ 
2H2 + 0.5CO2 ->  0.5CH4 + H2O   
delta G°'=  -68 kJ 
CH3CH2OH + 0.5CO2 ->  CH3COOH + 0.5CH4  
delta G°'=  -58 kJ

Question 38

Describe interspecies H2 transfer.

Answer 38

Secondary fermentation: Under standard conditions, these rxns are endergonic. All of these fermentations produce H2. It is a low H2 partial pressure (approx. 10^-4 atm ) typical of their environments, which allows these fermentations to proceed. The low H2 partial pressure makes these catabolic reactions exergonic under environmental conditions (∆G). Thus, the secondary fermenters are obligate syntrophs that depend on other H2–consuming syntrophs. This process is called interspecies H2 transfer.

Question 39

Describe energy conservation of endergonic fermentations.

Answer 39

The short secondary fermentation pathways do not involve SLP. There are no high-energy compounds as intermediates. And, even with a very low H2
partial pressure, the overall pathways do not yield enough energy to drive SLP. Thus, secondary fermenters must use a chemiosmotic mode of energy conservation. In most cases, secondary fermentations appear to drive formation of a sodium-motive force (analogous to a PMF). It further appears that the mechanism typically involves membrane-associated decarboxylases, which catalyze a
decarboxylation in the fermentation pathway and couple that reaction to pumping a sodium ion out of the cell (i.e., they are sodium pumps).

Question 40

Describe direct interspecies electron transfer.

Answer 40

Alternative to interspecies H2 transfer. Example: two Geobacter species couple ethanol oxidation, a secondary fermentation producing acetic acid plus electrons, to fumarate respiration, reduction of fumarate to succinate using the above electrons and acetic acid as electron donors. Cell aggregation important.

Question 41

Describe homoacetogens

Answer 41

Ferment sugars. Unique characteristic of fermenting glucose to acetate as the sole product. Characteristic capacity to grow on only CO2 + H2. Both processes involve the acetyl-CoA pathway. Most known
homoacetogens are members of phylum Firmicutes and are gram-positive, low-GC, endospore-forming bacteria. And, most of these are members of the genus, Clostridium.

Question 42

Describe the significance of homoacetogens.

Answer 42

Multiple roles. Primary fermenters, transforming sugars to acetate, and H2 consumers, enabling secondary fermenters to function. Relative to other H2
consumers, such as methanogens, homoacetogens gain less energy. Nevertheless, in certain environments,
homoacetogens are the main H2 consumers (gut of termites). Some termite species are acetogenic, whereas others are methanogenic. Termites play a very important role in decomposition of plant biomass and maintenance of soil fertility, particularly in tropical regions. Competition between acetogenic and methanogenic termites has a significant impact on global climate. Acetogenic termites emit less greenhouse gasses than methanogenic termites, which contribute a few percent of total terrestrial emissions of both CO2 and CH4.

Question 43

Describe homoacetogenic glucose fermentation.

Answer 43

Homoacetogens use the EMP pathway to transform glucose to 2 pyruvate. As in mixed acid fermentations, homoacetogens convert pyruvate to acetate plus CO2. Yield ATP via SLP. They also produce reductant (shown as H), which must be consumed by forming a reduced product. Homoacetogens have the unique ability to use their reductant to reduce CO2 to acetate, producing the third acetate molecule from glucose (Eqn 3). Homoacetogens also have the ability to conserve additional energy from Eqn 3, which is exergonic. Thus, homoacetogens have a very energetically efficient sugar fermentation, with the net reaction shown below.

Question 44

What are the equations of homoacetogenic glucose fermentation?

Answer 44

Eqn 1. C6H12O6 -> 2C3H4O3 + 4H (glycolysis to pyruvate)
Eqn 2. 2C3H4O3 + 2H20 -> 2CH3COOH + 2CO2 + 4H (like mixed acid ferm.)
Eqn 3. 2CO2 + 8H -> CH3COOH + 2H20 (key reaction)
Net rxn: C6H12O6 -> 3CH3COOH (balanced overall reaction)

Question 45

Describe the growth on H2 and CO2 of homoacetogens.

Answer 45

A defining characteristic of homoacetogens is the ability to grow on H2 + CO2, a type of chemolithotrophy (Eqn 4). Essentially, they can use exogenous H2 as the reductant for Eqn 3, independent of glucose fermentation. They do not need any other C source, so they are also autotrophs. Thus, homoacetogens are facultative chemolithoautotrophs.
Eqn 4. 4H2 + 2CO2 -> CH3COOH + 2H2O (similar to Eqn 3 above)
Growth on CO2 + H2 is what allows homoacetogens to function as H2 consumers. It was also the first evidence that homoacetogens can conserve energy from Eqn 3.

Question 46

Describe the acetyl-coA pathway.

Answer 46

Eqn 3 and 4 of homoacetogenesis.

Question 47

What is the key enzyme of the acetyl-coA pathway and what does it do?

Answer 47

carbon monoxide dehydrogenase (CODH).
This enzyme catalyzes a complex process including the reduction of CO2 to CO, the binding of the two C atoms and the binding of CoA. This reaction is reversible, proceeding from left to right in the homoacetogenic fermentation. The acetyl-CoA pathway also occurs in other diverse anaerobic Bacteria and Archaea, performing a range of important functions. In so organisms, it proceeds in the opposite direction to oxidize acetate to CO2.
Eqn 5. B12-CH3 + H2 + CO2 + HSCoA CH3CO-SCoA + H2O + B12

Question 48

Describe the evolution of the acetyl-coA pathway.

Answer 48

The reductive acetyl-CoA pathway is proposed to be the first catabolic pathway to evolve. This proposal is consistent with hypothesized geochemical conditions on ancient Earth. Under those conditions the pathway could yield energy and fixed carbon, both fundamental to the origin of life.

Question 49

Describe methanogens.

Answer 49

Methanogens are a very diverse group of Archaea that form methane as a product of catabolism. Reflecting that diversity, methanogens are adapted to a wide range of environmental conditions. Accordingly, the group ranges from psychrophilic to thermophilic organisms and from acidophilic to alkaliphilic ones. They also come in
diverse morphologies: rods, cocci, spirilla and sarcina (cuboidal packets of eight or more cells). One thing methanogens have in common is that they are strictly anaerobic and extremely oxygen-sensitive.

Question 50

What is the significance of methanogens?

Answer 50

Methanogens are often the main H2 consumers in anaerobic food webs, an essential process in the overall system. Other methanogens can be major acetate consumers. As such, methanogens are abundant and functionally important in many environments

Question 51

Describe some environments where methanogenesis may occur.

Answer 51

sediment systems (especially freshwater) and gut
communities. Often these are eutrophic environments that accumulate enough organic matter to cause depletion of oxygen and other respiratory electron acceptors. Reduction of CO2 to CH4 is relatively high on the electron tower, compared to most respiratory reductive half-reactions, so methanogenesis has a relatively low energy yield and tends not to occur in the presence of most respiratory electron acceptors. However, small populations of methanogens can also be found in a variety of environments, including aerobic soil, where their importance is unclear.

Question 52

Describe the mutualistic relationship between certain protozoans and methanogens.

Answer 52

This endosymbiosis occurs in termite guts and probably
elsewhere. The protozoan ferments cellulose from the termite’s diet. The methanogen consumes H2 from the fermentation, completing a simple but functional self-contained food web, including syntrophy.

Question 53

What human activities increase methane in the atmosphere?

Answer 53

certain crops (especially rice), ruminant livestock 
and landfills release globally significant amounts of methane. In landfills, accumulation of flammable methane can also be a fire hazard.

Question 54

What substrates do methanogens use?

Answer 54

CO2-type substrates, methyl substrates, acetotrophic substrates. Methanogens have unique biochemistry.

Question 55

What is the equation for methanogenesis from CO2?

Answer 55

CO2 + 4H2 -> CH4 + 2H20 (delta Go’ = -131 kJ)

Question 56

Describe the pathway for methanogenesis from CO2.

Answer 56

• The reduction of CO2 to a bound methyl group is analogous to what occurs during homoacetogenesis but involves different catalysts.
• Anabolism involves the acetyl-CoA pathway.
• Energy conservation is believed to occur via both sodium and proton translocation (both chemiosmotic mechanisms).
• The methyl reductase complex is a key component of this pathway; it is what defines all methanogens (i.e., produces methane); and, it is important for energy conservation.

Question 57

Describe acetoclastic methanogenesis.

Answer 57

acetate-splitting.
CH3COOH -> CH4 + CO2 (delta Go’ = -31 kJ)
Acetoclastic methanogens have a very low energy yield. Accordingly, they grow slowly, are difficult to culture and isolate, and so they have not been well studied. However, despite the limiting energetics, most biogenic methane is from acetate. So, acetoclastic methanogenesis is very important on the global scale.

Question 58

Describe the pathway for acetoclastic methanogenesis.

Answer 58

The pathway for acetoclastic methanogenesis is essentially the acetyl-CoA pathway.

• The direction of the pathway is reversed, compared to homoacetogenesis; acetate is disproportionated, being oxidized to CO2 and reduced to CH4.
• The methyl reductase complex is again present, and is believed to support proton translocation.
• It is also thought that the transfer of 2H from CODH to the methyl reductase complex may support additional proton translocation.
• Anabolism uses acetyl-CoA intermediate, so this is not autotrophy.

Question 59

Describe digester systems.

Answer 59

Anaerobic digesters treat organic matter (and other compounds) in waste streams. Used by industry, for example to treat pulp and paper manufacturing wastewater, agricultural wastes, or food processing waste streams. These digesters have an anaerobic food web, which are typically methanogenic. Ideally, such systems completely remove organic matter from the wastewater and convert it to CO2 + CH4. Ideally, they also remove fixed N and other components that would be harmful in discharged treated water. Biogas, mainly methane, is often collected and used as a fuel, essentially equivalent to natural gas.

Question 60

Describe solid digestate and liquid digestate.

Answer 60

Solid residual matter from the process (solid digestate) may be used for various applications. However, there are often concerns about potentially harmful effects of the solid digestate due to contaminants, such as metals, pathogens, or organic pollutants. Liquid digestate may be used for fertilization/irrigation, or if sufficiently pure, it may be discharged to the environment (effluent).

Question 61

Describe digester souring.

Answer 61

One serious response to perturbation is souring. Souring can result from a rapid increase in degradable organic matter in the digester influent, overloading the system and disrupting the food web. Increased organic loading initially increases fermentation and production of VFAs. However, methanogens cannot respond to the change as rapidly as the fermenters, so H2 accumulates. This inhibits secondary fermentation, so VFAs continue to increase and to lower the pH. This acidification causes a positive feedback loop by inhibiting the methanogens, which are more pH-sensitive than the fermentative organisms. Eventually, the pH becomes so low that it inhibits all guilds in the food web. Souring refers to the low pH.

Question 62

Describe perturbations of digester systems.

Answer 62

Ideally a digester treats a stable inflow of wastewater that does not greatly change in composition, pH, temperature, etc. Perturbations, can disrupt the digestion process, leading to excessive organic matter and, sometimes, even toxic compounds in the system effluent.

Question 63

Describe the rumen.

Answer 63

The rumen is part of the stomach system of ruminant animals, which functions in the digestion of a recalcitrant lignocellulosic diet. Ruminant animals include cattle, sheep, goats, deer, camels and giraffes. These animals would be unable to obtain sufficient nutrition from their diets without mutualistic symbioses with their rumen microbial community, which constitutes an anaerobic food web.

Question 64

Describe the significance of the rumen.

Answer 64

The rumen occurs before an acidic stomach and provides a very favourable environment for the microbial community essential to the host animal. It is anaerobic, 39°C and pH 6.5. The animal buffers the pH with bicarbonate in its saliva. This community is highly syntrophic. Bacteria and protozoans are highly abundant. Bacteria are involved in degradation of all plant polymers. Protozoans are believed to play an important role grazing prokaryotes in the system, and so, controlling prokaryotic populations. Protozoans also degrade some plant polymers, but their importance in lignocellulose degradation is not clear. Fungi are also present and are thought to play an important role in lignocellulose degradation. Methanogens are the main H2 consumers, and the animal produces a lot of methane, constantly belching (eructation) to remove the CO2 + CH4.

Question 65

Describe the food web of the rumen

Answer 65

• The animal physically breaks up the forage, facilitating microbial degradation.
• Protozoans, bacteria and fungi hydrolyze complex polymers, especially cellulose.
• Primary fermentations yield a range of products, as in the general anaerobic food web, largely via mixed acid fermentations.
• The host animal absorbs VFAs directly into its bloodstream, using them as a major
  nutrient for energy.
• Methanogens remove H2, which is still important despite the limited role of secondary ermenters.
• The animal subsequently digests microbial cells as they pass through the digestive tract, which provides a major source of protein.

Question 66

Why is the role of secondary fermenters in the rumen limited?

Answer 66

First, the animal takes up VFAs efficiently and outcompetes secondary fermenters for these substrates. Second, the retention time in the rumen is too short for the maintenance of large populations of slow-growing secondary fermenters.

Question 67

Describe the role of methanogenesis in the rumen.

Answer 67

Methanogenesis is still important in the food web to promote H2-producing processes other than secondary fermentation (eg. mixed fermentation). But, methane production represents energy lost to the host. There is great interest in engineering the rumen to reduce
methane production in order to increase host growth and to reduce greenhouse gas emissions. In particular, increased H2 consumption by homoacetogens would accomplish this goal.

Question 68

Describe acidosis.

Answer 68

Acidosis is a serious imbalance in the rumen that can result from a rapid change from poor to rich, easily-digestible forage, such as clover or grain. If the rumen community is not gradually acclimated to rich forage, it causes a sudden increase in acidic fermentation
products in the rumen. This overcomes the animal’s capacity to buffer pH and take up the VFAs. In a positive feedback loop, the lower pH selects for LAB that displace the normal fermenters. Thus, VFAs, particularly lactic acid, accumulate, and acidification continues. The animal can die from this condition.

Question 69

Describe mimosine.

Answer 69

a toxic compound occurring in tropical legumes. Cattle introduced to Australia became ill from grazing native plants containing mimosine. However, cattle in Hawaii were able to safely graze plants with mimosine. Transplants of the microbial communities from rumens of Hawaiian cattle to the rumens of Australian cattle made the latter resistant to mimosine. The resistance was shown to be due to degradation of mimosine by the rumen community.

Question 70

Describe fluoroacetate.

Answer 70

a toxic compound synthesized by plants worldwide, presumably to protect them from herbivores. In numerous cases, fluoroacetate has poisoned livestock, particularly in Australia, Brazil, and South Africa. Rumen bacteria were genetically engineered to degrade fluoroacetate and shown to protect livestock from fluoroacetate poisoning under experimental conditions, but the commercial use of these genetically modified organisms has not been permitted. Native rumen bacteria capable of fluoroacetate degradation have been isolated, and current efforts are focussed on selecting
for sufficient populations of these bacteria in the rumen to protect the host from fluoroacetate toxicity.

Question 71

Describe energy conservation of homoacetogens

Answer 71

Energy conservation: It is proposed that homoacetogens
can further conserve energy via a chemiosmotic mechanism, reducing the electron carrier ferredoxin and then coupling ferredoxin oxidation to ion translocation via membrane-associated pumps. Some homoacetogens appear to pump protons and others, sodium, yielding proton- or sodium-motive forces, respectively. Overall, energy conservation from this pathway has elements of fermentation and of respiration.