Microbial Metabolism Questions and Answers

Answer: C

The free-living photosynthetic nitrogen fixer is Rhodospirillum.

Rhodospirillum rubrum is a purple non-sulphur bacterium: it carries out anoxygenic photosynthesis, lives freely in water and mud, and has nitrogenase, so it fixes nitrogen without a host.

Frankia (an actinomycete with alder and other non-legumes) and Rhizobium (with legumes) fix nitrogen mainly in symbiotic root nodules and are not photosynthetic. Clostridium is free-living and fixes nitrogen, but it is an anaerobic heterotroph, not photosynthetic.

Answer: A

The component that is not an electron acceptor is Lactate.

In anaerobic respiration an inorganic (or oxidised organic) compound replaces oxygen as the terminal electron acceptor of an electron transport chain. Nitrate is reduced to nitrite or N₂ by denitrifiers, sulphate to sulphide by sulphate reducers, and carbonate (CO₂) to methane by methanogens or to acetate by acetogens.

Lactate is the reverse case: it is an end product of lactic acid fermentation and a common electron donor for anaerobes such as Desulfovibrio, which oxidise it while reducing sulphate.

Answer: D

The defective enzyme is likely Isocitrate lyase.

To grow on acetate, a two-carbon compound, a bacterium must make four-carbon compounds for gluconeogenesis without losing both carbons as CO₂ in the citric acid cycle. The glyoxylate shunt does this: isocitrate lyase splits isocitrate into succinate and glyoxylate, and malate synthase joins glyoxylate to acetyl CoA. Without isocitrate lyase there is no net synthesis from acetate. Glycerol enters glycolysis, so growth on it does not need the shunt.

Glyceraldehyde 3-phosphate dehydrogenase and isocitrate dehydrogenase are needed on glycerol too, so their loss would block growth on both sources. Pyruvate dehydrogenase is not needed on acetate, which enters as acetyl CoA directly.

Answer: A

The nitrogen-fixing pair is Clostridia and Rhizobia.

Clostridium pasteurianum is a free-living anaerobic nitrogen fixer, and its nitrogenase was among the first to be purified. Rhizobium fixes nitrogen symbiotically in the root nodules of legumes.

Lactobacillus, Enterococcus and Mycoplasma do not fix nitrogen, so every other pair contains a non-fixer. Some actinomycetes (Frankia) do fix nitrogen, but Mycoplasma does not.

Answer: A

Heterocyst formation involves synthesis of nitrogenase and retention of photosystem I.

Nitrogenase is destroyed by oxygen, so filamentous cyanobacteria such as Anabaena fix nitrogen in specialised heterocysts. During differentiation the cell builds a thick glycolipid and polysaccharide envelope that limits oxygen entry, switches on the nif genes to make nitrogenase, and dismantles photosystem II, the oxygen-evolving photosystem. Photosystem I is kept, because cyclic electron flow around it supplies the large amount of ATP that nitrogen fixation needs.

Losing photosystem I would leave the heterocyst short of ATP, so the options with loss of photosystem I are wrong, and the options with loss of nitrogenase contradict the whole purpose of the heterocyst, which is nitrogen fixation.

Answer: D

The methane comes from carbon dioxide produced by bacterial fermentation occurring in the gut.

In the rumen, bacteria, protozoa and fungi ferment cellulose and other plant material to volatile fatty acids, releasing H₂ and CO₂. Methanogenic archaea use the hydrogen to reduce CO₂ to methane, which the animal belches out.

CO₂ from the cow's own respiration or mitochondrial oxidation leaves through the lungs and is not converted to methane, since animal cells have no methanogenic pathway. Lactic acid fermentation in muscle makes lactate, not CO₂ for methanogens.

Answer: C

The terminal electron acceptor is carbon dioxide.

Methanococcus is a methanogenic archaeon. It oxidises H₂ (or formate) and passes the electrons to CO₂, reducing it to methane, CO₂ + 4H₂ → CH₄ + 2H₂O. CO₂ is therefore the final electron acceptor of its anaerobic energy metabolism.

Nitrate is the acceptor for denitrifying bacteria such as Pseudomonas, and sulfate for sulfate reducers such as Desulfovibrio. Oxygen is the acceptor of aerobic respiration, not anaerobic respiration, and methanogens are strict anaerobes.

Answer: C

Photosynthesis in cyanobacteria takes place on the thylakoids.

Cyanobacteria are prokaryotes with no chloroplasts. Their chlorophyll a, photosystems and electron transport chain sit in internal thylakoid membranes lying free in the cytoplasm, with phycobilisomes attached to their surface as light-harvesting antennae.

The chloroplast is the organelle of algae and plants, itself descended from an engulfed cyanobacterium. Chromatophores are the photosynthetic intracytoplasmic membranes of purple bacteria. Chlorosomes are the light-harvesting sacs of green sulfur bacteria and Chloroflexus.

Answer: B

The sequence is 2NO₃^- → 2NO₂^- → 2NO → N₂O → N₂.

Denitrification reduces nitrate stepwise to dinitrogen gas, one enzyme per step:

1. nitrate reductase: NO₃^- to NO₂^-;
2. nitrite reductase: NO₂^- to nitric oxide, NO;
3. nitric oxide reductase: two NO to nitrous oxide, N₂O;
4. nitrous oxide reductase: N₂O to N₂.

The oxidation state of nitrogen falls steadily, +5, +3, +2, +1, 0, which is a quick check on the order. Option A puts N₂O (+1) before NO (+2), option C puts NO before nitrite, and option D jumps from nitrate straight to N₂O and then back up to nitrite.

Answer: B

The organism is Desulfovibrio.

Desulfovibrio is a sulfate-reducing bacterium: under anaerobic conditions it uses sulfate as the terminal electron acceptor in respiration and reduces it to hydrogen sulfide, which gives anoxic sediments their smell and black iron sulfide.

Beggiatoa, Thiobacillus and Thiothrix do the opposite: they are sulfur oxidisers that oxidise hydrogen sulfide or elemental sulfur to sulfate as a source of energy.