Questions: Photosynthesis in Bacteria and Cyanobacteria

Question 1

Purple sulfur bacteria can photosynthesize and grow in the light but do not produce oxygen. Why not?

• A. They lack chlorophyll and therefore cannot drive the energetically demanding water-splitting reaction
• B. They use hydrogen sulfide (H₂S) rather than water as their electron donor, so they never split water or release O₂
• C. They have only one photosystem, which produces insufficient energy to drive any oxidation reaction
• D. They perform photosynthesis in anaerobic environments where oxygen would immediately react with sulfide

Answer: (B)

The key difference between anoxygenic and oxygenic photosynthesis is the electron donor. Purple sulfur bacteria oxidize H₂S → elemental sulfur (or H₂, or organic compounds), extracting electrons without ever splitting water. Since water-splitting is the only source of O₂ in biological photosynthesis, these organisms produce no oxygen. Option A is incorrect — bacteriochlorophyll efficiently absorbs light energy; the constraint is the electron source, not the pigment. Option C is partially true (they use one photosystem) but the real reason is the electron donor, not energy insufficiency.

Question 2

What made the evolution of cyanobacteria so ecologically transformative compared to the anoxygenic photosynthetic bacteria that preceded them?

• A. Cyanobacteria absorbed light more efficiently, outcompeting anoxygenic bacteria and occupying a broader range of habitats
• B. Cyanobacteria evolved the Z-scheme — two linked photosystems — enabling them to use water as an electron donor and release O₂, transforming Earth's atmosphere
• C. Cyanobacteria were the first organisms to fix CO₂, inventing carbon fixation where none had existed before
• D. Cyanobacteria evolved cell walls that protected them from the UV radiation previously responsible for limiting bacterial photosynthesis

Answer: (B)

The Z-scheme — Photosystem II and Photosystem I working in series — was the innovation that made oxygenic photosynthesis possible. PSII has sufficient oxidizing power to strip electrons from water (a very stable molecule), releasing O₂. A single photosystem (as in anoxygenic bacteria) lacks the energy to oxidize water; two photosystems working in series provide enough. The accumulated O₂ output triggered the Great Oxidation Event (~2.4 Gya), rusting dissolved iron from the oceans, transforming atmospheric chemistry, and establishing the oxygen-rich environment that all aerobic life depends on.

Question 3

Anoxygenic photosynthetic bacteria — the purple and green sulfur bacteria — are evolutionarily more ancient than cyanobacteria and dominated Earth's photic zone for over a billion years before oxygen-producing photosynthesis evolved.

• T. True
• F. False

Answer: True

This is correct. Life existed for over a billion years before the evolution of oxygenic photosynthesis. During this time, anoxygenic phototrophs dominated, and Earth's atmosphere was reducing — essentially devoid of free oxygen. Cyanobacteria evolved around 2.7 Gya (with their full ecological impact from ~2.4 Gya onward), fundamentally changing the planet. This history matters for understanding the origin of chloroplasts: plant chloroplasts derive from cyanobacteria via endosymbiosis, not from the earlier anoxygenic bacteria.

Question 4

Plant chloroplasts perform photosynthesis using fundamentally different molecular machinery from cyanobacteria, because they evolved independently within the eukaryotic lineage.

• T. True
• F. False

Answer: False

Chloroplasts did not evolve independently — they are the descendants of ancient cyanobacteria that were engulfed by a eukaryotic host cell through endosymbiosis. Over evolutionary time, most cyanobacterial genes were transferred to the nuclear genome or lost, but the core photosynthetic machinery — including Photosystem I, Photosystem II, the cytochrome b₆f complex, and ATP synthase — is directly homologous between cyanobacteria and plant chloroplasts. Modern cyanobacteria and chloroplasts use the same Z-scheme, the same pigments (chlorophyll a), and essentially the same protein complexes. The endosymbiotic origin is why they are so similar.

Question 5

Explain why two linked photosystems (the Z-scheme) are required for oxygenic photosynthesis but not for anoxygenic photosynthesis.

Model answer: Water is a very stable molecule with a high oxidation potential — stripping electrons from it requires delivering very high-energy, strongly oxidizing 'electron holes.' A single photosystem can generate enough energy to oxidize easier electron donors like H₂S or H₂, but not enough to oxidize water. The Z-scheme solves this by using two photons in series: Photosystem II generates the strong oxidant needed to split water, producing low-energy electrons that then pass through an electron transport chain to Photosystem I, which uses a second photon to boost these electrons to the high-energy state needed to reduce NADP⁺. Two sequential light-driven reactions achieve what one cannot.

This is the fundamental constraint distinguishing oxygenic from anoxygenic photosynthesis. Thermodynamically, the oxidation of water (E° = +0.82 V) requires a much stronger oxidizing agent than the oxidation of H₂S (E° ≈ −0.23 V). PSII generates an exceptionally strong oxidant (E° ≈ +1.1 V) specifically to accomplish water-splitting — the strongest biological oxidant known. Anoxygenic bacteria, using only one photosystem, can generate moderately strong oxidants sufficient for H₂S or H₂ but fall far short of the oxidizing power needed for water.