During the manufacture of food by plants, which of the following organism use energy from the sun?

Analyze the question: The question asks which organism uses energy from the sun (light energy) to manufacture food. That is photosynthesis (phototrophy), as opposed to chemosynthesis, which uses energy from chemical reactions.

Concept:

• Photosynthesis: organisms use sunlight, chlorophyll or other pigments, CO₂ and water to make food. Examples are green plants, algae and cyanobacteria.
• Chemosynthesis: organisms get energy from oxidizing inorganic compounds such as ammonia, nitrite and sulfur compounds. They don't need sunlight.

Evaluating each option:

• A. Anabaena: a cyanobacterium (blue-green alga). It contains chlorophyll a and carries out true photosynthesis using sunlight. Uses solar energy.
• B. Sulfur bacteria: this is the tricky one. Colorless sulfur bacteria such as Beggiatoa and Thiobacillus are chemosynthetic and oxidize hydrogen sulfide for energy. Purple and green sulfur bacteria do use light, but they are anoxygenic photosynthesizers. In the typical Nigerian syllabus, "sulfur bacteria" is taught as the chemosynthetic example, so it isn't the intended answer.
• C. Nitrosomonas sp: chemosynthetic. It oxidizes ammonia to nitrite.
• D. Nitrobacter sp: chemosynthetic. It oxidizes nitrite to nitrate.

Answer: Anabaena is the only option that is clearly and unambiguously photosynthetic.

Exam tip: Remember it this way:

• Anabaena = Algae-like = light (a blue-green alga, so photosynthetic).
• Nitrosomonas and Nitrobacter = Nitrogen cycle chemosynthesizers (they get energy from chemicals).
• Sulfur bacteria = usually listed as chemosynthetic in JAMB questions.

Whenever you see "nitrifying bacteria", think chemosynthesis, and whenever you see "blue-green algae/cyanobacteria", think photosynthesis.