Consider the following kinds of organisms: ​ 1. Copepods ​ 2. Cyanobacteria ​ 3. Diatoms ​ 4. Foraminifera ​ Which of the above are primary producers in the food chains of oceans? ​

The answer is B (2 and 3 only). A primary producer is an autotroph that fixes energy from light or chemicals into organic matter, so it forms the base of the food chain. In the ocean, that role belongs to photosynthetic phytoplankton.

• Cyanobacteria (2) are correct. These photosynthetic prokaryotes, such as Prochlorococcus and Trichodesmium, are among the most abundant primary producers in open-ocean and tropical waters, and some also fix nitrogen.
• Diatoms (3) are correct. These silica-shelled algae are major contributors to marine primary production and drive much of the carbon export to the deep sea.
• Copepods (1) are wrong. They are zooplankton and primary consumers that graze on phytoplankton.
• Foraminifera (4) are wrong. These are protist consumers, mostly heterotrophic, that feed on bacteria, algae and detritus. Their shells matter for the geological record, but they do not fix energy.

Mains connection: the efficiency of the biological carbon pump, ocean acidification effects on calcifiers, and fishery productivity all start with phytoplankton.

Trap: Candidates often pick copepods because they are visibly abundant in plankton samples. Abundance is not the same as being a producer.