The maximum bacterial congregation will be in regions P and R, because these receive blue (400 nm) and red (700 nm) light, which drive maximum photosynthesis and oxygen evolution.

Concept behind the question

• This question is based on Engelmann’s experiment on the action spectrum of photosynthesis using a filamentous alga and aerobic bacteria.

• Aerobic bacteria move towards regions with higher oxygen concentration, so they accumulate where the alga performs photosynthesis most efficiently.

In green algae, photosynthesis is most active in blue (around 400–500 nm) and red (around 600–700 nm) parts of the spectrum, matching the absorption peaks of chlorophyll.

Analysis of regions P, Q, R

• Region P: receives 400 nm and 500 nm light (violet–blue–green range). Blue light is strongly absorbed by chlorophyll and produces high photosynthetic rate and oxygen release, so bacteria will accumulate heavily here.

• Region Q: receives 500 nm and 600 nm light (green–yellow). Green light is least effective for photosynthesis because much of it is reflected or transmitted by chlorophyll; hence oxygen evolution and bacterial congregation are relatively low here.

• Region R: receives 600 nm and 700 nm light (orange–red). Red light is another peak of photosynthetic activity, so oxygen production is high and bacteria strongly congregate here as well.

Therefore, P and R are the regions with maximum oxygen concentration and thus maximum bacterial congregation, matching option (A).

Explanation of each option

• Option (A) P and R – Correct

  • Both regions receive blue or red light where the photosynthetic action spectrum shows sharp maxima, so these segments of the alga produce the most oxygen and attract the most bacteria.

• Option (B) P and Q – Incorrect

  • P is correct, but Q receives mainly green–yellow light, which has low photosynthetic efficiency; bacterial density here will not be maximal.

• Option (C) Only P – Incorrect

  • P shows high activity due to blue light, but ignoring R neglects the equally strong photosynthesis peak in red light, where bacteria also cluster.

• Option (D) Only Q – Incorrect

  • Q corresponds to the least effective part of the spectrum for chlorophyll, so it cannot have the maximum congregation when P and R exist.

Introduction

Bacterial congregation on photosynthetic filamentous alga reveals which wavelengths of light drive maximum oxygen evolution and photosynthesis, mirroring the discoveries of Engelmann’s classic action spectrum experiment. In a JAM-style problem where algal segments are illuminated with 400, 500, 600 and 700 nm light, this principle helps identify exactly which regions attract the most oxygen‑seeking bacteria.

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Wavelengths and photosynthetic efficiency

• Photosynthesis in green algae peaks in the blue region (around 400–500 nm) and red region (around 600–700 nm), where chlorophyll absorbs light most strongly and drives high rates of electron transport and oxygen evolution.

• In contrast, the green–yellow band (around 500–600 nm) is poorly absorbed, so photosynthetic yield and oxygen release are much lower there.

These wavelength‑dependent differences underpin the pattern of bacterial congregation on photosynthetic filamentous alga placed under discrete light beams.

Link to Engelmann’s experiment

• Engelmann illuminated a filamentous alga with a spectrum of visible light and added aerobic bacteria that migrate towards high oxygen concentrations.

• The bacteria accumulated in regions corresponding to blue and red light, leading to the conclusion that these colors are most effective in driving photosynthesis, forming the first action spectrum of photosynthesis.

Modern exam questions, like the one with regions P, Q and R under 400, 500, 600 and 700 nm light, directly apply this classic result to predict bacterial congregation on photosynthetic filamentous alga.

Why P and R regions show maximum congregation

• Region P is illuminated by 400 and 500 nm; the 400 nm component lies in the blue peak of the action spectrum, causing high oxygen production and strong attraction of aerobic bacteria.

• Region R is illuminated by 600 and 700 nm; the 700 nm red light also corresponds to a photosynthetic maximum, again generating high oxygen concentration and dense bacterial congregation.

Because both P and R contain wavelengths where photosynthesis is highly efficient, bacterial congregation on photosynthetic filamentous alga is maximal in these two regions, making option (A) the correct choice.