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Filamentous photosynthetic algae were placed on a microscopic slide and illuminated with light of different colors as illustrated.

Algal filament lit at Blue, Green, Yellow, Red

The bacteria that are known to migrate towards the region of high O2 were also added uniformly on the slide. Which one of the following options illustrates the distribution of bacteria along the length of the microscopic slide after illumination?

  • A.Bacterial concentration against position with peaks at Blue and Red and almost none at Green and Yellow
  • B.Bacterial concentration against position with peaks at Green and Red and a minimum at Yellow
  • C.Bacterial concentration against position with a tall peak at Green, a smaller broad peak at Yellow and low values at Blue and Red
  • D.Bacterial concentration against position with peaks at Blue and Yellow and low values at Green and Red

Answer: A

The bacteria gather in the blue and red regions, as in option (A).

This is Engelmann's experiment. The algae photosynthesise and release O2 fastest where they absorb light best, and chlorophyll absorbs most strongly in the blue and red parts of the spectrum. Aerotactic bacteria swim toward high oxygen, so they pile up under the blue and red lights and stay sparse under green and yellow light, which chlorophyll largely transmits or reflects.

Option (B) puts a peak at green, the region chlorophyll absorbs least. Options (C) and (D) place peaks at green or yellow and miss the red peak, which would require photosynthesis to be driven mainly by poorly absorbed wavelengths.

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