Question
Answers

Photoautotrophs lacking chlorophyll a are
A. Cyanobacteria
B. Red algae
C. Brown algae
D. Bacteria

Answer
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Hint: Chlorophyll is a chemical pigment present in almost all of the autotrophic organisms. It is found in the thylakoid membranes of the chloroplast. It helps organisms to create chemical energy by utilizing light energy. Chlorophyll a is the primary pigment of photosynthesis. It is not present in bacteria that do not absorb orange-red and violet-blue areas of visible light.

Complete answer: The photoautotrophs refer to those organisms that utilize light energy to make their food. Plants and some microbes are photoautotrophs. Chlorophyll is the key pigment that is responsible for the process of photosynthesis through which the photoautotrophs make their food. Chlorophyll is a chemical pigment that is found in the organelle called the chloroplast. Chlorophyll is majorly of two types. These are chlorophyll a and chlorophyll b. chlorophyll a is the primary pigment and chlorophyll b is the accessory pigment. The function of chlorophyll a is to absorb light in the range of orange-red and violet-blue. It is responsible for transferring the energy to the reaction center. It is a primary electron donor. Cyanobacteria are prokaryotic bacteria that are photosynthetic. They lack organized chloroplasts as present in eukaryotes. Their photosynthetic apparatus is distributed peripherally inside the cytoplasm. The blue-green bacteria do contain chlorophyll a and produce free oxygen as a photosynthesis byproduct. Most of the cyanobacteria produce cyanotoxins. These are dangerous chemical substances that are products of energy conversion.
So, from the above discussion, we can conclude that option A is correct.

Note: Cyanobacteria are thought to be the causative organisms of the great oxygenation event in the early Earth’s environment. They are considered to be the first organisms that converted the early oxygen-poor and reduced atmosphere into an oxidizing one.