Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic (both fresh water and marine) organisms. They occur in a variety of other habitats: moist stones, soils and wood. Some of them also occur in association with fungi (lichen) and animals (e.g., on sloth bear).
Assertion (A): Algae play a major role in global carbon dioxide fixation. Reason (R): Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic organisms.
Correct answer: A — If both assertion and reason are true and the reason is the correct explanation of the assertion.
NCERT states “At least a half of the total carbon dioxide fixation on earth is carried out by algae through photosynthesis,” making the Assertion true. NCERT defines “Algae are chlorophyll-bearing, simple, thalloid, autotrophic and largely aquatic organisms.” The Reason explains WHY algae can fix CO2 - being autotrophic with chlorophyll enables photosynthetic CO2 fixation. This shifts from structural definition to ecological function.
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