At least _______ of the carbon dioxide fixation on earth is carried out by algae through photosynthesis.

The correct option is a half.

Algae are chlorophyll-bearing, simple, thalloid, autotrophic, and largely aquatic (both fresh water and marine) organisms. They play a critical role in the global carbon cycle and energy flow.

Through the process of photosynthesis, algae absorb carbon dioxide
${\mathrm{CO}}_2$
from their surroundings and convert it into organic compounds using solar energy. It is estimated that at least a half (about 50%) of the total carbon dioxide fixation on Earth is carried out by algae. This makes them one of the most important primary producers of food and oxygen in aquatic ecosystems.