Question

How does the bulk fixation of carbon dioxide through photosynthesis take place?

Answer 1

Photosynthesis is the process in which the autotrophs, which are the animals that make their own food, entrap solar energy, carbon dioxide and water from the surroundings and synthesize food (organic matter), which is a source of energy. The autotrophs by this process actually fix carbon in the environment.
Here the question actually asks us to highlight “where” or “in which place” such a huge amount of carbon is fixed. This is what the term ‘bulk fixation’ is meant here.

Complete answer:
The bulk fixation of carbon by photosynthesis occurs in the oceans. This is due to the fact that the oceans cover 3/4th of this planet. In such a large area, huge numbers of planktons, cyanobacteria, dinoflagellates, seaweeds, etc. are present that primarily occupy the ocean surface and perform photosynthesis. Thus, oceans are the place where bulk fixation of carbon dioxide occurs.
Now, the question of ‘how this fixation occurs through photosynthesis’ demands us to describe the mechanism of photosynthesis. This is an overall another question of a different topic, but we can definitely speak up about it within a few sentences.
Photosynthesis basically occurs and covers two sets of reactions:
Light Reactions
Dark Reactions
The light reaction occurs in the grana of chloroplast and involves 4 steps:
Activation of chlorophyll (PS-I & PS-II)
Photolysis of water
Formation of $H_2O$, $O_2$, NADPH + $H^+$ ↔ $NADPH_2$
Formation of ATP through cyclic and non-cyclic photophosphorylation.
The Dark reaction mainly involves the Calvin cycle (C3 cycle) where a compound named RuBP is synthesized and which ultimately leads to the formation of starch or glucose. Certain plants may also show the C4 mechanism which is more efficient than the Calvin cycle as it lacks a definite process called ‘photorespiration’.

Note:
One point to be noted here is that although bulk fixation of carbon dioxide occurs in the oceans, the annual net primary productivity (NPP) of oceans is only 55 billion tons and that of the terrestrial ecosystem is 115 billion tons.
This is due to the fact that only the surface and a few hundred meters depth of the ocean receives sunlight. So, most of the part of the oceans remain in perpetual darkness. So, fewer autotrophs are there in the ocean to fulfill the needs of the whole marine population.