Multiple choice The amount of carbon fixed annually by photosynthesis is: 70 billon tonnes 80 billon tonnes 50 billon tonnes 20 billons tonnes Reveal answer Fill a bubble to check yourself A Correct answer Explanation Approximately 70 billion tonnes of carbon is fixed globally through photosynthesis each year, representing the net primary production. This is the total amount of carbon dioxide converted into organic compounds by plants, algae, and cyanobacteria.