Solution:
In the oceans, the primary producers are above all algae, which form phytoplankton. Phytoplankton is essential in the production of oxygen. In fact, more than 50 percent of the oxygen on our planet is produced in the sea through photosynthesis triggered by phytoplankton, an autotrophic microorganism that is essential for life on the planet.
Now, if iron is a fundamental nutrient for phytoplankton and scientists are working to make this nutrient more available for this photosynthetic microorganism, which can use carbon dioxide and generate oxygen in exchange, this is because they want to establish a reduction of the greenhouse gas carbon dioxide in the atmosphere.
So that, we can conclude that the correct answer is:
Reduction of the greenhouse gas carbon dioxide in the atmosphere.