How can algae be responsible for the production of more oxygen than land plants?

Question

How can algae be responsible for the production of more oxygen than land plants?

Answer 1

Simply put, there is much more algae living on earth than there are land plants. If you added up the mass of all the algae, it would weigh more than the mass of all the land plants.  

Algae is also very efficient at photosynthesis, in terms of the amount of oxygen produced as compared to a plant's size.  (In other words, if you compared the O2 output of a 100 pound tree with that of 100 pounds of algae, the algae would produce more O2)

Answer 2

Algae and cyanobacteria do not spend a portion of the year dormant as temperate plants must. They may even produce more oxygen per unit weight because they don't construct non-photosynthesizing plant parts like bark, roots, branches, fruit etc. They need to use a lot of the oxygen they release from photosynthesis in their mitochondria's respiration to carry out the work to grow and maintain themselves. This is the cost of more elaborate body plans plants needed to live on land.

Last there is more water than land for the algae and cyanobacteria to occupy. They are not just marine, both can be found in fresh water. So with about 70% of Earth's surface covered in water aquatic photosynthesizers just have more space than terrestrial ones.