Phenomenal movie, but the existence of algae ruins the entire plot. Is it just me?

The entire plot is that blight is wiping out crops causing a collapse in agriculture and since the blight thrives on nitrogen, oxygen was being removed due to plant life being wiped out.

Except one little thing ruins the plot. One little word, algae. Most oxygen comes from algae, the ocean's ecosystem depends on algae not land plants.

Earth's plants are being wiped out by blight? Stockpile as much seed as possible every day of every year and let the blight burn itself out, then after research, start crop re-introduction . In the meantime setup land based algae cultivation for land locked regions, and coastal areas focus on large scale fish farms.

The plot around nitrogen replacing oxygen is non-sensical as most oxygen comes from the oxygen.

One of my favorite movies ever, but once I started doing strategy for agricultural companies, I started going "wait what?".