I think you are on the right track. Phytoplankton perform a very important task of providing most of the world's oxygen, sequestering CO2 and seeding clouds by creating condensation nuclei, from the sulphur compounds they produce, that end up above the ocean surface, increasing the planet's albedo. They help reflect UV rays away from the Earth and keep it cooler. They are a very critical part of the Earth's homoestatic mechanism from what I can see.
Algae are the most fundamental organism and every single higher organism relies on them, directly or indirectly. Even viruses need them because if they have no hosts they cannot replicate. The solution to helping mitigate climate change and the burden of increasing human population seems to be to increase their presence.