The algae fern Azolla, whose mass proliferation around 50 million years ago contributed to our current cold period, is an impressive example of how a simple plant can influence the entire Earth's climate.

Similar to us humans today, only in reverse, the fern drastically changed the CO2 content in the atmosphere and thus the global temperature within a short period of time through photosynthesis. Due to cooling and ice formation, large parts of its habitat became uninhabitable for it.

Azolla lives in symbiosis with the blue-green microalga Anabaena azollae. The alga fixes nitrogen, which Azolla needs for its growth, while the fern provides the alga with a protected habitat and carbohydrates. Blue-green algae can form so-calledtoxic water blooms as a result of their proliferation.

Kelp forests are like water lungs for the ocean. They mostly consist of brown algae and grow incredibly fast.

With plenty of nutrients and light, kelp can grow several centimeters a day, some species about 1–2 meters per year. Macrocystis, also known as giant kelp, is also called the Sequoia of the oceans.

Blue carbon means that kelp uses photosynthesis to take CO2 from the water and store it in its biomass. Some of that carbon ends up in sediments on the seabed when the kelp dies, helping to lower atmospheric CO2 levels. According to studies, kelp forests absorb about as much CO2 worldwide as the Amazon rainforest. Also, kelp absorbs two to three times more CO2 than any other algae or or terrestrial plant.