The little pink-edged ferns above are Azolla filiculoides, and they're smaller than a fingernail. Scientists just made it the first fern to get its genome sequenced because of its potential for fertilizing and even cooling the planet. Fifty million years ago, it was so abundant as ocean blooms that it helped cool the earth's atmosphere. Via Quartz:
This great Azolla boom was so successful that it lasted for 800,000 years, and is now known to paleobotanists as the "Azolla event." Green plants suck up carbon dioxide; Azolla is particularly good at doing so. Over that period, researchers believe it sequestered about 10 trillion tons of carbon dioxide from the Earth's atmosphere, or well over 200 times the total amount of carbon dioxide humans currently release into the atmosphere every year.
During the Azolla boom, global temperatures plummeted, suggesting the diminutive fern "played a key role in transitioning Earth from a hot house to the cool place it is today," Fay-Wei Li, a plant evolutionary biologist at Cornell University, said in a press release. As Yale's E360 pointed out, scientists have wondered for years if Azolla could be harnessed to cool the planet again.