Short answers, pulled from the story.
Algae are any of a large and diverse group of photosynthetic organisms, excluding the land plants known as embryophytes. They range from microscopic unicellular microalgae, including cyanobacteria and phytoplankton, to multicellular seaweeds that may grow up to 50 metres in length.
Algae are polyphyletic because they do not share a common ancestor. Eukaryotic algae acquired their chloroplasts through separate endosymbiotic events, with some carrying primary chloroplasts from cyanobacteria and others carrying secondary chloroplasts from engulfed red or green algae.
As of January 2024, the database AlgaeBase documents 50,605 living and 10,556 fossil algal species, classified into 15 phyla or divisions. Scientists regard the true total as unknown because most estimates omit microscopic algae such as phytoplankton.
Algae are eaten in many countries, with China consuming more than 70 species including fat choy, Japan over 20 species such as nori and aonori, Ireland eating dulse, and Chile eating cochayuyo. Wales uses laver to make laverbread, known there as bara lawr.
Reef-building stony corals require endosymbiotic algae from the genus Symbiodinium to stay healthy, as dinoflagellates inside coral cells generate sugar and oxygen through photosynthesis. When corals lose their Symbiodinium, the result is coral bleaching, which leads a reef to deteriorate.
Algae can treat sewage and capture fertilizer runoff. Agricultural Research Service scientists found that a horizontal algal turf scrubber of shallow 100-foot nylon raceways can capture 60 to 90% of nitrogen runoff and 70 to 100% of phosphorus runoff from manure effluents.