Single-celled photosynthetic eukaryotes, they occupy a position that makes it hard to call them plants (they’re not embryophytes) but also makes it hard to not call them plants (they’re the closest relatives of land plants and share a large number of features with them) A widespread and extremely prolific group, microalgae are individually tiny but grow in such numbers that one can see algal blooms from space. When such groups die, the decay of absurd quantities of algae may deplete oxygen in the seawater and result in a small-scale collapse of the entire marine ecosystem, those events are known as red tides. They may also produce toxins responsible for some types of shellfish poisoning.
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