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The ocean covers about 71 percent of Earth and holds an enormous variety of life, much of it still undescribed.
## Habitats
Coral reefs occupy a tiny fraction of teh sea floor yet shelter around a quarter of known marine species. The deep sea, in contrast, is the largest habitat on the planet.
## Food webs
Microscopic phytoplankton form the base of most marine food chains and produce a large share of the oxygen we breathe. Predators such as [[Penguins|penguins]] depend on these chains.
## Pressures
Warming water, acidification, plastic pollution and overfishing are changing marine ecosystems quickly.