Approximately what percentage of Earth's living organisms live in the ocean?

• A. 50%
• B. 75%
• C. 90%
• D. 10%

Answer: (C)

The ocean hosts the vast majority of Earth’s living organisms because it is an enormous, diverse, and ongoing reservoir of life, especially microorganisms. Water covers about 71% of the planet and harbors countless habitats—from sunlit surface waters to the dark depths and the seabed—where bacteria, archaea, plankton, and other organisms thrive. Because many of the most abundant life forms on Earth are microscopic and widespread in marine environments, the total number of living organisms found in the ocean far exceeds that on land. While exact percentages are hard to pin down, a rough, widely cited estimate is that about 90% of Earth’s living organisms are in the ocean. This helps explain why the ocean is such a central part of Earth's biodiversity, even though land areas host many unique species as well.