Marine Biology
Marine biology is the scientific study of the organisms and ecosystems of the ocean, one of the largest and most diverse habitats on Earth. The ocean covers more than 70% of the planet's surface and contains 97% of its water[^c1], and marine organisms — from microscopic plankton to the blue whale, the largest animal that has ever lived[^c2] — occupy every part of it, from sunlit surface waters to the deepest trenches.
The discipline traces its origins to ancient observations of marine life and matured in the 19th century with the Challenger Expedition (1872–76), the first expedition to explore the deep sea successfully on a global scale[^c3], which demonstrated that life exists throughout the deep ocean and helped establish oceanography as a modern science. The ocean is organised into distinct environments — the intertidal zone, coastal habitats such as coral reefs, mangroves and seagrass beds, the open ocean, polar seas and the deep sea — each with characteristic communities and physical conditions. Among the most productive of these coastal habitats are kelp forests, which line about a quarter of the world's coastline[^c7], and salt marshes, whose wetland ecosystems rank among the most productive on Earth[^c8].
Marine organisms are enormously diverse and ecologically central. Marine microbes are the ocean's most abundant life forms: in a litre of seawater or a gram of sediment, bacteria typically make up more than 99% of the DNA in the sample[^c6]. Phytoplankton, the ocean's primary producers, are responsible for producing over 50% of the world's oxygen while forming the foundation of oceanic food webs[^c4], and marine invertebrates, fishes, [[Marine Reptiles]], [[Seabirds]] and mammals occupy the higher levels. The field studies how these organisms function and adapt to the physical and chemical challenges of ocean life, how they interact in food webs and biogeochemical cycles, and how they are investigated using technologies ranging from field sampling and remote sensing to environmental DNA.
Marine biology is also a conservation science. As the discipline developed, so did the recognition of the threat that puts our oceans at danger[^c5]: overfishing, pollution, climate change, ocean acidification and habitat destruction all endanger marine life. Marine protected areas, fisheries management and restoration are used to protect and rebuild ocean ecosystems, including coastal habitats such as kelp forests and salt marshes whose recovery has become a focus of active intervention. Understanding the ocean is increasingly urgent, because it regulates the global climate and sustains human societies worldwide.