Marine life

Marine life, also called sea or ocean life, includes all organisms living in salt water. This encompasses animals, plants, algae, fungi, protists, and single-celled microorganisms. It also includes associated viruses within saline habitats. These habitats range from the vast oceans to coastal lagoons and estuaries.

Marine life is incredibly diverse and vast. Scientists have documented over 242,000 marine species so far. However, they estimate that two million species may still be undiscovered. On average, researchers describe about 2,300 new species every year. By volume, the oceans provide about 90% of the living space on Earth. This makes the ocean a vital sanctuary for life throughout geological history.

Organisms in the ocean are categorized by how they obtain energy. These roles are known as autotrophs and heterotrophs. Autotrophs are the "producers" of the ecosystem. They include photosynthetic organisms like cyanobacteria, algae, and seagrass. These organisms use sunlight to create energy. Some rare autotrophs are chemoautotrophs. They use energy from chemical oxidation to convert inorganic molecules into organic compounds.

Heterotrophs are the "consumers" that must eat other organisms to survive. This group includes all animals, fungi, and many protists. Marine animals are often divided into vertebrates and invertebrates. Vertebrates include fish, marine mammals, reptiles, and seabirds. Invertebrates include everything else, such as many mollusks and crustaceans.

Marine life plays a critical role in regulating the planet. Primary producers like chloroplastic algae and cyanobacteria perform photosynthesis. This process produces oxygen and sequesters carbon. This activity significantly influences the chemistry of the atmosphere. Other species, like migratory fish, move energy between different global regions. These are often called keystone species because they are vital to their ecosystems. Some organisms even shape the physical world. For example, corals build reefs that can create new land through accumulation.

The history of marine life began billions of years ago. The earliest life forms were anaerobic prokaryotes. These were simple cells like archaea and bacteria. They lived in the Archean oceans near deep-sea hydrothermal vents. Later, the Great Oxygenation Event changed marine chemistry. This event likely caused many anaerobes to go extinct. However, it also allowed eukaryotes to evolve through symbiogenesis. This is a process where surviving organisms live together in a cooperative way.

Complex life eventually emerged during the Neoproterozoic era. This led to the Avalon Explosion of sessile macrofaunae. Later, the Cambrian Explosion saw a rise in actively moving animals. These marine organisms eventually moved into fresh waters, helping terrestrial ecosystems develop.

Water is the fundamental requirement for all marine life. It is often called the "universal solvent" because it dissolves many substances. The Earth's hydrosphere includes oceans, lakes, and even underground waters. The world ocean covers 71% of the Earth's surface. It has a mean depth of about 3,700 meters.

The total mass of the world ocean is 1.35 metric tons. This is about 1/4400 of the Earth's total mass. Most of the water on Earth is saline. About 97.5% of Earth's water is salt water. The average salinity is about 3.5% salt per kilogram. Most of this salt comes from the weathering of rocks on land.

Understanding the ocean also helps us search for life in space. Some moons in our solar system may have marine environments. Europa, a moon of Jupiter, likely has a large saltwater ocean. This ocean exists beneath a thick crust of solid ice. It is estimated that Europa's ocean could be twice the volume of Earth's. If hydrothermal vents are active there, it might support multicellular microorganisms. Saturn's moon, Enceladus, also appears to have an underground ocean. This moon actively vents warm water from its surface. These discoveries show how important water is for potential life.

Scientists study these complex systems through marine biology and biological oceanography. They use many tools to understand how life interacts with the physical sea. They look at everything from microscopic phytoplankton to massive blue whales. Phytoplankton can be as small as 0.02 micrometers. In contrast, large cetaceans reach incredible lengths. By studying the connections between species and their environment, we learn how the ocean protects our shorelines and regulates our climate. The ocean acts as a massive heat reservoir for the entire planet.