Brackish water in the context of "Salt marsh"

Marine life, sea life or ocean life is the collective ecological communities that encompass all aquatic animals, plants, algae, fungi, protists, single-celled microorganisms and associated viruses living in the saline water of marine habitats, either the sea water of marginal seas and oceans, or the brackish water of coastal wetlands, lagoons, estuaries and inland seas. As of 2023, more than 242,000 marine species have been documented, and perhaps two million marine species are yet to be documented. An average of 2,332 new species per year are being described. Marine life is studied scientifically in both marine biology and in biological oceanography.

By volume, oceans provide about 90% of the living space on Earth, and served as the cradle of life and vital biotic sanctuaries throughout Earth's geological history. The earliest known life forms evolved as anaerobic prokaryotes (archaea and bacteria) in the Archean oceans around the deep sea hydrothermal vents, before photoautotrophs appeared and allowed the microbial mats to expand into shallow water marine environments. The Great Oxygenation Event of the early Proterozoic significantly altered the marine chemistry, which likely caused a widespread anaerobe extinction event but also led to the evolution of eukaryotes through symbiogenesis between surviving anaerobes and aerobes. Complex life eventually arose out of marine eukaryotes during the Neoproterozoic, and which culminated in a large evolutionary radiation event of mostly sessile macrofaunae known as the Avalon Explosion. This was followed in the early Phanerozoic by a more prominent radiation event known as the Cambrian Explosion, where actively moving eumetazoan became prevalent. These marine life also expanded into fresh waters, where fungi and green algae that were washed ashore onto riparian areas started to take hold later during the Ordovician before rapidly expanding inland during the Silurian and Devonian, paving the way for terrestrial ecosystems to develop.

The Messinian salinity crisis (also referred to as the Messinian event, and in its latest stage as the Lago Mare event) was an event in which the Mediterranean Sea went into a cycle of partial or nearly complete desiccation (drying-up) throughout the latter part of the Messinian age of the Miocene epoch, from 5.96 to 5.33 Ma (million years ago). It ended with the Zanclean flood, when the Atlantic reclaimed the basin.

Sediment samples from below the deep seafloor of the Mediterranean Sea, which include evaporite minerals, soils, and fossil plants, show that the precursor of the Strait of Gibraltar closed about 5.96 million years ago, sealing the Mediterranean off from the Atlantic. This resulted in a period of partial desiccation of the Mediterranean Sea, the first of several such periods during the late Miocene. After the strait closed for the last time around 5.6 Ma, the region's generally dry climate at the time dried the Mediterranean basin out nearly completely within a thousand years. This massive desiccation left a deep dry basin, reaching 3 to 5 km (1.9 to 3.1 mi) deep below normal sea level, with a few hypersaline pockets similar to today's Dead Sea. Then, around 5.5 Ma, wetter climatic conditions resulted in the basin receiving more fresh water from rivers, progressively filling and diluting the hypersaline lakes into larger pockets of brackish water (much like today's Caspian Sea). The Messinian salinity crisis ended with the Strait of Gibraltar finally reopening 5.33 Ma, when the Atlantic rapidly filled up the Mediterranean basin in what is known as the Zanclean flood.

Marine microorganisms are defined by their habitat as microorganisms living in a marine environment, that is, in the saltwater of a sea or ocean or the brackish water of a coastal estuary. A microorganism (or microbe) is any microscopic living organism or virus, which is invisibly small to the unaided human eye without magnification. Microorganisms are very diverse. They can be single-celled or multicellular and include bacteria, archaea, viruses, and most protozoa, as well as some fungi, algae, and animals, such as rotifers and copepods. Many macroscopic animals and plants have microscopic juvenile stages. Some microbiologists also classify viruses as microorganisms, but others consider these as non-living.

Marine microorganisms have been variously estimated to make up between 70 and 90 percent of the biomass in the ocean. Taken together they form the marine microbiome. Over billions of years this microbiome has evolved many life styles and adaptations and come to participate in the global cycling of almost all chemical elements. Microorganisms are crucial to nutrient recycling in ecosystems as they act as decomposers. They are also responsible for nearly all photosynthesis that occurs in the ocean, as well as the cycling of carbon, nitrogen, phosphorus and other nutrients and trace elements. Marine microorganisms sequester large amounts of carbon and produce much of the world's oxygen.

Saline water (more commonly known as salt water) is water that contains a high concentration of dissolved salts (mainly sodium chloride). On the United States Geological Survey (USGS) salinity scale, saline water is saltier than brackish water, but less salty than brine. The salt concentration is usually expressed in parts per thousand (permille, ‰) and parts per million (ppm). The USGS salinity scale defines three levels of saline water. The salt concentration in slightly saline water is 1,000 to 3,000 ppm (0.1–0.3%); in moderately saline water is 3,000 to 10,000 ppm (0.3–1%); and in highly saline water is 10,000 to 35,000 ppm (1–3.5%). Seawater has a salinity of roughly 35,000 ppm, equivalent to 35 grams of salt per one liter (or kilogram) of water. The saturation level is only nominally dependent on the temperature of the water. At 20 °C (68 °F) one liter of water can dissolve about 357 grams of salt, a concentration of 26.3 percent by weight (% w/w). At 100 °C (212 °F) (the boiling temperature of pure water), the amount of salt that can be dissolved in one liter of water increases to about 391 grams, a concentration of 28.1% w/w.

An estuary is a partially enclosed coastal body of brackish water with one or more rivers or streams flowing into it, and with a free connection to the open sea. Estuaries form a transition zone between river environments and maritime environments and are an example of an ecotone. Estuaries are subject both to marine influences such as tides, waves, and the influx of saline water, and to fluvial influences such as flows of freshwater and sediment. The mixing of seawater and freshwater provides high levels of nutrients both in the water column and in sediment, making estuaries among the most productive natural habitats in the world.

Most existing estuaries formed during the Holocene epoch with the flooding of river-eroded or glacially scoured valleys when the sea level began to rise about 10,000–12,000 years ago. Estuaries are typically classified according to their geomorphological features or to water-circulation patterns. They can have many different names, such as bays, harbors, lagoons, inlets, or sounds, although some of these water bodies do not strictly meet the above definition of an estuary and could be fully saline.

Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include non-salty mineral-rich waters, such as chalybeate springs. Fresh water may encompass frozen and meltwater in ice sheets, ice caps, glaciers, snowfields and icebergs, natural precipitations such as rainfall, snowfall, hail/sleet and graupel, and surface runoffs that form inland bodies of water such as wetlands, ponds, lakes, rivers, streams, as well as groundwater contained in aquifers, subterranean rivers and lakes.

Water is critical to the survival of all living organisms. Many organisms can thrive on salt water, but the great majority of vascular plants and most insects, amphibians, reptiles, mammals and birds need fresh water to survive.