page_title: Marine prokaryotes
section_title: Background
hierarchical_section_title: Marine prokaryotes / Background
caption_attribution_description: English: Ocean mist and spray
context_page_description: Marine prokaryotes are marine bacteria and marine archaea. They are defined by their habitat as prokaryotes that live in marine environments, that is, in the saltwater of seas or oceans or the brackish water of coastal estuaries. All cellular life forms can be divided into prokaryotes and eukaryotes. Eukaryotes are organisms whose cells have a nucleus enclosed within membranes, whereas prokaryotes are the organisms that do not have a nucleus enclosed within a membrane. The three-domain system of classifying life adds another division: the prokaryotes are divided into two domains of life, the microscopic bacteria and the microscopic archaea, while everything else, the eukaryotes, become the third domain.
  Prokaryotes play important roles in ecosystems as decomposers recycling nutrients. Some prokaryotes are pathogenic, causing disease and even death in plants and animals. Marine prokaryotes are responsible for significant levels of the photosynthesis that occurs in the ocean, as well as significant cycling of carbon and other nutrients.
  Prokaryotes live throughout the biosphere.
context_section_description: The words prokaryote and eukaryote come from the Greek where pro means "before", eu means "well" or "true", and karyon means "nut", "kernel" or "nucleus". So etymologically, prokaryote means "before nucleus" and eukaryote means "true nucleus".
  The division of life forms between prokaryotes and eukaryotes was firmly established by the microbiologists Roger Stanier and C. B. van Niel in their 1962 paper, The concept of a bacterium. One reason for this classification was so what was then often called blue-green algae (now called cyanobacteria) would cease to be classified as plants but grouped with bacteria.
  In 1990 Carl Woese et al.  introduced the three-domain system. The prokaryotes were split into two domains, the archaea and the bacteria, while the eukaryotes become a domain in their own right. The key difference from earlier classifications is the splitting of archaea from bacteria.
  The earliest evidence for life on earth comes from biogenic carbon signatures and stromatolite fossils discovered in 3.7 billion-year-old rocks. In 2015, possible "remains of biotic life" were found in 4.1 billion-year-old rocks. In 2017 putative evidence of possibly the oldest forms of life on Earth was reported in the form of fossilized microorganisms discovered in hydrothermal vent precipitates that may have lived as early as 4.28 billion years ago, not long after the oceans formed 4.4 billion years ago, and not long after the formation of the Earth 4.54 billion years ago.
  Microbial mats of coexisting bacteria and archaea were the dominant form of life in the early Archean Eon and many of the major steps in early evolution are thought to have taken place in this environment. The evolution of photosynthesis around 3.5 Ga resulted in a buildup of its waste product oxygen in the atmosphere, leading to the great oxygenation event beginning around 2.4 Ga. 
  The earliest evidence of eukaryotes dates from 1.85 Ga, and while they may have been present earlier, their diversification accelerated when they started using oxygen in their metabolism. Later, around 1.7 Ga, multicellular organisms began to appear, with differentiated cells performing specialised functions.
  A stream of airborne microorganisms, including prokaryotes, circles the planet above weather systems but below commercial air lanes. Some peripatetic microorganisms are swept up from terrestrial dust storms, but most originate from marine microorganisms in sea spray. In 2018, scientists reported that hundreds of millions of viruses and tens of millions of bacteria are deposited daily on every square meter around the planet.
  Microscopic life undersea is diverse and still poorly understood, such as for the role of viruses in marine ecosystems. Most marine viruses are bacteriophages, which are harmless to plants and animals, but are essential to the regulation of saltwater and freshwater ecosystems. They infect and destroy bacteria and archaea in aquatic microbial communities, and are the most important mechanism of recycling carbon in the marine environment. The organic molecules released from the dead bacterial cells stimulate fresh bacterial and algal growth. Viral activity may also contribute to the biological pump, the process whereby carbon is sequestered in the deep ocean.
caption: Sea spray containing marine microorganisms, including prokaryotes, can be swept high into the atmosphere and may travel the globe before falling back to earth.
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