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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. In 2018 it was estimated the total biomass of all prokaryotes on the planet was equivalent to 77 billion tonnes of carbon (77 Gt C). This is made up of 7 Gt C for archaea and 70 Gt C for bacteria. These figures can be contrasted with the estimate for the total biomass for animals on the planet, which is about 2 Gt C, and the total biomass of humans, which is 0.06 Gt C. This means archaea collectively have over 100 times the collective biomass of humans, and bacteria over 1000 times. There is no clear evidence of life on Earth during the first 600 million years of its existence. When life did arrive, it was dominated for 3,200 million years by the marine prokaryotes. More complex life, in the form of crown eukaryotes, didn't appear until the Cambrian explosion a mere 500 million years ago. Evolution The Earth is about 4.54 billion years old. The earliest undisputed evidence of life on Earth dates from at least 3.5 billion years ago, during the Eoarchean Era after a geological crust started to solidify following the earlier molten Hadean Eon. Microbial mat fossils have been found in 3.48 billion-year-old sandstone in Western Australia. Past species have also left records of their evolutionary history. Fossils, along with the comparative anatomy of present-day organisms, constitute the morphological, or anatomical, record. By comparing the anatomies of both modern and extinct species, paleontologists can infer the lineages of those species. However, this approach is most successful for organisms that had hard body parts, such as shells, bones or teeth. Further, as prokaryotes such as bacteria and archaea share a limited set of common morphologies, their fossils do not provide information on their ancestry. Prokaryotes inhabited the Earth from approximately 3–4 billion years ago. No obvious changes in morphology or cellular organisation occurred in these organisms over the next few billion years. The eukaryotic cells emerged between 1.6 and 2.7 billion years ago. The next major change in cell structure came when bacteria were engulfed by eukaryotic cells, in a cooperative association called endosymbiosis. The engulfed bacteria and the host cell then underwent coevolution, with the bacteria evolving into either mitochondria or hydrogenosomes. Another engulfment of cyanobacterial-like organisms led to the formation of chloroplasts in algae and plants. The history of life was that of the unicellular prokaryotes and eukaryotes until about 610 million years ago when multicellular organisms began to appear in the oceans in the Ediacaran period. The evolution of multicellularity occurred in multiple independent events, in organisms as diverse as sponges, brown algae, cyanobacteria, slime moulds and myxobacteria. In 2016 scientists reported that, about 800 million years ago, a minor genetic change in a single molecule called GK-PID may have allowed organisms to go from a single cell organism to one of many cells. Soon after the emergence of these first multicellular organisms, a remarkable amount of biological diversity appeared over a span of about 10 million years, in an event called the Cambrian explosion. Here, the majority of types of modern animals appeared in the fossil record, as well as unique lineages that subsequently became extinct. Various triggers for the Cambrian explosion have been proposed, including the accumulation of oxygen in the atmosphere from photosynthesis. Background 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 .... Discover the Rj Scott V L Locey popular books. Find the top 100 most popular Rj Scott V L Locey books.