marine prokaryotes marine prokaryotes 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. flagellum nucleoid region chromosome (dna) ribosome cellmembrane cell wall capsule pili parts of a prokaryotediagram above contains clickable links cellular life forms can be divided into prokaryotes and eukaryotes. prokaryotes are bacteria or archaea, and the diagram shows some (clickable) parts shared by both. but bacteria and archaea also have fundamental differences, as indicated by their placement in different domains. part of a series of overviews onmarine life marine habitats marine microorganisms marine microbiomes marine viruses marine prokaryotes marine protists marine fungi marine invertebrates marine vertebrates marine primary production marine food web marine carbon pump marine biogeochemical cycles human impact on marine life marine conservation marine life portal 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 timeline of prokaryote domination−4500 —–−4000 —–−3500 —–−3000 —–−2500 —–−2000 —–−1500 —–−1000 —–−500 —–0 —no clearevidenceof lifemarineprokaryotescrowneukaryotes ←earth (−4540)←earliestundisputed life←earliest oxygen←atmospheric oxygen←oxygen crisis←sexual reproduction←earliest plants←earliest animals←ediacara biota←cambrian explosion←tetrapods←earliest apesphanerozoicproterozoicarcheanhadeanpongolahuroniancryogenian(million years ago)this timeline contains clickable links marine prokaryotes have been the dominant life form for most of earth's history, perhaps because water protected them from ionizing radiation 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. phylogenetic and symbiogenetic tree of living organisms, showing a view of the origins of eukaryotes and prokaryotes 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. a 2016 metagenomic representation of the tree of life using ribosomal protein sequences. the tree includes 92 named bacterial phyla, 26 archaeal phyla and five eukaryotic supergroups. major lineages are assigned arbitrary colours and named in italics with well-characterized lineage names. lineages lacking an isolated representative are highlighted with non-italicized names and red dots. eukaryote versus prokaryote a view of the phylogenetic tree based on the three-domain system, showing the divergence of modern species from their common ancestor in the centre. the three domains are coloured, with bacteria blue, archaea green and eukaryotes red. 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 range of sizes shown by prokaryotes (bacteria and archaea) and viruses relative to those of other organisms and biomolecules 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. microbial matsmicrobial mats are the earliest form of life on earth for which there is good fossil evidence. the image shows a cyanobacterial-algal mat.stromatolites are formed from microbial mats as microbes slowly move upwards to avoid being smothered by sediment. external video how bad was the great oxidation event? sea spray containing marine microorganisms, including prokaryotes, can be swept high into the atmosphere where they become aeroplankton, and can travel the globe before falling back to earth. 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. estimates of microbial species counts in the three domains of life bacteria are the oldest and most biodiverse group, followed by archaea and fungi (the most recent groups). in 1998, before awareness of the extent of microbial life had gotten underway, robert m. may estimated there were 3 million species of living organisms on the planet. but in 2016, locey and lennon estimated the number of microorganism species could be as high as 1 trillion. 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. marine bacteria different bacteria shapes (cocci, rods and spirochetes) and their sizes compared with the width of a human hair. a few bacteria are comma-shaped (vibrio). archaea have similar shapes, though the archaeon haloquadratum is flat and square. the unit μm is a measurement of length, the micrometer, equal to 1/1,000 of a millimeter pelagibacter ubique of the sar11 clade is the most abundant bacteria in the ocean and plays a major role in the global carbon cycle. scanning electron micrograph of a strain of roseobacter, a widespread and important genus of marine bacteria. for scale, the membrane pore size is 0.2 μm in diameter. vibrio vulnificus, a virulent bacterium found in estuaries and along coastal areas