Genus of bacteria
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Polaribacter is a genus in the family "Flavobacteriaceae". They are gram-negative, aerobic bacteria that can be heterotrophic, psychrophilic or mesophilic. Most species are non-motile and species range from ovoid to rod-shaped. "Polaribacter" forms yellow- to orange-pigmented colonies. They have been mostly adapted to cool marine ecosystems, and their optimal growth range is at a temperature between 10 and 32 °C and at a pH of 7.0 to 8.0. They are oxidase and catalase-positive and are able to grow using carbohydrates, amino acids, and organic acids.
There is evidence of two life strategies for members of the genus, "Polaribacter". Some "Polaribacter" species are free-living and consume amino acids and carbohydrates, as well as have proteorhodopsin that enhances living in oligotrophic seawaters. Other species of "Polaribacter" attach to substrates in search of protein polymers.
In the context of climate change, algal blooms are becoming increasingly prevalent. Members of the genus "Polaribacter" decompose algal cells and thus may be important in biogeochemical cycling, as well as influence seawater chemistry and the composition of microbial communities as temperatures continue to rise. This may impact the efficiency of the biological pump in sequestering atmospheric carbon.
"Polaribacter" is a genus that is being continuously researched and to date there are 25 species that have been validly published under the International Code of Nomenclature of Prokaryotes (ICNP): "P. aquimarinus, P. atrinae," "P. butkevichii", "P. dokdonensis", "P. filamentus", "P. franzmannii", "P. gangjinensis, P. glomeratus", "P. haliotis, P. huanghezhanensis, P. insulae, P. irgensii, P. lacunae, P. litorisediminis, P. marinaquae, P. marinivivus, P. pacificus, P. porphyrae, P. reichenbachii, P. sejongensis, P. septentrionalilitoris, P. staleyi, P. tangerinus, P. undariae, P. vadi."
The genus is sometimes incorrectly referred to as "Polaribacer"; "Polarobacter" or "Polaribacteria".
Phylogeny.
This phylogeny is based on rRNA gene sequencing.
Distribution and abundance.
Members in the genus "Polaribacter" are abundant in polar oceans and are important in the export of dissolved organic matter (DOM). A small percentage of the bacterial community is responsible for the DOM uptake rate.
In northern latitude waters, the fraction of cells using glucose (fraction of active cells) is higher in summer than winter, and high abundances may occur after phytoplankton blooms, although a study in southern high-latitude waters found lower abundances of "Polaribacter" after an "in situ" diatom bloom.
Within the Arctic Ocean, there is no obvious pattern in the relative abundance between summer and winter. In the Chukchi Sea, the fraction of cells using leucine is higher in the winter than in summer. In the Beaufort Sea, the fraction of cells using leucine does not differ between seasons. In the coastal waters of Fildes Peninsula, "Polaribacter" dominated cells in the phylum Bacteriodetes.
Habitat.
Microorganisms in the genus "Polaribacter" are widely distributed and various species are capable of living in a plethora of different environments. Some "Polaribacter" species have been isolated from brine pools in the Arctic Ocean. in addition to hypersaline environments, numerous "Polaribacter" species inhabit extreme environments ranging from -20 °C to 22 °C. In the past, it was thought that "Polaribacter" only flourished in cold waters as the members of the species that were first discovered ("P. irgensii, P. filamentus," and "P. franzmannii") in the Arctic and Southern Oceans could only survive in water with temperatures ranging from -20 °C to 10 °C. Subsequently, members of the genus "Polaribacter" have been shown to be very versatile microorganisms and can survive in oligotrophic and in copiotrophic environments. "Polaribacter" have also been found in sediments. For example, SM1202T, a phylogenetically close strain to "Polaribacter" was isolated from marine sediment in Kongsfjorden, Svalbard. "Polaribacter" have also been experimentally isolated from red macroalgae ("Porphyra yezoensis) a"nd green macroalgae ("Ulva fenestrate)".
Role in ecosystem.
Isolates of related Flavobacteria are able to degrade High-Molecular Weight (HMW) DOM. and "Polaribacter" may be among the first organisms to degrade particulate organic matter and break-down polymers into smaller particles that can be used by free-living bacterial heterotrophs. This suggests that they likely remineralize primary production matter within the food web.
In the Southern Ocean.
The Antarctic Peninsula exhibits strong seasonal changes, which influences how bacteria respond to and live in these environmental conditions. The Antarctic spring is especially important as it brings about significant changes, including sea ice melting, thermal stratification due to warming surface waters, and increased dissolved organic matter (DOM) production. All these physical changes also result in phytoplankton blooms which are important in supporting higher trophic levels.
In the Southern Ocean, flavobacteria dominate bacterial activity, particularly flavobacteria in the genus "Polaribacter". Typically, these bacteria are prevalent in sea ice; however, during seasonal melting in the summer, they dominate coastal waters as sea ice retreats. In the Southern Ocean, when phytoplankton blooms occur, Flavobacteria, and particularly members in the genus "Polaribacter," are among the first bacterial taxa to respond to phytoplankton blooms, breaking down organic matter by direct attachment and the use of exoenzymes. Both particle-attached and free-living members of the family Rhodobacteraceae were also found in close association with phytoplankton blooms; however, bacteria in this family were found to use lower molecular weight substrates. This suggests that they're secondary in the microbial succession of substrates, using the byproducts of degradation by flavobacteria, which also includes members of the genus "Polaribacter". The relative abundance of free-living bacteria belonging to the genus "Polaribacter" and in the family Rhodobacteraceae peaked at different points during phytoplankton blooms, suggesting a niche specialization contributing to successive degradation of phytoplankton-derived organic matter. Bacteria in the genus "Polaribacter" and family Rhodobacteraceae were found in clusters, with "Polaribacter" clusters forming earlier in the bloom, which further suggests a successive ecological interaction between various bacterial taxa.
For both the Arctic Ocean and the North Sea, "Polaribacter" exhibited similar trends pertaining to phytoplankton blooms in the summertime as well as assuming particular niches for organic matter degradation.
Metabolism.
Members of the genus "Polaribacter" are metabolically flexible depending on their physiology, lifestyle and seasonality of the region they inhabit. Many research studies have found that "Polaribacter" can alternate between two lifestyles as a mechanism for adaptation in surface waters where nutrient concentrations are low and light exposure is high. Sequenced strains of the genus "Polaribacter" show a high prevalence of peptidase and glycoside hydrolase genes in comparison to other bacteria in the Flavobacteriaceae, indicating they contribute to degradation and uptake of external proteins and oligopeptides.
In the pelagic water column, some species are well equipped to attach to particles and substrates to search for and degrade polymers. They are amongst the first organisms to degrade particulate organic matter and break-down polymers into smaller particles. Studies have shown that they will colonize and attach to particles, glide to search for substrates, and degrade them for carbon and nutrients. Once they've degraded these molecules, the bacterium may then search for new particles to colonize, forcing them to freely-swim in environments where nutrients and organic carbon is not easily available.
CAZymes.
Genetic sequencing found that strains contain numerous genes which encode for CAZymes that are involved in polysaccharide degradation. For example, strain DSW-5 (a strain genetically very similar to strain MED-152), contains 85 genes encoding to CAZymes and 203 peptidases, which suggests its role as a free-living heterotrophs. However, the ratio of peptidases to glycoside hydrolase genes varies depending on the environmental conditions the strain is subjected to. For example, "Polaribacter" sp. MED134 lives in environmental conditions with extended starvation conditions and expresses twice as many peptidases as CAZymes. On the other hand, macroalgae-colonizing species that live in stable, eutrophic environments may express greater proportions of CAZymes than peptidases.
Proteorhodopsin.
"Free-living" species have the proteorhodopsin gene, which allows them to complete inorganic-carbon fixation using light as an energy source. By utilizing their proteorhodopsin to use light energy, "Polaribacter" can grow in oligotrophic environmental conditions.
Genome.
General genome characteristics.
The genome of bacteria in the genus "Polaribacter" vary in size from 2.76 Mb ("P. irgensii") to 4.10 Mb ("P. reichenbachii") and the number of genes ranging from 2446 in "P. irgensii" to 3500 in "P. reichenbachii", but have a fairly constant G+C content of approximately 30 mol%. Some notable features of the genome include genes for agar, alginate, and carrageenan degrading enzymes in "Polaribacter" species which colonize the surface of macroalgae. Agar degrading enzymes have also been found in strains of "Polaribacter" that colonize the gut of the comb pen shell. Proteases are also commonly found in the genomes of species that preferentially grow on solid substrates and degrade protein instead of using free amino acids and living a pelagic lifestyle. Some members of the genus encode proteorhodopsin, which has been implicated in supporting their central metabolism through photophosphorylation.
DNA sequencing of "Polaribacter".
DNA sequencing has commonly been used to identify new strains of "Polaribacter" and help place species on a phylogenetic tree. DNA sequencing has also been used to help understand, or predict a species role in an environment due to the presence of certain genes. Members of the family "Flavobacteriaceae" can be identified through the specific quinone, Menaquinone 6, also known as Vitamin K2; however, differentiating species can be much more difficult. Species such as "Polaribacter vadi" and "Polaribacter atrinae" were identified as new species based on their similar but unique genome when compared to other members of the genus "Polaribacter". New species can be identified through DNA hybridization or through the sequencing and comparison of a common gene such as 16S rRNA. This has allowed scientists to create phylogenetic trees of the genus based on genomic similarity, as seen in the phylogeny section, as well as identify common features in the genome.
Life strategies of "Polaribacter" based on genome analysis.
Genomic analysis has allowed scientists to examine the relationships between different species of "Polaribacter". However, by combining genomic analysis with other analytical techniques such as chemotaxonomic and biochemical, scientists can theorize how a species might fit into an environment or how they believe a species is adapted to survive. A genomic analysis of the "Polaribacter" strain MED152, found a considerable amount of genes that allow for surface or particle attachment, gliding motility and polymer degradation. These genes fit with the current understanding of how marine bacteroidetes survive through attaching to a surface and moving over it to look for nutrients. However the researchers also noticed that the organism had a proteorhodopsin gene as well as other genes which could be used to sense light and found that under light the species increased carbon dioxide fixation. This led the researchers to theorize that "Polaribacter" strain MED152 has two different life strategies, one where it acts like other marine bacteroidetes, attaching to surfaces and searching for nutrients and, another life strategy where, if the strain was in a well lit, low nutrient area of the ocean, it would use carbon fixation to synthesize intermediates of metabolic pathways.
Another example of this comes from the "Polaribacter" strains Hel1_33_49 and Hel1_85. The strain Hel1_33_49 has a genome which contains proteorhodopsin, fewer polysaccharide utilization loci and no mannitol dehydrogenase, which the researchers associate with a pelagic lifestyle. Hel1_85 on the other hand, has a genome which contains twice as many polysaccharide utilization loci, a mannitol dehydrogenase and no proteorhodopsin, pointing to a lifestyle with lower oxygen availability such as a biofilm.
Viral pathogens.
Only two species of lytic phage are known to infect members of this genus, and both have double stranded DNA with virions that include isometric heads and non-contractile tails (class "Caudoviricetes", morphotype: siphoviruses). Viral lysis has been implicated as a major driver of changes in genus-level composition of microbial communities.
Applications/uses.
Cold water enzymes contained in psychrophilic bacteria like "Polaribacter" are valuable for biotechnology applications since they do not require high temperatures that may other enzyme systems do.
Psychrophilic enzymes.
"Polaribacter" is a psychrophilic bacterium that lends itself to a variety of applications in both academic and industrial settings. These cold dwelling bacteria are an abundant source of psychrophilic enzymes which have an interesting ability to retain higher catalytic activity at temperatures below 25 °C. This is due to the highly malleable nature of these enzymes as this allows for better substrate - active site binding at colder temperatures. This is important as enzymes that operate at lower temperatures not only make the industrial processes more efficient, but they also minimize the chance of side reactions occurring. More of the substrate can directly be converted into the desired product all the while requiring less energy to do so. Psychrophilic enzymes can also aid with heat labile or volatile compounds, allowing reactions to occur without significant product loss. Another unique application for these enzymes is the ability to be inhibited without