Genus of bacteria
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Dokdonia is a genus of bacteria in the family "Flavobacteriaceae" and phylum Bacteroidota.
The genus "Dokdonia" was first described in 2005 by Yoon et al. near Liancourt Rocks in the Sea of Japan. "Dokdonia" is named after Dokdo, the Korean name for the Liancourt Rocks which sovereignty is disputed between Japan and South Korea. Yoon et al. isolated the bacterium from seawater and identified the first species as "Dokdonia donghaensis."
There are 10 classified species ("D. aurantiaca, D. diaphoros, D. donghaensis, D. eikasta, D. flava, D. genika, D. lutea, D. pacifica, D. ponticola", and "D. sinensis") and many unclassified strains under the "Dokdonia" genus based on the NCBI taxonomy database. The International Committee on Systematics of Prokaryotes (ICSP) currently recognizes nine groups of "Dokdonia" described to species level with "D. ponticola" considered not validly published.
The general characteristics of "Dokdonia" species include gram-negative, non-motile, aerobic, catalase- and oxidase-positive, non-spore-forming rods or elongated rods. Species are usually considered relatively halophilic as they are cultivated optimally with 2% w/v sea salts (NaCl).
Ecology and significance.
"Dokdonia" species have a relatively wide distribution in the water column. They have been isolated from surface seawater, marine sediment, and seaweed. "Dokdonia" have been found across a wide range of marine environments around Korea, China, and Japan, but have also been found in Baltic and Mediterranean waters. They tend to live a planktonic lifestyle, drifting in the water column, but they can occupy a wide range of ecological niches.
"Dokdonia" cells are primarily heterotrophic and sustain off dissolved organic carbon in the water column. It has also been shown that phototrophy can be induced in some strains under laboratory conditions, implying that bacteria in the genus "Dokdonia" are not obligate heterotrophs but are mixotrophic. This shift in carbon source is induced by lower levels and quality of carbon source as well as lower light levels. As a planktonic mixotrophic microbe, "Dokdonia" cells can provide a source of organic matter and carbon for higher trophic level organisms, contribute to the ocean's primary productivity, and also play an important role in transforming elements and nutrient cycling.
Bacteria in the genus "Dokdonia" has been seen to congregate in biofilms with other bacterial species, collectively improving their resistance to bacterial predation and producing antimicrobial agents.
Among the "Flavobacteriaceae", "Dokdonia" have a high requirement for copper in order maintain regular growth and metabolism. Most information known about the genus is from strains of the type species, "Dokdonia donghanensis". As research and cultivation "Dokdonia spp." continues, insights into their diversity and adaptations contribute greatly to public knowledge of marine bacteria as a whole.
Genome.
Nine species of "Dokdonia" have been added to core genomic databases such as Uniprot and Genbank but not all have undergone formal review.
Genome properties of "D. donghaensis" DSW-1T.
The complete genome sequence of "D. donghaensis DSW-1T" can be accessed from GenBank under the accession number CP015125. The complete circular genome contains 3,923,666 base pairs, 55 RNA genes, and 2,881 protein genes. The sequencing was established using the PacBio sequencing platform and funded by the National Research Foundation of Korea.
Genome properties of "Dokdonia" sp. strain MED134.
Whole-genome sequencing of "Dokdonia" sp. strain MED134 was done by J.Craig Venter Institute using Sanger method. The genome size of "Dokdonia" sp. strain MED134 is 3,301,953 bp which is relatively small compared to other Bacteroidota"." The number of conserved genes is similar to other Bacteroidota and it contains 170 core genes for bacteria. Genome analysis shows that proteorhodopsins(PR)-containing marine Bacteroidota contains significantly fewer paralogous genes which form through intragenomic duplication events. This result suggests that phototrophic Bacteroidota have similar number of gene families but less genes for each paralogous gene family which results in a reduced genome size. This finding is consistent among phototrophic bacteria, suggesting that Bacteroidota have evolved to retain only select genes from each paralogous gene family while also establishing other genetic features common among PR-containing marine Bacteroidota.
Several key genes involved in PR-phototrophy have been identified through further genomic analysis of "Dokdonia" sp. strain MED134 and comparison to heterotrophic marine bacteria. "Dokdonia" have a relatively high abundance of peptidases and greater proportion of peptidases to total proteins compared to other bacteria. This property may indicate that the degradation of peptides may be the main carbon and nitrogen source instead of polysaccharides. Phylogenetic analysis of features like conjugative transposons genes highlight horizontal gene transfer events with other marine flavobacteria. Successful incorporation of PR genes into the genome has been seen to contribute to the overall fitness and survival of bacteria in the marine environment.
Metabolism.
Metabolic processes of "Dokdonia".
"Dokdonia" species are known to be chemoheterotrophs, acquiring energy from organic molecules. They show maximal growth in conditions with a high concentration of dissolved organic matter. "Dokdonia" and other members of the Bacteroidota phylum are important in the degradation of organic materials, especially during sporadic nutrient increase. They have a mechanism that allows them to attach and degrade polymeric substances efficiently.
Some strains of "Dokdonia" are facultative double mixotrophs as they can utilize both heterotrophic and phototrophic metabolism. These strains contain proteorhodopsins (PRs) which act as light-dependent proton pumps. PR has a simple structure compared to other light-harvesting molecules like chlorophyll. They are a single membrane protein with retinal as its prosthetic group. Retinal is a polyene chromophore (light-sensitive pigment) which acts as the light-absorbing molecule in "Dokdonia" species. PRs allow cells to harvest energy from sunlight. When exposing to light, PRs pump protons across membrane and build proton gradient that can be used to generate ATP that powers various cellular activities. The energy generated by PRs can support cell growth, degradation of complex and recalcitrant organic matter, and allow cells to uptake amino acids and peptides at lower concentrations.
"Dokdonia" sp. strain MED134 ("Dokdonia donghaensis" MED134).
This strain uses aerobic heterotrophic metabolism; it primarily uses amino acids as carbon and nitrogen sources via expression of peptidases which break peptides down into amino acids. "Dokdonia" sp. strain MED134 has a Na+ ion pump on its membrane that can generate Na+ gradient to produce ATP for other cellular activities.
"Dokdonia" sp. strain MED134 has complete Embden-Meyerhof-Farnas pathway (glycolysis), gluconeogenesis pathway, and tricarboxylic acid (TCA) cycle. The anaplerotic reaction which connects glycolysis with TCA cycle utilizes several enzymes including PEP carboxykinase and PEP carboxylase.
The PRs in "Dokdonia" sp. strain MED134 are predicted to be heptahelical integral membrane proteins that pump H+ across membranes to build a proton gradient which generates ATP. Depends on the organism's depth in water column, the PR's maximum absorbance wavelength changes. In near surface waters, the maximum absorption wavelength of PR is around 530 nm (green light). In deeper water, the maximum absorption wavelength is 490 nm (blue light).
"Dokdonia" sp. PRO95.
"Dokdonia" sp. PRO95 carries two types of rhodopsins: PR as well as a rhodopsin that is related to xanthorhodopsins (XRs). XRs are light-harvesting proton pumps that are more efficient compared to PRs. The PRs of this strain are light-driven sodium-motive pumps (Na+-rhodopsins or NaRs) which pump Na+ from the cytoplasm to external medium. They can also pump H+ when Na+ is absent. Unlike "Dokdonia" sp. strain MED134, no light-induced growth is observed in "Dokdonia" sp. PRO95 despite they have high genome similarity.
Ecological significance of proteorhodopsin-containing "Dokdonia".
The presence of PRs allows "Dokdonia" to grow better in light compared dark conditions, especially when there is low or intermediate levels of nutrients available. Cells with PRs are less dependent on the amount and quality of organic carbon sources. Being able to harvest energy from light gives them an advantage during nutrient deficient periods, making the retention of PRs favoured by selection.
Alpha- and gammaproteobacteria that contain PRs have enhanced cell growth due to cellular functions such as survival in environments with deficient nutrients that are promoted by light . The correlation between PR phototrophy and increased cell growth was first observed in "Dokdonia" sp. strain MED134.
Expression of proteorhodopsin encoding gene.
In "Dokdonia", light induces the expression of PR genes. There is a significant increase in the expression levels of PR genes in light compared to dark conditions as light can increase the strength of the PR gene promoter. The light-dark cycle can induce the upregulation of PR genes in Flavobacteriia that lead to population growth.
Behavior.
Biofilm.
In the Tyrrhenian Sea off the coast of Naples, species of "Dokdonia" are the most abundant in biofilms on plastic debris. 4.76 ± 7.1% of genera forming biofilms belong to "Dokdonia."
Given that "Dokdonia" was found on plastics but not in the sediment or water column, it is possible that the habitat in biofilms on plastics is preferable to planktonic growth for species of "Dokdonia" in the Mediterranean Sea. As microbial mortality is reduced in the more stable microenvironment of a biofilm, population maintenance is less dependent on environmental factors such as nutrient concentration and grazers.
Response to varying nutrient availability.
Due to variable expression of PR, "Dokdonia" sp. strain MED134 growth rate is more positively influenced by light exposure when metabolizing an energy-poor carbon source such as alanine compared to that which was observed in the presence of glucose.
Growth rates of "Dokdonia" sp. Dokd-P16 are significantly affected by variations in dissolved copper concentration. The strain exhibited an 80% reduction in growth rates in 0.6 nM copper compared to those observed in 50 nM copper. This suggests that the presence of copper is crucial to "Dokdonia" metabolism.
Iron is a common limiting nutrient in the ocean. "Dokdonia" sp. strain MED134 has various mechanisms make use of different forms of iron. It contains ATP-binding cassette-type transport system that can transport iron into the cell. Also, it can store iron and recycle iron from heme.
References.
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