Higher order taxa
Bacteria; Actinobacteria; Actinobacteria; Actinomycetales; Micrococcaceae1
Description and significance
A. rhombi are short and rod-shaped or spherical, much like a coryneform morphology 2. Two strains, CCUG 38812 and CCUG 38813, were isolated from abdominal organs of Greenland halibut from the Flemish Cap, Newfoundland, caught at depths of 300-800 meters and water temperature of 3.5- 4.0°C 2.
It is important because it contains enzymes that are efficient at reducing hexavalent chromium [Cr(VI)] to trivalent chromium, Cr(III) 3. Cr(VI) is carcinogenic and highly toxic; it is released into the environment as a byproduct of many industries including production of dyes, petroleum refining, wood preservation, and leather tanning3. Cr(III) is more stable and much less toxic than Cr(III)3.
It also is capable of growth in the presence of heavy metals 4. This is significant because it may give insight into using them for bioremediation of soils contaminated with heavy metals5. Soil can become contaminated with heavy metals through fertilizers, pesticides, burning of fossil fuels, batteries, and sewage6.
The genomes for these two strains have not been sequenced. However, the 16S rRNA genes for both strains, CCUG 38812 and CCUG 38813, have been sequenced and logged in GenBank2. The accession numbers are Y15885 and Y15884, respectively2. These genes are linear and 1423 base pairs long7.
Cell and colony structure
A. rhombi is a Gram-positive obligate aerobe2. It is not spore-forming and non-motile2. Cells are short and rod-shaped or spherical, much like coryneform morphology2. A. rhombi has an A4α peptidoglycan type L-Lys-L-Ala-D-Glu in its cell wall 2. Colonies are round, convex, and yellowish or whitish in color2.
A. rhombi grows on brain heart infusion agar2. It is a strict aerobe that has been found to use glycerol, galactose, D-glucose, D-fructose, D-mannose, mannitol, amygdalin, arbutin, aesculin, salicin, cellubiose, maltose, lactose, melibiose, sucrose, trehalose, β-gentiobiose, D-turanose, gluconate, and D-arabitol as sole carbon sources2. It cannot use erythritol, D-arabinose, L-arabinose, ribose, D-xyose, adonitol, methyl β-xyloside, L-sorbose, dulcitol, sorbitol, methyl α-D-glucoside, insitol, N-acetyl-β-glucosamine, inulin, melezitose, D-raffinose, starch, glycogen, xylitol, D-lyxose, D-tagatose, D-fucose, L-fucose, L-arabitol, 2-ketogluconate, and 5-ketolgluconate as sole carbon sources2.
Growth still occurs with arsenate, cadmium, chromium, zinc, mercury, lead, cobalt, copper, and nickel in its growth medium4. It may be a hyperaccumulator, an organism that can accumulate and tolerate high levels of heavy metals6.
It is catalase and oxidase positive2.
A. rhombi produces the enzyme chromium reductase, which is efficient at reducing the carcinogenic and toxic Cr(VI)3. Cr(VI) is produced during the manufacturing of dyes, leather tanning, and wood preservation3. This enzyme reduces Cr(VI) to a more stable and less toxic form of chromium, Cr(III)3. The reduction of Cr(VI) occurs through a soluble NADH-dependent enzyme3.
Sodium pyruvate, NADH, and propionic acid were found to be effective electron donors for enhanced chromium reductase activity3. The addition of Ca2+ enhanced enzyme activity, while addition of Hg2+, Cd2+, Ba2+, and Zn2+ inhibited enzyme activity3.
A. rhombi can grow in the presence of some heavy metals4. This may give insight into a way to degrade artificially heavy metal-contaminated soil5. Some soils naturally contain heavy metals, namely serpentine soils4. Soils can be contaminated through fertilizers, pesticides, burning of fossil fuels, and sewage6.
A. rhombi was found in abdominal organs of Greenland Halibut living at depths of 300-800 meters, in water 3.5°C-4.0°C2. It can grow at 4°C and 30°C, in both 1% and 10% NaCl2. It produces an enzyme, chromium reductase, which can be used to reduce hexavalent chromium to Cr(III)3. It also can grow in the presence of some heavy metals5. Metagenomic Data Link.
A. rhombi is not pathogenic to any organisms.
1 Arthrobacter rhombi.(n.d.) Retrieved Feb. 9, 2013, from http://www.ncbi.nlm.nih.gov/Taxonomy/Browser/wwwtax.cgi?id-71253
2 Orsorio, Carlos R., Barja, Juan L., Huston, Roger A., and Collins, Matthew D. (1999). Arthrobacter rhombi sp. nov., isolated from Greenland halibut (Reinhardtius hippoglossoides). International Journal of Systematic Bacteriology, 49, 1217-1220. doi: 10.1099/00207713-49-3-1217
3 Elanvogvan, R., Philip, Ligy, and Chandraraj, K. (2010). Hexavalent chromium reduction by free and immobilized cell-free extract of Arthrobacter rhombi-RE. Applied Biochemistry and Biotechnology, 160(1), 81-97. doi: 10.1007/s12010-008-8515-6.
<sup.4 Abou-Shanab, R.A.I., van Berkum, P., and Angle, J.S. (2007). Heavy metal resistance and genotypic analysis of metal resistance genes in gram-positive and gram-negative bacteria present in Ni-rich serpentine soil and in the rhizosphere of Alyssum murale. Chemosphere, 68(2), 360-367.
5 Rajkumar, M., Ma, Y., and Freitas, H. (2008). Characterization of metal-resistant plant-growth promoting Bacillus weihenstephanensis isolated from serpentine soil in Portugal. Journal of basic microbiology. 48(6) 500-508.
6 Rajkumar, M., Prasad, M., Freitas, H., and Ae, N. (2009). Biotechnological applications of serpentine soil bacteria for phytoremediation of trace metals. Critical Reviews in Biotechnology. 29(2). 120-130. doi: 10.1080/07388550902913772
7 Rodriguez-Osorio, C. (1999). Arthrobacter rhombi 16S rRNA gene, strain F58.3CB57. GenBank Y15884.
8 Garbisu, C., Alkorta, I., Llama, MJ., and Serra, JL. (1998). Aerobic chromate reduction by Bacillus subtilis. Biodegradation. 9. 133-141.
Edited by Beatrice Byrne of Dr. Lisa R. Moore, University of Southern Maine, Department of Biological Sciences, http://www.usm.maine.edu/bio