Domain: Bacteria, Phylum: Actinobacteria, Class: Actinobacteria, Subclass: Actinobacteridae, Order: Actinomycetales, Suborder: Micrococcineae, Family: Microbacteriaceae, Genus: Rhodoluna
Rhodoluna lacicola (Genome), Candidatus Rhodoluna limnophila, Candidatus Rhodoluna planktonica, Rhodoluna sp. KAS3, Rhodoluna sp. KAS7, Rhodoluna sp. KAS8, Rhodoluna sp. KAS9
Strains of Rhodoluna were first discovered in 2002 in mixed cultures (4), submitted as Candidatus in 2009 (5), and later the novel genus was presented as part of the novel species R. lacicola in pure culture of strain MWH-Ta8 (6). Species of the genus Rhodoluna are small (0.05µm^3) freshwater planktonic ultramicrobacteria found worldwide with red pigmentation, selenoid (moon-shaped) morphology, and loose concave colonies (5). These cells are so small that the Gram staining technique is nearly impossible without magnification beyond x1000, which can prevent accurate viewing of color change (6).
Researchers study Rhodoluna because this genus and other Actinobacteria make up a significant but poorly studied proportion of freshwater lake bacterioplankton (11). Although small, their influence should not be underestimated. Rhodoluna is emblematic of the changes in microbiological research before and after the development of high-throughput genetic sequencing technology. Microbiologists have shifted from culture-dependence to culture-independence, which expands their reach in classifying uncultivable microbial species while at the same time sacrificing some taxonomic rigor. Culturing remains vital to identifying and quantifying many traits of a microbial species because although the "Luna 1 lineage" was known for many years, the GC content of Rhodoluna could not be determined (2) until its pure culture (6,3). Candidatus status is a helpful way to use well-documented genetic determinants of relatedness and simpler morphotypes in place of full organismal characterizations before a pure culture can be maintained (8,9). Phospholipid analysis is one method that has been used on Candidatus species of Rhodoluna (10), but it remains only a metric of distinguishing major bacterial groups without species- or genus-level specificity.
Genome and Metabolism
Rhodoluna species have one chromosome with a sequence length of 1,430,433 bp and GC content of 51.5 mol%. 16S rRNA and RpoB genes suggest Rhodoluna it is in the Microbacteriaceae family (1) and its closest relative is Pontimonas salivibrio. Because of its streamlined genome, Rhodoluna is largely dependent on microbial satellites (7), but the genome of R. lacicola has several unique traits, including the presence of a phosphotransferase system for fructose but not glucose, a complete citrate cycle, several ABC transporters, a complete respiratory electron transport chain, as well as . Missing from the genome was nitrite and nitrate reductases as well as any machinery for flagellum synthesis (6).
1. Aizenberg-Gershtein Y, Vaizel-Ohayon D, Halpern M. 2012. Structure of bacterial communities in diverse freshwater habitats. Can J Microbiol 58:326–335.
2. Ghai R, Mcmahon KD, Rodriguez-Valera F. 2012. Breaking a paradigm: Cosmopolitan and abundant freshwater actinobacteria are low GC. Environ Microbiol Rep 4:29–35.
3. Ghai R, Mizuno CM, Picazo A, Camacho A, Rodriguez-Valera F. 2014. Key roles for freshwater Actinobacteria revealed by deep metagenomic sequencing. Mol Ecol 23:6073–6090.
4. Hahn MW, Lünsdorf H, Wu Q, Höfle MG, Boenigk J, Stadler P, Lu H, Schauer M, Ho MG. 2003. Isolation of Novel Ultramicrobacteria Classified as Actinobacteria from Five Freshwater Habitats in Europe and Asia. Appl Environ Microbiol 69:1442–1451.
5. Hahn MW. 2009. Description of seven candidate species affiliated with the phylum Actinobacteria, representing planktonic freshwater bacteria. Int J Syst Evol Microbiol 59:112–117.
6. Hahn M, Schmidt J, Taipale SJ, Doolittle WF, Koll U. 2014. Rhodoluna lacicola gen. nov., sp. nov., a planktonic freshwater bacterium with stream-lined genome. Int J Syst Evol Microbiol 3254–3263.
7. Kulichevskaya IS, Belova SE, Komov VT, Dedysh SN, Zavarzin G a. 2011. Phylogenetic composition of bacterial communities in small boreal lakes and ombrotrophic bogs of the upper Volga basin. Microbiology 80:549–557.
8. Murray RG, Schleifer KH. 1994. Taxonomic notes: a proposal for recording the properties of putative taxa of procaryotes. Int J Syst Bacteriol 44:174–176.
9. Murray RG, Stackebrandt E. 1995. Taxonomic note: implementation of the provisional status Candidatus for incompletely described procaryotes. Int J Syst Bacteriol 45:186–187.
10. Taipale S, Kankaala P, Hahn M, Jones R, Tiirola M. 2011. Methane-oxidizing and photoautotrophic bacteria are major producers in a humic lake with a large anoxic hypolimnion. Aquat Microb Ecol 64:81–95.
11. Glockner, F. O., E. Zaichikov, N. Belkova, L. Denissova, J. Pernthaler, A. Pernthaler, and R. Amann. 2000. Comparative 16S rRNA analysis of lake bacterioplankton reveals globally distributed phylogenetic clusters including an abundant group of actinobacteria. Appl Environ Microbiol 66:5053– 5065.
Page authored by Michael Braus, student of Prof. Katherine Mcmahon at University of Wisconsin - Madison.