Pelagibacterales (SAR11)

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Classification

Domain: Bacteria

FIGURE 1. Scanning electron microscope image of SAR11 (1).

Phylum: Proteobacteria

Class: Alphaproteobacteria

Subclass: Rickettsidae

Order: Pelagibacterales


Description and Significance

Pelagibacterales (SAR11) is an order in the Alphaproteobacteria composed of free-living heterotrophic bacteria. (4)(5) They are most abundant group of planktonic cells in marine systems and possibly the most numerous bacterium in the world. Typically accounting for ~25% of prokaryotic cells in seawater worldwide.(3) The order was originally named SAR11 following its discovery in the Sargasso Sea in 1990 by Professor Stephen Giovannoni and colleagues, from Oregon State University.(2)(5) It was first placed in the order of Rickettsiales, but after rRNA gene-based phyogenetic analysis in 2013, it was raised to the rank of order, and then placed as sister order to the Rickettsiales in the subclass Rickettsidae.(5)

It is rod or crescent shaped and one of the smallest free cells known, with a length of 0.37-0.89 µm and a diameter of only 0.12-0.20 µm. 30% of the cell's volume is taken up by its genome.[5] It is gram negative.[6] It recycles dissolved organic carbon. It undergoes regular seasonal cycles in abundance - in summer reaching ~50% of the cells in the temperate ocean surface waters. Thus it plays a major role in the Earth's carbon cycle.

Genome Structure

An industrial standard strain of O. oeni, PSU-1, has a circular genome of 1780517 basepair nucleotides, 1691 protein-coding genes, and 51 RNA genes (7). 43 tRNA sequences that represent 20 amino acids and 14 different insertion sequence transposase genes are present. Comparing different strains of O. oeni genomes, the general number of nucleotides tends to be very similar across the specie (7). The critical malolactic fermentation operates from the mleA gene, coding for the malolactic enzyme that breaks down the malic acid in the environment. Morphological and genomic evidence established the grounds to reclassify this bacteria as Oenococcus oeni.

FIGURE 3. Genome of Oenococcus oeni PSU-1. Inner most circle shows ORF classification: (1) Information storage, (2) cellular processes, (3) metabolism, (4) poorly characterized, (5) uncharacterized. The next circle outwards shows ORF orientation. Next circle next shows tRNA (green) and rRNA (blue). The red dots are transposase genes. The green (low) and orange (high) bands show deviation in GC content. (8).

Cell Structure, Metabolism and Life Cycle

Oenococcus oeni is a facultative anaerobe. It is able to use oxygen for cellular respiration but can also gain energy through fermentation. It characteristically grows well in the environments of wine, being able to survive in acidic conditions below pH 3.0 and tolerant of ethanol levels above 10% (2). Optimal growth occurs on sugar and protein rich media, like grape or tomato juice. The cocci are ellipsoidal to spherical in shape, usually grow in chains or pairs, and are typically non-sporulating. Lactic acid bacteria, like Oenococcus oeni, perform malolactic fermentation (also known as malolatic conversion). It occurs after (or sometimes during) primary fermentation. The main function of malolatic fermentation is converting glucose and malic acid to lactic acid. This is occurs by the uptake of malate, decarboxylation of malate to L-lactic acid and carbon dioxide, and the export of end products. O. oeni is heterofermentative, meaning it can create multiple end products from fermenting the sugars. In O. oeni’s case, it produces carbon dioxide, ethanol, and acetate, as well as characteristic flavor molecules like diacetyl. Strain variation of Oenoccocus oeni cellular processes can have significant effects on the community dynamics, fermentation, and overall quality of wine. Strain variation exists in sugar utilization pathways, phosphototransferase enzyme II systems, bacteriophage integration, and cell wall exopolysaccharides (3).


Ecology

Oenococcus oeni stabilizes wine communities by consuming available nutrients and lowering potential growth of other microbes, but its malolactic fermentation can be beneficial or detrimental to the production of wine depending on grapes, climate, and style of wine. Variations between strains and fermentation conditions have the potential to impact general quality and production of wine. Industrial winemakers use a standardized strain of O. oeni, but the many external and environmental variables will dictate the success of the wine. O. oeni is not the only lactic acid bacteria that can perform secondary fermentation. There are a variety of lactic acid bacteria that can dominate bacterial community in response to temperature, nutrients, sulfur dioxide content, pH, ethanol levels, and inoculation densities. O. oeni commonly dominates secondary fermentation from its extreme tolerance to pH and ethanol levels. The molecule diacetyl is produced as a byproduct of lactic acid bacteria in secondary fermentation. In wine, the levels of diacetyl create buttery and caramel flavor notes. It is generated when there is little or no malic acid to be consumed so citric acid is used. This byproduct is sought out by some winemakers while it is avoided by others. Rarely do the diacetyl levels reach a point of spoiling the wine (10).

Because of its heterofermentive properties, Oenoccocus may be viewed as an ecosystem engineer. O. oeni plays a major role in establishing the environment for which other microbes will interact. Its end products and life strategies positively feedback into creating a more harsh environment for other microbes like yeasts and fungi while making the conditions more ideal for other lactic acid bacteria (5).


References

(1) "Rebounding bacteria". 2013

(2) “SAR11 bacteria thrive — despite viruses”.“Nature”

(3) "Why Sequence SAR11 Genome Evolution?". "Joint Genome Institute" Stephen Giovannoni, Oregon State University and Michael Rappé, Hawaii Institute of Marine Biology

(4) Grote J, et al. 2012. Streamlining and core genome conservation among highly divergent members of the SAR11 clade. mBio 3(5):e00252-12. doi:10.1128/mBio.00252-12.

(5) Ferla MP, Thrash JC, Giovannoni SJ, Patrick WM (2013) New rRNA gene-based phylogenies of the Alphaproteobacteria provide perspective on major groups, mitochondrial ancestry and phylogenetic instability. PLoS ONE 8: e83383 doi:

(6) A war without end - with Earth’s carbon cycle held in the balance


(7) Kakarova, K., et al. “Comparative genomics of the lactic acid bacteria” . “Proc. Natl. Acad. Sci. USA.”. 2006. Volume 103. P. 15611-6

(8) Mills, D. A., Rawsthorne, H., Parker, C., Tamir, D., Makarova, K. “Genomic analysis of Oenococcus oeni PSU-1 and its relevance to winemaking”. “FEMS Microbiology Reviews”. 2005. Volume 29. P. 465-475

(9) phys.org “Raise your glass to Oenococcus oeni, a real wine bug”. “Science X Network”. 2014.

(10) “Gibson, G., Farkas, M. “Flaws and Faults in wine”. ”British Columbia Amateur Winemakers Association”. Accessed 28 April 2014

Author

Page authored by Digvinder Singh Mavi student of Prof. Katherine Mcmahon at University of Wisconsin-Madison.