Zoogloea ramigera
Matt Banks

Zoogloea ramigera is an aerobic Gram-negative bacillus found primarily in organically enriched aqueous environments. In addition, this bacterium is nonsporeforming, polarly flagellated (amphitrichous), approximately 0.5-1.3 um x 1.0-3.6 um in size and utilizes oxidative metabolism for energy. Although Z. ramigera relies on oxidizing inorganic metals, such as Manganese (Mn), for its primary energy source, it does utilize organic compounds for its carbon source, thus classifying its energy needs as chemolithotrophic. Z. ramigera also have the ability to flocculate, that is to form flocs. A floc is a collection of cells that grow in an aggregated form. Flocs are formed by the intertwining of extracellular fibrillar strands contained within a Zoogloeal matrix. The Genus name, Zoogloea, literally translates to mean "animal glue," and it is this gooey matrix that makes Z. ramigera unique.

The Zoogloeal matrix surrounds the floc and is a collection of extracellular polymer strands composed of polysaccharides. The chemical composition and structure of the polysaccharides determine the overall physical and chemical properties of the matrix and hence each Zoogloea species. The Genus contains 9 species, 4 of which are variations of Z. ramigera. The most studied species and most versatile representative of the Genus is Z. ramigera 115. The polysaccharides composing the matrix of 115 are weakly acidic, slightly water soluble, stable in pH ranges of 3-10 and temperature ranges of –15° to 90°C, do not precipitate in the presence of salts and are highly viscous. The main constituents making up the polysaccharides include glucose, galactose and pyruvic acid. The polymer chains form B-1,4-linkages, are highly branched and do not have regular repeating subunits. The approximate molecular weight of a given polymer unit is 105 g/mol. Other species and variation within the Genus may contain mannose, hexoses, arabinose, rhamnose, glucosamine, fucosamine or uronic acids.

The matrix and flocculation properties of Z. ramigera make this bacterium versatile and unique in three ways: poly-B-hydroxybutyrate (PHB) production, water treatment and biosorption of metals. PHB is a key component in the manufacturing of biodegradable plastics. PHB is produced by Z. ramigera whenever carbon sources are present in abundance and researchers have determined that the enzyme B-ketothiolase necessary in carrying out this process. In addition, researchers believe that they have isolated the segment of DNA responsible for coding this enzyme and think it may be possible to transduce the gene to another suitable organism, such as E. coli. Water treatment is perhaps what Z. ramigera is best known for. (Recall that Z. ramigera prefers "organically enriched" aqueous environments…aka sewage treatment.) Through the formation of flocs and oxidative metabolism, Z. ramigera is useful in reducing the Biological Oxygen Demand (BOD). This will later be attributed to the "natural pumping system." The biosorption of metals is also an exciting and unique use of Z. ramigera. In addition to the ability to utilize Mn-oxidation metabolism, the matrix polysaccharides are well suited to adsorb and concentrate metals and transuranic elements from contaminated water sources. Although theoretically any metal could potentially bind within the matrix, this has only been tested on Al, Ca, Co, Fe, Mg, Mn, Ni, Si, U, Cu and Cd in acidic mine water samples. Z. ramigera forms soluble salts with both monovalent and divalent cations and actually precipitates out trivalent aggregates. These manufacturing and treatment processes prove to be cost-effective since the biomass may be reused for multiple treatments. Furthermore, certain metals may be "selected" for binding by altering the environment. For instance, in attempting to eliminate Cd, Cu and U from water sources, an acid treatment causes the metal aggregates to precipitate out of solution.

I originally selected Z. ramigera because I thought that its ability to metabolize Mn via oxidation was unique. However, I quickly learned that many bacteria are capable of this metabolism. Are there any advantages to Mn-oxidizing metabolism? You bet! First off, Mn is a relatively abundant element, albeit in trace amounts. But considering that Mn usually is found in concentrations of 1-5 uM and that the toxic concentration of Mn in bacteria is 10 uM in most species, this trace amount means that Mn-oxidizing bacteria have an energy source virtually anywhere where water is found (in other words, over approximately 70% of the world). What amazed me the most about Mn-oxidation was that it is actually thermodynamically favorable to oxidize Mn(II) to Mn(III) or (IV) under aerobic conditions. In fact, the free energy associated with this oxidation process is approximately –16 kJ/mol. This oxidation step does have a high activation energy (Ea), but this is easily managed in one of two ways: either by elevating the pH in the oxidizing environment or by the presence of pre-existing Mn-oxides, which then serve as catalyst (a form of autooxidation). Since bacteria such as Z. ramigera oxidize Mn regularly in their metabolism, Mn-oxides are already present.

In addition to catalyzing the metabolism of Mn, the Mn-oxides also perform several other important functions. Mn-oxides are potent chelators of other metals, which helps explain why Z. ramigera functions so well in the role of water treatment. These oxides are also excellent electron acceptors for anaerobic respiration and may stimulate respiratory carbon mineralization in sedimentary environments through the overall process of precipitating Mn-oxides and allowing them to settle to the bottom. This cycling process of the Mn-oxides between the sediments and precipitates is the "natural pumping system" of Z. ramigera. Finally, as mentioned, Mn is toxic to most bacteria in concentrations around 10 uM. Mn-oxidizing bacteria are usually prevalent in areas where extreme concentrations of Mn are found. By converting the Mn(II) to Mn-oxides, the harmful concentrations of Mn are lowered and potentially beneficial Mn-oxide salts are often times formed. Therefore, Mn-oxidizing bacteria provides a protective mechanism and fills a niche.

Hopefully, you now see that Z. ramigera is in fact a useful, versatile and unique bacterium. And hopefully Dr. Westenberg does not know 3 facts about this wonderful microbe!

Reference:

  1. Dugan, Patrick R. et al. The Genus Zoogloea. The Prokaryotes. Second Edition, Chapter 220. pgs 3952-3962.
  2. Nealson, Kenneth H. The Manganese-Oxidizing Bacteria. The Prokaryotes. Second Edition, Chapter 114. pgs 2310-2319.
*Disclaimer - This report was written by a student participaring in a microbiology course at the Missouri University of Science and Technology. The accuracy of the contents of this report is not guaranteed and it is recommended that you seek additional sources of information to verify the contents.

 

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