Marinomonas sp. MWYL1
   
   
 

Marinomonas MWYL1 was isolated from the root surface of the salt marsh grass Spartina anglica, growing near the North Norfolk (England) village of Stiffkey. Sequencing of its 16S rRNA gene placed it in the genus Marinomonas ( 1 ) van Landschoot and de Ley, 1983, J . Gen . Microbiol ., 1983, 129 , 3057-3074 ), the most closely related (97% identical 16S) named species being M. pontica, which was isolated from the Black Sea ( 2 ). The genus Marinomonas comprises a widespread group of g -proteobacteria that exist in coastal waters, and which had been earlier been included in the genus Alteromonas. Some Marinomonas strains are pigmented strains due to melanin and some produce metabolites that may be of biotechnological value ( 3 ).

However, the interest in Marinomonas MWYL 1 was that it could grow on the betaine molecule Dimethylsulphoniopropionate (DMSP) as sole carbon source and, when it did do, it released large amounts of the gas dimethyl sulphide. DMSP is a compatible solute that is used by many marine phytoplankton and seaweed macroalgae as an osmoticum and an anti-stress compound ( 4 ). In addition, a few known land angiosperms make DMSP and these include certain species of Spartina ( 5 ) - hence the choice of these plants as a source for DMSP-degrading bacteria. Indeed, others had shown previously that the DMSP-catabolising bacteria isolated from Spartina root surfaces included Marinomonas strains ( 6 ).

The catabolism of DMSP is of major importance, for several reasons. First, it represents a colossal biotransformation of marine sulfur and carbon - as much as 1 billion tonnes being catabolised, worldwide, each year ( 7 ). Secondly, the process can lead to the production of DMS, much of which (~ 35 million tons per annum ) is released into the atmosphere, where it can be oxidised to form products that act as cloud condensation nuclei. These are responsible for forming much of the cloud cover over the oceans, on such a scale that the light incidence on the Earth and hence the climate is significantly affected ( 8 ).

Despite its importance, nothing was known, till very recently, of the molecular genetics of the process of DMSP-dependent DMS production. It had been surmised that it involved a "DMSP lyase" that cleaved the DMSP, liberating the DMS and forming acrylate plus a proton. However, in the first-ever study on the genes that were involved, Todd et al ( 9 ) identified the key gene in Marinomonas MWYL1 as being dddD , which specified an Acyl CoA transferase, which was proposed to add CoA to DMSP, prior to its subsequent cleavage. The deduced DddD protein was found in other sequenced strains of Marinomonas and also in some more distantly related a -proteobacteria that were already known to make DMS form DMSP. More surprisingly, some "terrestrial" bacteria, including strains of Burkholderia and Rhizobium also had a functional dddD gene ( 9 ).   Thus the DMS-dependent synthesis of the climate-changing gas DMS is likely determined by genes that are subject to horizontal gene transfer.

References:

1. Van Landschoot, A., and de Ley, J. 1983 Intra- and intergeneric similarities of the rRNA cistrons of Alteromonas , Marinomonas (gen. nov.) and some other gram-negative bacteria. J . Gen . Microbiol . 129: 3057-3074.

2. Ivanova, E. P., Onyshchenko, O. M., Christen, R., Lysenko, A. M., Natalia V. Zhukova, N. V., Shevchenko, L. S., and Kiprianova, E. A . 2005. Int. J. Syst. Evol. Microbiol. 55: 275-279 .

3. Lucas-Elío, P ., Gómez, D ., Solano, .F , Sanchez-Amat, A . 2008 The antimicrobial activity of marinocine, synthesized by Marinomonas mediterranea , is due to hydrogen peroxide generated by its lysine oxidase activity. J. Bacteriol.   188: 2493-2501.

4. Simó R . 2001. Production of atmospheric sulfur by oceanic plankton: biogeochemical, ecological and evolutionary links. Trends Ecol Evol. 16:287-294.

5. Otte, M. L., Wilson, G., Morris, J. T., and Moran, B. M. 2004. Dimethylsulphoniopropionate (DMSP ) and related compounds in higher plants. J. Exp Bot. 55:1919-1925.

6. Ansede J. H., Friedman, R., and Yoch, D. C. 2001. Phylogenetic analysis of culturable dimethyl sulfide-producing bacteria from a Spartina -dominated salt marsh and estuarine water. Appl. Env. Microbiol. 67: 1210-1217.

7. Kettle, A. J., and Andreae, M. O. 2000 Flux of dimethylsulfide from the oceans: A comparison of updated data sets and flux models, J. Geophys. Res.105: 26,793- 26,808.

8. Ch arlson, R ., Lo velock, J., Andreae, M., and Wa rren, S. 1987. Oceanic phytoplankton, atmospheric sulphur, cloud albedo and climate. Nature , 326: 655-661.

9. Todd, J. D., Rogers, R., Li, Y. G., Wexler, M., Bond, P. L., Sun, L., Curson, A. R. J., Malin, G., Steinke, M., and Johnston, A. W. B. 2007. Structural and regulatory genes required to make the gas dimethyl sulfide in bacteria. Science 315: 666-669.