Date Published: April 1, 2013
Publisher: Public Library of Science
Author(s): Julianne Megaw, Alessandro Busetti, Brendan F. Gilmore, Michael M. Meijler. http://doi.org/10.1371/journal.pone.0060806
The aim of this study was to isolate and identify marine-derived bacteria which exhibited high tolerance to, and an ability to biodegrade, 1-alkyl-3-methylimidazolium chloride ionic liquids. The salinity and hydrocarbon load of some marine environments may induce selective pressures which enhance the ability of microbes to grow in the presence of these liquid salts. The isolates obtained in this study generally showed a greater ability to grow in the presence of the selected ionic liquids compared to microorganisms described previously, with two marine-derived bacteria, Rhodococcus erythropolis and Brevibacterium sanguinis growing in concentrations exceeding 1 M 1-ethyl-3-methylimidazolium chloride. The ability of these bacteria to degrade the selected ionic liquids was assessed using High Performance Liquid Chromatography (HPLC), and three were shown to degrade the selected ionic liquids by up to 59% over a 63-day test period. These bacterial isolates represent excellent candidates for further potential applications in the bioremediation of ionic liquid-containing waste or following accidental environmental exposure.
With growing restrictions on acceptable limits of worker exposure and environmental release of carcinogenic, mutagenic, toxic, persistent and bioaccumulative compounds , there is an increasing global demand for the adoption of policies of green chemistry and sustainability in research and industry , . This has necessitated the search for alternative, safer compounds, provoking the exploration of ionic liquids as viable replacements for many conventional and volatile organic solvents. A major advantage of ionic liquids is their ‘tuneable’ nature, with simple structural modifications enabling many of their physicochemical properties to be altered, which is particularly beneficial in chemical processes which may be limited by the available solvents . Consequently, ionic liquids have found numerous and diverse applications –. Other benefits in terms of operational and environmental safety include their negligible vapour pressure, non-flammability, recyclability and thermostability, which have led to ionic liquids frequently being referred to as ‘green’ solvents.
Fourteen bacterial isolates were obtained using the procedures described above (Table 1), nine on the ionic liquid-supplemented M9 minimal salts medium. As the ILs provided the only available carbon source, any isolates obtained in this manner theoretically had the ability to degrade them. While not used for isolation by Deive et al., their 0.1% peptone medium with 1 M [C2mim]Cl  yielded one isolate (an immediate indication of high ionic liquid tolerance), which was later identified as Brevibacterium sanguinis. The nature of the sampling site – a saline environment in close proximity to a harbour – could account for the presence of ionic liquid-tolerant/biodegrading isolates in this environment. This is in keeping with the findings of Deive et al., who noted that saline and hydrocarbon-polluted soil yielded ionic liquid-tolerant and degrading isolates, when unpolluted soil did not. Taken together, these data indicate that, for biodegradation and tolerance, it is sufficient for ionic liquid naïve microbes to have been previously exposed to compounds structurally analogous to the ionic liquids under test. A strain of Kocuria palustris which had been isolated previously from a marine environment was also included in the study as bacteria of the genus Kocuria are known for their ability to degrade hydrocarbons , .
The isolation of microorganisms from the environment using minimal media, where the sole carbon sources are 1-alkyl-3-methylimidazolium chloride ionic liquids, is an effective and facile method for the selection of bacteria which exhibit an ability to biodegrade or tolerate high concentrations of these compounds, as evidenced by their rapid growth in concentrations exceeding those previously described in the literature. This is likely due to selective pressures imposed by the conditions in the environment from which they were isolated. The three isolates which were identified as being particularly effective ionic liquid biodegraders are potential candidates for the remediation of 1-ethyl- and 1-butyl-3-methylimidazolium chlorides. The ability of two of these bacteria to form biofilms may also provide good candidate organisms for further studies of the role of biofilms in the ultimate bioremediation of ionic liquid-containing waste.