Intensive exploitation, poor recycling, low repeatable use, and unusual resistance of plastics to environmental and microbiological action result in accumulation of huge waste amounts in terrestrial and marine environments, causing enormous hazard for human and animal life. In the last decades, much scientific interest has been focused on plastic biodegradation. Due to the comparatively short evolutionary period of their appearance in nature, sufficiently effective enzymes for their biodegradation are not available. Plastics are designed for use in conditions typical for human activity, and their physicochemical properties roughly change at extreme environmental parameters like low temperatures, salt, or low or high pH that are typical for the life of extremophilic microorganisms and the activity of their enzymes. This review represents a first attempt to summarize the extraordinarily limited information on biodegradation of conventional synthetic plastics by thermophilic, alkaliphilic, halophilic, and psychrophilic bacteria in natural environments and laboratory conditions. Most of the available data was reported in the last several years and concerns moderate extremophiles. Two main questions are highlighted in it: which extremophilic bacteria and their enzymes are reported to be involved in the degradation of different synthetic plastics, and what could be the impact of extremophiles in future technologies for resolving of pollution problems.

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Halophilic microorganisms are producers of a lot of new compounds whose properties suggest promising perspectives for their biotechnological exploration. Moderate halophilic bacterium Chromohalobacter canadensis 28 was isolated from Pomorie salterns as an extracellular polymer substance (EP) producer. The best carbon source for extracellular polymer production was found to be lactose, a sugar received as a by-product from the dairy industry. After optimization of the culture medium and physicochemical conditions for cultivation, polymer biosynthesis increased more than 2-fold. The highest level of extracellular polymer synthesis by C. canadensis 28 was observed in an unusually high NaCl concentration (15% w/v). Chemical analysis of the purified polymer revealed the presence of an exopolysaccharide (EPS) fraction (14.3% w/w) and protein fraction (72% w/w). HPLC analysis of the protein fraction showed the main presence of polyglutamic acid (PGA) (75.7% w/w). EPS fraction analysis revealed the following sugar composition (% w/w): glucosamine 36.7, glucose 32.3, rhamnose 25.4, xylose 1.7, and not identified sugar 3.9. The hydrogel formed by PGA and EPS fractions showed high swelling behavior, very good emulsifying and stabilizing properties, and good foaming ability. This is the first report for halophilic bacterium able to synthesize a polymer containing PGA fraction. The synthesized biopolymer shows an extremely high hydrophilicity, due to the simultaneous presence of PGA and EPS. The analysis of its functional properties and the presence of glucosamine in the highest proportion in EPS fraction clearly determine the potential of EP synthesized by C. canadensis 28 for application in the cosmetics industry.

New alkaliphilic Bacillus producers of cyclodextrin glucanotransferase (CGTase, EC 2.4.1.19) were isolated from 17 Bulgarian alkaline and normal habitats (springs and soils) by three steps of a selection. None of the isolates obtained, producing CGTase, appeared to be thermophilic in character. One hundred and thirty-seven strains were estimated for CGTase activity by batch cultivation in a liquid alkaline medium. Twenty-seven of them had a detectable CGTase activity in their culture supernatants under the enzyme assay conditions, despite of the significant growth of all isolates. The phenotypic properties of three selected strains (20RF, 8SB and 24WE) were determined. They were aerobic endospore-forming Bacillus strains: two of them were obligated alkaliphiles (20RF and 8SB) and one, alkalitolerant (24WE). Both obligated alkaliphiles were further characterised by 16S rRNA analysis. According to the full 16S rRNA gene sequences obtained and deposited to the NCBI GenBank database, both isolated obligated alkaliphiles 20RF and 8SB were clustered into the group of alkaliphilic Bacillus species. The exhibited CGTase production by them (230-250 U ml(-1) for 20RF and 130-160 U ml(-1) for 8SB) defined these new isolates as promising producers of the enzyme, especially Bacillus sp. 8SB synthesising thermostable alkaline beta-CGTase. Both new enzymes from 20RF and 8SB Bacillus strains formed only two types of cyclodextrins, beta and gamma, which could be of interest for their easy separation and industrial production.