Synthesis and production of polyhydroxyalkanoates by halophiles: current potential and future prospects.

Biodegradable materials with plastic or elastomeric properties are in great demand for a variety of applications. Polyhydroxyalkanoates (PHAs), polyesters synthesized by microorganisms, possess such desired features. Industrial production of PHAs is currently achieved using recombinant Escherichia coli. Nevertheless, recent research on halophiles, salt requiring microorganisms, has shown a remarkable potential for biotechnological production of PHAs. The halophilic archaeon Haloferax mediterranei accumulates a co-polymer, i.e., poly(3-hydroxybutyrate-co-3-hydroxyvalerate) in large amounts using glucose, starch, and hydrolyzed whey as carbon sources. Chemical composition and molecular weight of PHAs produced by H. mediterranei can be modified depending on the substrate utilized as precursor. Phylogenetic studies on haloarchaeal enzymes able to polymerize the components of PHAs (i.e., PHA synthases) reveal a novel cluster, with a close relationship with PHA polymerases of bacteria and archaea found in marine-related niches. On the other hand, sequences of PHA synthases of two halophilic bacteria are more closely affiliated to synthases of Proteobacteria. Several bacterial species of the family Halomonadaceae accumulate PHAs. Halomonas boliviensis reached PHA yields and volumetric productivities close to the highest reported so far. Furthermore, H. boliviensis and other Halomonas species are able to co-produce PHA and osmolytes, i.e., ectoines and hydroxyectoine, in one process.