Abstract

Microbial polyhydroxyalkanoates (PHA) have been developed as bioplastics for the past many years with limited successes for marketing. Commercial PHA are normally poly-3-hydroxybutyrate (PHB), copolyesters of 3-hydroxybutyrate and 3-hydroxyhexanoate (PHBHHx), copolyesters of 3-hydroxybutyrate and 4-hydroxybutyrate (P3HB4HB), as well as copolyesters of 3-hydroxybutyrate and 3-hydroxyvalerate (PHBV). Due to the higher production cost, PHA can not compete with petrochemical plastics. Therefore, it is very important to develop low cost production technology for PHA marketing. Recently, we have succeeded in isolating halophile Halomonas spp. that are able to grow in seawater utilizing mixed substrates such as kitchen waste under unsterile and continuous processes. We have developed genetic engineering tool to construct pathways for effective production of PHBV from glucose as the only C-source. The PHA production by Halomonas spp. based on seawater allows several important advantages including fresh water saving, energy saving, reduction on process complexity and increasing on process efficiency. This process can at least save 50% PHA production cost. At the moment we are conducting process scale up in an industrial setting. Recently, by engineering the genomes of some microorganisms, especially Pseudomonas ssp., it becomes possible to produce a series of novel PHA including homopolymers, random copolymers and block copolymers as well as functional polymers. For example, poly-3-hydroxypropionate (P3HP), poly-4-hydroxybutyrate (P4HB), poly-3-hydroxydecanoate (P3HD) and poly-3-hydroxydodecanoate (P3HDD) et al. Random copolymers containing defined monomer compositions can also be microbially produced. More importantly, block copolymerization containing various block such as diblock copolymers of PHB-b-P4HB, PHB-b-PHHHx, P3HP-b-P4HB, PHBHHx-b-PHDD et al have been produced. They remarkably increased the diversity of PHA structures and properties. By manipulating the block compositions, by grafting functional groups of PHA, the polymer properties can be easily controlled. Now, PHA industry will be entering a functional polymer era, which allows PHA competitiveness not by the low cost but by its functionality. We will soon experience ultra-strong, shape memory, gas selective permeability and other environmentally responsive PHA.