Polyhydroxyalkanoate (PHA) Production from Carbon Dioxide by Recombinant Cyanobacteria

Abstract

Global warming is the urgent issue of our time, and the carbon dioxide is a greenhouse gas of the major concern. There are various research activities for carbon dioxide mitigation, such as CO2 recovery from the flue gas of industrial sites, underground and undersea CO2 storage, and also chemical/biological conversion of CO2 into the industrial materials [1]. On-site CO2 fixation by bioprocess is based on the activities of photosynthetic organisms. The fixation of CO2 by photosynthetic microorganisms can be an efficient system for the CO2 mitigation, but one of the major problems of this system is the effective utilization of the fixed biomass. The biomass produced by photosynthetic microorganisms must be utilized as a resource, or it will be easily degraded by microorganisms into CO2 again. Cyanobacteria are procaryotic photosynthetic microorganisms and can provide a simple genetic transformation system for the production of useful materials from CO2. We have established an efficient vector-promoter system for the introduction and expression of foreign genes in the marine cyanobacterium Synechococcus sp. PCC7002, and examined the production of biodegradable plastic, polyhydroxyalkanoate (PHA), by genetically engineered cyanobacteria. The PHA is a biopolymer accumulated by various microorganisms as reserves of carbon and reducing equivalents. PHAs are linear head to tail polyesters composed of 3-hydroxy fatty acid mono‐ mers (Figure 1), have physical properties similar to those of polyethylene, and can replace the chemical plastics in some applications, such as disposable bulk materials in packing films, containers, and paper coatings [2]. PHA applications as implant biomaterials, drug delivery carrier, and biofuel have also been investigated [3]. The commercial mass production of PHA from corn sugar by using the genetically enigineered microorganism was started in 2009 in the United State and China [3].