Photo-autotrophic Production of Poly(hydroxyalkanoates) in Cyanobacteria

Replacing conventional by biodegradable plastics is claimed to be beneficial for the environment, especially when the drastic negative consequences of litter are considered. Such plastics are called biobased if the basic chemicals derive from organisms, preferably from autotrophic organisms, such as higher plants, algae or cyanobacteria.1 Established biotechnological processes convert carbohydrates by heterotrophic bacteria into the polymer storage substance poly(hydroxyalkanoates) (PHA). These PHAs, especially poly(hydroxybutyrate) (PHB) and poly(hydroxyvalerate) (PHV), belong to the most promising polymers replacing conventional plastic, as they are thermoplastic, hydrophobic and biodegradable at usual ambient conditions. Monomeric sugar is used as carbon and energy source for microbial growth. The sugars derive from plants, such as sugar cane or from corn (starch) or wood (cellulose) after hydrolysis. Process optimization and the use of carbon sources from industrial waste (e.g. food or slaughterhouse waste) have been reported, mainly for reduction of production costs.2,3 All those plants grow on agricultural or forestry areas, and the carbohydrates are highly demanded for food, feed, fuels, biobased materials and chemicals as well. Industry claims more and more biomass as their raw material responding to the rising demand for alternative production. However, even small and medium-scale solutions will occupy more agricultural land area as ethically reasonable. Unfortunately, there will not be enough biomass available when plants are harvested only from areas which could not be used for food and feed production.4,5 One of the possibilities to avoid ethical conflicts and dodge a disadvantageous development on the resources market is the production of biomass by photo-autotrophic microorganisms, such as algae and cyanobacteria. Closed photobioreactor types may be installed on building fronts and roofs or may cover traffic areas – if the local climate allows their operation. PHA is a natural energy and carbon storage product of prokaryotes. That includes heterotrophic bacteria, such as Cupriavidus sp. and autotrophic bacteria such as cyanobacteria (aerobic) or purple bacteria (anaerobic).6 For production in eukaryotes the responsible genes have to be transferred from bacteria to plants or algae cells. However, genetically modified plants face some critics, are widely alPhoto-autotrophic Production of Poly(hydroxyalkanoates) in Cyanobacteria