Can cyanobacteria be a potential PHA producer?

Polyhydroxyalkanoates (PHAs) are microbial polyesters produced by various microorganisms. Many of these bacteria when fed with suitable carbon sources can produce PHAs up to about 30 wt% of the cell dry weight (CDW) and a few such as Ralstonia eutropha can accumulate PHAs up to almost 90 wt% of the CDW. In cyanobacteria (blue-green algae), the PHA content is usually about 5 wt% of the CDW. In this study, we have investigated various aspects of PHA biosynthesis in the cyanobacterium Synechocystis PCC6803. PHA biosynthesis can be improved by introducing multicopies of heterologous PHA synthase gene. Nile blue A staining and freeze-fracture electron microscopy revealed the presence of many PHA inclusions in the cell cytoplasm. Based on the sizes and number of these inclusions, the amount of PHAs produced by cyanobacteria is comparable to that produced by most bacteria. The relatively low wt% of PHA in cyanobacteria when compared to other bacteria is probably due to the larger size and mass of the former. Our investigations show that the PHA synthesizing ability of cyanobacteria may in fact be quite similar to that shown by most bacteria in nature.