Cultures of cyanobacteria with the ability to produce PHAs
Cyanobacteria are oxygen evolving photoautotrophic bacteria. Production of polyhydroxyalkanoates (PHAs) by cyanobacteria is especially beneficial because atmospheric carbon dioxide is converted into a biopolymer using sunlight as the energy source. Since PHA is completely biodegradable to carbon dioxide and water, the entire cycle of the PHA production and disposal is a sustainable process where PHA becomes part of the biological carbon cycle.
Three strains of Spirulina platensis isolated from different locations were investigated for its PHA production ability in our laboratory. The results revealed all three strains were capable of synthesizing P(3HB) under nitrogen starved condition with a maximum accumulation of up to 10 wt% of the cell dry weight under mixotrophic culture condition. The subsequent utilization of the accumulated P(3HB) was also studied. Mobilization of P(3HB) granules by S. platensis was initiated by restoration of nitrogen source and the process was affected by both illumination and culture pH. The mobilization of P(3HB) was better under illumination (80 % degradation) than in dark condition (40 % degradation) over a period of 4 days. Alkaline condition (pH 9-11) was optimal for both biosynthesis and mobilization of P(3HB) at which 90 % of the accumulated P(3HB) was mobilized.
TEM revealed that mobilization of P(3HB) involved in changes in granules morphology. P(3HB) granules became irregular in shape and the boundary region was less defined. In contrast to bacteria, in S. platensis the intracellular mobilization of P(3HB) seems to be faster than the biosynthesis process. This study has successfully identified the factors that affect P(3HB) biosynthesis and mobilization in S. platensis, which includes nutrient limitation, pH culture medium, carbon substrates and illumination.