Bioreactor experiments
A 2L dished bottom bioreactor Z6110/Coob (Cole-Parmer Instrument) was used,
which consisted of 3 liter vessel equipped with lipseal stirrer assembly, automatic pH
controller, automatic dissolved O2 controller, CO2 controller, automatic temperature
controller, foam controller and multi-channel peristaltic pump (for feeding). The
PHAs producing bacteria were grown in the bioreactor as batch, two-stage batch and
fed-batch (pulsed, continuous and high cell density) cultivation. During all
fermentations, temperature, dissolved O2 and agitation speed were kept at 30°C,
20% of saturation and 500 rpm, respectively. Initial pH was adjusted to 7±0.1 which
was not controlled during the fermentation period. Samples (10 ml) were aseptically
withdrawn from the fermentation vessel periodically. The samples were centrifuged
at 15000 x g for 4 min at 4°C. The sediment (biomass) was washed twice with
distilled water, and then dried at 70°C to constant weight. Polymer in bacterial cells
was determined using the chloroform-sodium hypochlorite method (Hahn et al.,
1994). Residual carbon was determined in supernatant according to Walinga et al.
(1992). PHAs and copolymer parameters were calculated. The parameters of
polymer production; yield (%), conversion coefficient (%) and carbon utilization
efficiency were calculated according to Ramadan et al. (1985). Polymer content (%)
and productivity were calculated according to Lee & Choi (1998) and Lee (1996),
respectively.
Bioreactor as a batch culture (one-stage)
In this experiment, the fermentation vessel containing 1950 ml productive
medium was autoclaved at 121°C for 20 min. The bioreactor was inoculated with
1% standard inoculum (5 x 108 cfu/ml) of Ps. fluorescens S48. The final working
volume was 2 liter.
Bioreactor as two-stage culture
The production of PHAs was carried out by two-stage cultivation. In the first
stage, 1 liter of nutrient broth inoculum culture of Ps. fluorescens S48 was
centrifuged at 15000 x g for 4 min at 4°C and the bacterial cells were collected
and suspended in additional sterile productive medium to inoculate the
fermentation vessel to give a final working volume of 1 liter.