Table 2.

Factors affecting the accumulation of poly-β-hydroxyalkanoates.

FACTOR	CONDITIONS	AFFECT	MOLECULAR MECHANISM	GROUP	ORGANISM	REFERENCE
Nutrient	No nutrient limitation	PHA 302 mg/L culture	High molecular weight PHA was produced due to decreased activity of PhaC and PhaZ	Marine PNSB	Rhodovulum visakhapatnamense	[120]
Nutrient	Addition of 1−2 μm of iron under photoheterotrophic micro-aerobic conditions	PHA accumulation of ∼20−30% cell weight	During log phase, phaC and phaZ expression were relatively the same. However, during log phase the expression of phaC1 transcript and phaZ increased which consequently led to a decrease in PHA accumulation.	Marine PNSB	Rhodovulum sulfidophilum	[147]
Nutrient	Syngas devoid of carbon monoxide and phosphorous, in fed- batch cultivation with controlled acetate input	30 % w/w PHB produced.	CO was converted to carbon dioxide via the action of CO dehydrogenase. Carbon dioxide was then fixed through the Calvin Benson cycle and consequently led to the assimilation of acetate as PHB [148]	PNSB	Rhodospirillum rubrum	[141]
Light	3 wavelengths – 730, 800 and 850 nm. Low light intensity of 8 W/m2 and high light intensity of 50 W/m2.	A wavelength of 800 nm showed increased PHA production compared to 730 and 850 nm. Low intensities of light allowed for PHA accumulation (17−50 wt%) compared to high light intensities (15−30% wt)	Although there were no significant changes in the expression levels of IDH (isocitrate dehydrogenase), PhaC, PhaP and PhaZ, the changes in the TCA cycle and biosynthetic pathway of PHA could not account for low accumulation of PHA under high light intensities. However, a >3-fold elevation in the expression of PdhR – (pyruvate dehydrogenase complex regulator) under high light conditions indicated the involvement of the GntR family transcription factor. This was possible via an alternative metabolic pathway in order to protect cells from photooxidation caused due to high light intensities.	Marine PNSB	Rhodovulum sulfidophilum, Rhodovulum euryhalinum, Rhodovulum imhoffii, and Rhodovulum visakhapatnamense	[146]
Light	Three illumination cycles (light: dark): 15:15, 30:30 and 60:60 minutes	PHB accumulation highest under 30:30 min light:dark cycles, (308 ± 2 mgPHB/gdw) (24% wt) compared to continuous illumination	Highest PHB accumulation occurred when H2 production was the lowest. This was due to the rerouting of protons and electrons towards PHB accumulation instead of it being used for H2 production. With regards to the light/ dark - cycling due to the inhibition of nitrogenase activity which is required for H2 production in the dark, the activity of the TCA cycle was limited [149] and resulted in increased PHB accumulation.	PNSB	Rhodobacter capsulatus	[150]
Oxygen	PHA production under anaerobic vs aerobic conditions	Under anaerobic conditions, PHA content was ∼ 43 % wt. However, under aerobic conditions, the PHA accumulation was only between 0.4−1.8% wt. When acetate was added to the culture, under aerobic conditions, in the case of R. sulfidophilum 33% wt PHA was produced	Addition of acetate under aerobic conditions increased IDH expression by 10-fold, due to increased activity of the TCA cycle. External addition of acetate to the media resulted in the direct production of acetyl-CoA and an enhancement of the ethylmalonyl-CoA (EMC) pathway [151]. Induction of the EMC pathway (shared PhaA and PhaB enzymes with the PHA pathway) therefore enhanced PHA accumulation.	Marine PNSB	Rhodovulum sulfidophilum, Rhodovulum imhoffii, and Rhodovulum visakhapatnamense	[146]
pH	pH 7, 8 and 9 tested	At pH 8, P3HB accumulation was highest at 412 ml/L but pH 9 caused a drop in the P3HB accumulation.	As pH increased >7, P3HB accumulation increased. However, as pH increased >8, PHA depolymerase was activated and resulted in the reduction of P3HB.	Marine PNSB	Rhodovulum sulfidophilum DSM-1374	[152]
pH	Acetate concentrations – 10, 25, 50 and 65 mM.	At high concentrations of acetate (65 mM), the pH was >7.2 and resultant PHB accumulation was also at its highest- 234.7 mg/L (>20% cdw)	A high acetate (carbon) concentration of 65 mM, led to an increase in the mRNA transcripts in the organism. This resulted in an increase in enzymes involved in PHA biosynthesis, particularly pha C, which enhanced PHB production. Low pH (<7), was optimal for H2 production. Thus, as the pH increased, H2 production decreased significantly and PHB accumulation increased	PNSB	Rhodobacter capsulatus DSM 1710	[145]
Temperature	25, 30 and 35 °C	Highest P3HB production was observed at 25 °C - 428 ± 11 mg/L (192 h of cultivation).	BChl concentration was the lowest for cultures grown at 25 °C. Generally, PHA accumulation occurred when cell growth was inhibited. Hence as the BChl concentration decreased, PHA accumulation increased.	Marine PNSB	Rhodovulum sulfidophilum DSM-1374	[152]
Salinity	NaCl - 1.5, 2.5, 3.5 and 4.5 %	Highest P3HB obtained when cultures were growth at salinity of 4.5 % (820 ± 50 mg/L) and lowest observed at 1.5 % (388 ± 32 mg/L)	Rhodovulum sulfidophilum DSM-1374 is a marine PNSB with a high tolerance to salt. Therefore, highly saline conditions were ideal for its general metabolism and PHA accumulation.	Marine PNSB	Rhodovulum sulfidophilum DSM-1374	[152]