Table 5.

Recent studies using purple non-sulfur bacteria for carotenoid production.

FACTOR	SUBSTRATE	CONDITIONS	EFFECT	ORGANISM	REFERENCE
Light	Artificial sugar wastewater.	Photo-anaerobic conditions with 4 light: dark cycles- 3:3 h, 6:6 h, 12:12 h and 24:24h	Carotenoids were stable under different cycles which indicated that cycling of light was vital for carotenoid production. The maximum carotenoid output was during the 12 h dark:12 h light cycle (∼1.4 mg/L)	Mainly Rhodopseudomonas	[222]
Substrate – different agro-industrial waste	Soy, coconut and cassava meal	Photo-anaerobic conditions	Carotenoid productivity was the highest with cultivation in soybean meal medium 71.25 mg/L (over a period of 10 days). High soybean meal concentrations contains large amounts of protein which affects pH and in turn affects the carotenoid formation, due to reduced bacterial growth [223]. Hence soybean meal at 50% concentration was the ideal substrate for carotenoid production. Carotenoids such as lycopene 1,2-dihydrolycopene, cis-1,2-dihydrolycopene and 1,2-dihy- dro-3,4-dedihydrolycopene were synthesized.	Rhodopseudomonas faecalis PA2	[136]
Additives -yeast	Artificial brewery wastewater	Natural light and micro-aerobic conditions without aeration	Yeast extract at 400 mg/L was the best for the growth of the bacteria, and at this concentration of yeast the carotenoid production was 2.53 mg/g. The addition of 400 mg/L yeast increased dehydrogenase activity, which was indicative of biomass growth [224].	Rhodopseudomonas	[194]
High salinity	Synthetic highly saline wastewater	Bioreactor based photo-anaerobic cultivation, under 20, 50 and 100 g/L NaCl concentrations.	Carotenoid production was optimal (3 mg/L) when 50 g/L NaCl was used, this was higher than the control. However, when the concentration of NaCl was further increased to 100 g/L, carotenoid production was the lowest. A similar trend was observed in the case of bacteriochlorophyll concentrations. Dehydrogenase activity was the highest at 50 g/L NaCl, as a consequence of high biomass yield (over 144 h) and carotenoid production.	Rhodopseudomonas strain Z16	[225]
Light intensity and agitation speed	Domestic wastewater sourced from a pond	Cultivation in bioreactors under photo-anaerobic conditions. 4 different light intensities – 2000−5000 lx, and 3 different agitation speeds -150, 300 and 600 rpm	The highest carotenoid yield was 7.2 mg/g, while the highest carotenoid production rate was 74.3 mg/L and carotenoid productivity was 40.9 mg/L. Among the different light intensities tested, 4000 lx showed the highest carotenoid yield, production and productivity. As the light intensity increased, the photosynthetic rate increased resulting in high biomass productivity and production. However, when the intensity was >4000 lx, biomass and carotenoid decreased as a result of photoinhibition. In the case of different agitation speeds, a speed of 150 rpm resulted in maximum biomass productivity and production and highest carotenoid yield, production and productivity. Agitation of the culture led to an increase in mass transfer rates and decreased light transfer limitations.	Rhodopseudomonas faecalis PA2	[191]
Dissolved oxygen (DO)	Artificial sugar wastewater	Photo-bioreactors illuminated by natural light. Agitators were used to set different dissolved oxygen (DO) concentrations - <0.5, 0.5–1, 1–2, 2–4, 4–8 mg/L.	DO of <0.5 mg/L showed highest carotenoid production over a period of 96 h, ∼ 1.7 mg/L, and could be attributed to the photophosphorylation metabolic pathway adopted by the organism [226] (oxidative phosphorylation at DO concentrations >0.5 mg/L). Dehydrogenase activity was also highest at DO of <0.5 mg/L.	Rhodopseudomonas	[227]
Biostimulant Bacillus thuringiensis/ cereus L2	Synthetic peptone wastewater	Cultivation in photo-bioreactors under micro-aerobic conditions. Different volumes of biostimulants were used- 0, 5, 10, 20, 40 and 80 μL	Co-culture of R. sphaeroides strain (ATCC17023) with the addition of 40 μL of L2 had the highest carotenoid yield - 3.24 mg/g biomass and a concentration of 11.8 mg/L. Intracellular peroxidase activity (encoded by RSP_3419 gene in R. sphaeroides and degrades intracellular carotenoid content) was the lowest when 40 μL L2 was added to the culture. Therefore, the addition of L2 at an optimal volume of 40 μL inhibited peroxidase activity and thus increased carotenoid yield.	Rhodobacter sphaeroides strain (ATCC17023)	[228]
Additives – Magnesium ion (Mg2+)	Artificial sugar wastewater	Cultivation in bioreactors under photo-micro aerobic conditions. Different concentration of Mg2+ were added - 0, 1, 5, 10, 15 and 20 mmol/L	The highest carotenoid yield of 4.83 ± 0.14 mg/g was obtained when 15 mmol/L Mg2+ was added. The same concentration of Mg2+ upregulated the expression of the crtBDA genes and increased carotenoid yield (CRT genes are responsible for the biosynthesis of carotenoids [229])	Rhodobacter sphaeroides strain (ATCC17023)	[230]
Additives – magnesium ion (Mg2+)	Soybean wastewater	Bioreactor based photo-anaerobic cultivation with. different concentration of Mg2+ - 5, 10 and 15 mg/L	Addition of 10 mg/L of Mg2+ to the cultures resulted in highest bacteriochlorophyll content and ATP production - ∼11.5 μg/mg and 35 μg/mg respectively. Mg2+ function as active sites of enzymes and pigments, it plays a vital role in capturing solar energy and converting it to electrons. [231]. Thus, the addition of an optimal dose of the additive improved bacteria growth and increased bacteriochlorophyll content which triggered an increase in ATP production (bacteriochlorophyll content regulates ATP/ energy production from photosynthesis)	Rhodobacter sphaeroides Z08	[232]
Type of light	Sugar wastewater	Cultivation was in photo bioreactor with DO concentration <0.5 mg/L. Different light sources were used - red LED (650 nm, 18 W), a yellow LED 595 nm, 18 W), blue LED (470 nm, 18 W), white LED (9 W), and incandescent lamp (400–780 nm, 80 W).	Compared to incandescent lamps, carotenoid and bacteriochlorophyll production were highest for the cultures exposed to LED lights. Highest carotenoid and bacteriochlorophyll production were reported for the yellow LED cultures ∼950 mg/L and 900 mg/L respectively. Since carotenoids strongly absorb light in the range of 450−550 nm and also the visible part of the spectrum, it was synthesized at a higher concentration when compared to bacteriochlorophyll.	Rhodopseudomonas	[208]