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Since the 1960s, scientists have studied the application of photosynthetic bacteria (PSB) in treating olive mill wastewater, dairy wastewater, soy sauce wastewater, poultry slaughterhouse wastewater, and palm oil mill effluent wastewater (Lu et al., 2013 and Kaewsuk et al., 2010). This method showed high efficiency in the removal of COD, phosphate, nitrate, and hydrogen sulfide. In particular, PSB can convert organic carbon in wastewater into a valuable biomass.
PSB are a good source of single cell proteins, carotenoid, bacteriochlorophyll, biopolymers, antimicrobial agents, and pantothenic acid (Kuo et al., 2012). Among these valuable products from PSB, carotenoid is especially attractive. Carotenoid is lipid-soluble pigments responsible for the color of a wide variety of foods. Carotenoid is of increasing demand and has wide applications. Carotenoid has been applied to food coloring agents, precursors of vitamin A, β-carotene and astaxanthin in industry, and additives to cosmetics (Aksu and Eren, 2005). More and more research is now focusing on determining how to achieve efficient and safe carotenoid production. PSB synthesis might be the answer. Therefore, the application of PSB to treat wastewater in together with biomass recycling and carotenoid production is very attractive.
PSB are mostly cultivated in photo-bioreactor, which constitutes a system providing an artificial environment including light and culture medium. Research regarding biological carotenoid synthesis has been previously conducted in a culture medium (Kuo et al., 2012). The cost of culture medium accounts for a significant proportion for PSB production and carotenoid synthesis. One approach to reduce the cost is to substitute the culture medium with non-hazardous wastewater. An extra benefit of utilizing wastewater is the potential for pollutants removal. However, there are few studies on the biological synthesis of PSB cells and carotenoid using wastewater.
Light intensity is a very important factor in PSB growth since PSB convert light energy into chemical energy through anaerobic photosynthesis. Research has shown that in a certain culture medium, light was a limiting factor for PSB growth. In previous studies, an increase in light intensity from 2000 to 4000 lux resulted in a considerable increase in PSB growth, but further increases in light intensity exceeding 5000 lux resulted in a substantial decrease in PSB growth (Shi and Yu, 2005 and Nath and Das, 2009). Carotenoid is important antioxidant, active component, and auxiliary light-harvesting pigment in photosynthetic organisms such as PSB, algae, and plants (Chen et al., 2006 and Sheehan et al., 2012). Research has shown that carotenoid acted as light-harvesting pigment when exposed to weak light, and they mainly played an important role in photo-protection when exposed to intense light (Schagerl and Muller, 2006). Currently, there is limited research focused on the effects of light intensity on PSB wastewater efficiency. The wastewater COD removal increased with increasing light intensity below a specific threshold, but it fell at higher light intensity due to light inhibition of PSB (Zhang et al., 2014 and Qu et al., 2011).
In summary, the PSB biomass production and biological carotenoid synthesis from wastewater should be feasible, which might be strongly influenced by light intensity. This study aimed to demonstrate the feasibility of biomass, carotenoid and bacteriochlorophyll accumulation in PSB wastewater treatment. The primary aim of this work was to reduce pollutants. The biomass production was of secondary importance. In addition, the carotenoid and bacteriochlorophyll accumulations were also optimized, and the mechanisms of light intensity on PSB growth, carotenoid and bacteriochlorophyll synthesis were investigated.
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