Abstract
The synthesis of polyhydroxybutyrate (PHB) by photosynthetic non-sulfur bacteria is a potential approach for producing biodegradable plastics. In this work, acetate was used as a single carbon source to study the effect on PHB formation in Rhodopseudomonas sp. cultured in a cylindrical four-liter photobioreactor under semi-continuous mode. The cultivation process is divided into a symmetrical growth phase and a PHB accumulation phase separated temporally. The symmetrical growth phase (nutrient sufficient conditions) was followed by a sulfur-limited phase to promote PHB accumulation. The main novelty is the progressive lowering of the sulfur concentration into Rhodopseudomonas culture, which was obtained by two concomitant conditions: (1) sulfur consumption during the bacterial growth and (2) semi-continuous growth strategy. This caused a progressive lowering of the sulfur concentration into Rhodopseudomonas culturedue to the sulfur-free medium used to replace 2 L of culture (50% of the total) that was withdrawn from the photobioreactor at each dilution. The PHB content ranged from 9.26% to 15.24% of cell dry weight. At the steady state phase, the average cumulative PHB was >210 mg/L. Sulfur deficiency proved to be one of the most suitable conditions to obtain high cumulative PHB in Rhodopseudomonas culture.
Highlights
Environmental pollution, in addition to recent pressure on fossil energy resources, is forcing the research society to find a green, circular economy solution that uses mainly renewable resources [1]
The main novelty is the progressive lowering of the sulfur concentration into Rhodopseudomonas culture, which was obtained by two concomitant conditions: (1) sulfur consumption during the bacterial growth and (2) semi-continuous growth strategy
PHAs are synthesized under stressful conditions by several heterotrophic bacteria and many photosynthetic microorganisms such as cyanobacteria and photosynthetic non-sulfur bacteria (PNSB) [11]
Summary
Environmental pollution, in addition to recent pressure on fossil energy resources, is forcing the research society to find a green, circular economy solution that uses mainly renewable resources [1]. Application of photosynthesis could be a possible solution to address environmental problems and to overcome the fossil-fuel based economy. Photosynthetic microorganisms, such as microalgae and photosynthetic non-sulfur bacteria (PNSB), are promising alternative candidates for providing value-added compounds such as biofuels and biomaterials [2,3,4,5]. Plastic pollution is one of the main environmental problems worldwide. Replacement of petroleum-based plastics with biodegradable bioplastics could be a realistic solution to this environmental problem [6]. Research efforts are rising in the bioplastics sector to reduce the problem of recycling of petroleum-based plastics and minimize their environmental impact. Despite the environmental benefits of bioplastics, their actual worldwide production is only ca. 1 Mt/y [7]
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