CULTURE CONDITIONS OF AN ANOXYGENIC PHOTOSYNETHTIC MIXED CULTURE FOR PRODUCTION OF POLYHYDROXYALKANOATES (PHAS) FROM INDUSTRIAL WASTEWATERS.

Abstract

Plastics are one of the most notable materials invented during the 21st century because of their low cost, lightweight, ease of manufacturing and relative strength among many other desirable properties. Consequently, they are one of the most utilized materials in our day to day lives. However, one of their key benefits – their durability, is also the cause of one of their leading environmental impacts. When disposed to the environment, the extremely long decomposition time of plastics leads to their accumulation, causing deaths to animals through ingestion and entrapment, along with potential bioaccumulation risks in organisms. Although recycling methods exist, much of the plastic waste generated does not reach recycling facilities. Moreover, the production of plastics from petroleum resources is associated with significant greenhouse gas emissions. To overcome these issues, much interest exists to develop bioplastics derived from biomass and that are readily biodegradable. Polyhydroxyalkanoates (PHA) are one group of bioplastics that meet both criteria. They are a completely biodegradable bio-polyesters produced in nature by microorganisms. They are typically accumulated intracellularly in the form of inclusion bodies (PHA granules) during imbalance of carbon and/or nutrients, or oversupply of reducing equivalents. Two of the primary issues in producing PHAs commercially is the high cost of maintaining pure cultures, including the costly substrates required, and energy required for aeration. This project addresses these issues through the use of purple nonsulfur bacteria (PNSB) mixed cultures and their integration of PHA production with treatment of wastewater from the fuel synthesis industry. PNSB are subgroup of the anoxygenic phototrophic bacteria, a group of non-phylogenetically related bacteria, that can easily be enriched even in mixed microbial consortia under anaerobic illuminated conditions, and that are capable of producing PHA. Thus, they provide an opportunity to minimize the burden and cost of sterilization, substrate and aeration. This study evaluated the effectiveness of PNSB to simultaneously treat fuel synthesis wastewater (FSW) and produce PHA, by investigating the effect of nutrients limitation (nitrogen and phosphorus), illumination intensity and media pH. For the study on nutrient limitation, the maximum growth and organic removal was achieved simultaneously at N-high and p-High conditions. However, best PHA yield was obtained at N- low and N-deficient conditions. In phosphorus, highest PHA yield was obtained at P-high condition. PNSB appeared more sensitive to nitrogen than phosphorus deficiency, despite their ability to fix nitrogen from the atmosphere. This was most apparent when transitioning from nitrogen present to nitrogen deficient conditions, which require upregulation of nitrogenase. In the study of illumination intensity, the highest growth was achieved at a moderate intensity of 3000 lux. However, this corresponded with the lowest organic removal and lowest PHA content. The highest PHA content was obtained at high light intensities of 4500 lux and 6000 lux. For pH, the maximum biomass growth and organic removal were achieved simultaneously at pH 8. The condition also had the most rapid growth until plateauing, leaving pH 7 with the highest growth over the experiment. Highest yield of PHA was observed at pH 7 under controlled conditions, followed closely by, pH 7 and pH 8 (both uncontrolled). The study shows PNSB has the ability to treat FSW while producing PHA. Further study is warranted on microbial community dynamics, PHA quantity per cell and monomer types before moving to continuous flow systems.

Date of Award	2021
Original language	American English
Awarding Institution	HBKU College of Health & Life Sciences