Polyhydroxyalkanoates Production by Purple Non-sulfur Bacteria Employing Fuel Synthesis Wastewater

Abstract

The rising environmental burden of plastic pollution calls for sustainable alternatives to petroleum-based plastics. Polyhydroxyalkanoates (PHA) are biodegradable plastics with promising potential to replace conventional plastics. However, their commercialization is currently limited by their high costs of production. Purple non-sulfur bacteria (PNSB) are a group of versatile bacteria with the ability to accumulate PHA using various sources of organics carbon. Fuel synthesis wastewater (FSW), a carbon-rich effluent from the oil and gas sector in Qatar, presents a viable feedstock for this purpose. This work investigates the use of mixed PNSB for PHA production from FSW, exploring their physiological responses to environmental and operational variations, with a focus on optimizing biomass growth and PHA accumulation. Two distinct types of FSW, collected from different stages of the process, were characterized and evaluated for microbial compatibility. The influence of environmental pH, dilution levels, and inhibitory compounds was assessed to determine the optimal cultivation conditions. Macronutrient concentration reduction strategies, including individual and dual reductions of ammonium (NH₄⁺) and phosphate (PO₄³⁻), were investigated. Micronutrient depletion was also examined, focusing on both trace elements and B-group vitamins, assessed as mixtures and individually. The absence of magnesium (Mg²⁺) and select vitamins (biotin, thiamine, and cobalamin) significantly inhibited PNSB growth and metabolic activity, confirming their essential roles. A two-stage cultivation system was also investigated under nutrient-reduction conditions. This was further combined with continuous exposure to different light wavelengths. Far-red and infrared light spectra promoted biomass growth, while full-spectrum light containing UV and IR improved PHA accumulation, particularly under nitrogen-reduced conditions. A two-stage NH₄⁺ reduction yielded the highest PHA content without compromising PNSB biomass productivity, with an accumulation reaching 12.6% CDW under lights supplemented with IR wavelengths. In contrast, similar setup with PO₄³⁻ reduction resulted in slightly reduced but promising PHA accumulation, reaching 10.0% CDW. Overall, this work demonstrated a viable strategy for converting industrial effluents into value-added bioplastics using PNSB. It provides a foundation for sustainable wastewater valorization and offers insights for scaling up microbial bioplastic production through environmental and process control.

Date of Award	2025
Original language	American English
Awarding Institution	HBKU College of Science and Engineering