Fantahun Bitew | ALES Graduate Seminar

Event details: A graduate exam seminar is a presentation of the student’s final research project for their degree.
This is an ALES MSc Final Exam Seminar by Fantahun Bitew. This seminar is open to the general public to attend.

MSc with Dr. David Bressler

Thesis Topic: Bioconversion of lignin and glycerol to polyhydroxyalkanoates using Pseudomonas putida KT2440 under nutrient limited conditions

Abstract:

Polyhydroxyalkanoates (PHAs) are a type of bioplastic synthesized by several bacterial species using various carbon sources. PHAs are biocompatible, sustainable, and biodegradable, making them attractive alternatives to traditional petroleum-based plastics. However, the price of PHAs is currently higher than that of petroleum-based plastics due to the expensive carbon sources required, and the yields of PHAs are often low, necessitating large quantities of substrates for relatively small amounts of PHAs. The type of carbon sources and bacterial strains and fermentation condition used can significantly affect PHA yield. To reduce production costs, low-cost and abundant carbon sources such as crude glycerol and lignin have been the focus of much attention in recent years as potential feedstocks for PHA production. Production of PHA from these sources typically requires excess carbon sources and limiting essential nutrients such as nitrogen or phosphorus. This study aimed to investigate how nutrient limitation and lignin concentration affect cell growth and PHA yield in Pseudomonas putida KT2440.

In the initial stage, a study was conducted to determine the optimal mixtures of lignin monomers from various lignin monomeric units such as coniferyl alcohol derivative (vanillin and ferulic acid), p-coumaryl alcohol derivative (p-coumaric acid and 4-hydroxy methyl benzoate), and sinapyl alcohol derivative (syringic acid and syringol), during co-feeding glycerol and mixtures of lignin monomers under nitrogen-limited conditions. The findings revealed that the combination of glycerol with coniferyl alcohol derivatives, specifically vanillin and ferulic acid, resulted in increased cell growth and PHA yield when compared to using glycerol alone or co-feeding glycerol with other derivatives.

In a second study, the impact of the sulfur limitation on PHA yield and cell growth was examined by varying the carbon-to-sulfur molar ratio of the media using coniferyl alcohol derivatives (vanillin and ferulic acid) and glycerol. The results showed that the carbon-to-sulfur molar ratio highly influenced both PHA yield and cell growth. The PHA yield increased significantly when Pseudomonas putida KT2440 was cultivated in limited sulfur conditions, and the highest PHA content was achieved when the carbon-to-sulfur molar ratio was 3000.

The third study investigated the impact of different lignin concentrations on PHA production by varying the molar ratio of lignin in the media under a carbon to sulfur molar ratio of 3000. Results revealed that under sulfur-limited conditions, the addition of small amounts of lignin derivatives (10% mol/mol) significantly increased both PHA content and cell growth. However, increasing the lignin monomer level further caused a decline in PHA yield.

In conclusion, the findings suggested that a combination of glycerol with coniferyl alcohol derivatives could enhance PHA yield and cell growth. Furthermore, sulfur limitation condition can significantly increase the PHA accumulation in Pseudomonas putida KT2440. These findings have significant implications for the development of efficient and sustainable PHA production methods, which could help to reduce the final price of PHA.