Improvement of desulfurizing activity of haloalkaliphilic Thialkalivibrio versutus SOB306 with the expression of Vitreoscilla hemoglobin gene

Rational engineering of Halomonas salifodinae to enhance hydroxyectoine production under lower-salt conditions

Hydroxyectoine is an important compatible solute that holds potential for development into a high-value chemical with broad applications. However, the traditional high-salt fermentation for hydroxyectoine production presents challenges in treating the high-salt wastewater. Here, we report the rational engineering of Halomonas salifodinae to improve the bioproduction of hydroxyectoine under lower-salt conditions. The comparative transcriptomic analysis suggested that the increased expression of ectD gene encoding ectoine hydroxylase (EctD) and the decreased expressions of genes responsible for tricarboxylic acid (TCA) cycle contributed to the increased hydroxyectoine production in H. salifodinae IM328 grown under high-salt conditions. By blocking the degradation pathway of ectoine and hydroxyectoine, enhancing the expression of ectD, and increasing the supply of 2-oxoglutarate, the engineered H. salifodinae strain HS328-YNP15 (ΔdoeA::PUP119-ectD p-gdh) produced 8.3-fold higher hydroxyectoine production than the wild-type strain and finally achieved a hydroxyectoine titer of 4.9 g/L in fed-batch fermentation without any detailed process optimization. This study shows the potential to integrate hydroxyectoine production into open unsterile fermentation process that operates under low-salinity and high-alkalinity conditions, paving the way for next-generation industrial biotechnology. KEY POINTS: • Hydroxyectoine production in H. salifodinae correlates with the salinity of medium • Transcriptomic analysis reveals the limiting factors for hydroxyectoine production • The engineered strain produced 8.3-fold more hydroxyectoine than the wild type.

Utilization of gene manipulation system for advancing the biotechnological potential of halophiles: A review

Halophiles are salt-loving microorganisms known to have their natural resistance against media contamination even when cultivated in nonsterile and continuous bioprocess system, thus acting as promising cell factories for Next Generation of Industrial Biotechnology (NGIB). NGIB - a successor to the traditional industrial biotechnology, is a more sustainable and efficient bioprocess technology while saving energy and water in a more convenient way as well as reducing the investment cost and skilled workforce requirement. Numerous studies have achieved intriguing outcomes during synthesis of different metabolite using halophiles such as polyhydroxyalkanoates (PHA), ectoine, biosurfactants, and carotenoids. Present-day development in genetic maneuverings have shown optimistic effects on the industrial applications of halophiles. However, viable and competent genetic manipulation system and gene editing tools are critical to accelerate the process of halophile engineering. With the aid of such powerful gene manipulation systems, exclusive microbial chassis are being crafted with desirable features to breed another innovative area of research such as synthetic biology. This review provides an aerial perspective on how the expansion of adaptable gene manipulation toolkits in halophiles are contributing towards biotechnological advancement, and also focusses on their subsequent application for production improvement. This current methodical and comprehensive review will definitely help the scientific fraternity to bridge the gap between challenges and opportunities in halophile engineering.

Identification of the Biosynthetic Pathway of Glycine Betaine That Is Responsible for Salinity Tolerance in Halophilic Thioalkalivibrio versutus D301

Thioalkalivibrio versutus D301 has been widely used in the biodesulfurization process, as it is capable of oxidizing hydrogen sulfide to elemental sulfur under strongly halo-alkaline conditions. Glycine betaine contributes to the increased tolerance to extreme environments in some of Thioalkalivibrio species. However, the biosynthetic pathway of glycine betaine in Thioalkalivibrio remained unknown. Here, we found that genes associated with nitrogen metabolism of T. versutus D301 were significantly upregulated under high-salt conditions, causing the enhanced production of glycine betaine that functions as a main compatible solute in response to the salinity stress. Glycine betaine was synthesized by glycine methylation pathway in T. versutus D301, with glycine N-methyltransferase (GMT) and sarcosine dimethylglycine N-methyltransferase (SDMT) as key enzymes in this pathway. Moreover, substrate specificities of GMT and SDMT were quite different from the well characterized enzymes for glycine methylation in halophilic Halorhodospira halochloris. Our results illustrate the glycine betaine biosynthetic pathway in the genus of Thioalkalivibrio for the first time, providing us with a better understanding of the biosynthesis of glycine betaine in haloalkaliphilic Thioalkalivibrio.

Development of whole-cell catalyst system for sulfide biotreatment based on the engineered haloalkaliphilic bacterium

Microorganisms play an essential role in sulfide removal. Alkaline absorption solution facilitates the sulfide’s dissolution and oxidative degradation, so haloalkaliphile is a prospective source for environmental-friendly and cost-effective biodesulfurization. In this research, 484 sulfide oxidation genes were identified from the metagenomes of the soda-saline lakes and a haloalkaliphilic heterotrophic bacterium Halomonas salifodinae IM328 (=CGMCC 22183) was isolated from the same habitat as the host for expression of a representative sequence. The genetic manipulation was successfully achieved through the conjugation transformation method, and sulfide: quinone oxidoreductase gene (sqr) was expressed via pBBR1MCS derivative plasmid. Furthermore, a whole-cell catalyst system was developed by using the engineered strain that exhibited a higher rate of sulfide oxidation under the optimal alkaline pH of 9.0. The whole-cell catalyst could be recycled six times to maintain the sulfide oxidation rates from 41.451 to 80.216 µmol·min⁻¹·g⁻¹ dry cell mass. To summarize, a whole-cell catalyst system based on the engineered haloalkaliphilic bacterium is potentiated to be applied in the sulfide treatment at a reduced cost.

Recent advances in microbial capture of hydrogen sulfide from sour gas via sulfur-oxidizing bacteria

Biological desulfurization offers several remarkably environmental advantages of operation at ambient temperature and atmospheric pressure, no demand of toxic chemicals as well as the formation of biologically re-usable sulfur (S0), which has attracted increasing attention compared to conventionally physicochemical approaches in removing hydrogen sulfide from sour gas. However, the low biomass of SOB, the acidification of process solution, the recovery of SOB, and the selectivity of bio-S0 limit its industrial application. Therefore, more efforts should be made in the improvement of the BDS process for its industrial application via different research perspectives. This review summarized the recent research advances in the microbial capture of hydrogen sulfide from sour gas based on strain modification, absorption enhancement, and bioreactor modification. Several efficient solutions to limitations for the BDS process were proposed, which paved the way for the future development of BDS industrialization.

Recent Advances in the Physicochemical Properties and Biotechnological Application of Vitreoscilla Hemoglobin

Vitreoscilla hemoglobin (VHb), the first discovered bacterial hemoglobin, is a soluble heme-binding protein with a faster rate of oxygen dissociation. Since it can enhance cell growth, product synthesis and stress tolerance, VHb has been widely applied in the field of metabolic engineering for microorganisms, plants, and animals. Especially under oxygen-limited conditions, VHb can interact with terminal oxidase to deliver enough oxygen to achieve high-cell-density fermentation. In recent years, with the development of bioinformatics and synthetic biology, several novel physicochemical properties and metabolic regulatory effects of VHb have been discovered and numerous strategies have been utilized to enhance the expression level of VHb in various hosts, which greatly promotes its applications in biotechnology. Thus, in this review, the new information regarding structure, function and expressional tactics for VHb is summarized to understand its latest applications and pave a new way for the future improvement of biosynthesis for other products.