Online monitoring applying the anaerobic respiratory monitoring system reveals iron(II) limitation in YTF medium for Clostridium ljungdahlii

Sampling-free investigation of microbial carbon source preferences on renewable feedstocks via online monitoring of oxygen transfer rate

The transition towards sustainable bioprocesses requires renewable feedstocks to reduce dependency on finite resources. While plant-based feedstocks offer significant potential, their complex composition poses new challenges. The microorganisms often exhibit polyauxic growth when presented with multiple carbon sources simultaneously, consuming them in a distinct order according to their carbon source preferences. The traditional investigation of polyauxic growth involves laborious sampling and offline analysis, hindering high-throughput screenings. This study introduces an efficient method for identifying carbon source consumption and their order of metabolization by various microorganisms using the respiration activity monitoring system (RAMOS) in shake flasks. As aerobic carbon metabolization and oxygen consumption are strictly correlated, the characteristic phases of polyauxic growth are visible in the oxygen transfer rate (OTR) and can be assigned to the respective carbon sources. An extended 16-flask RAMOS enables real-time monitoring of microbial respiration on up to seven carbon sources and one reference cultivation simultaneously, thus providing crucial insights into their metabolization without extensive sampling and offline analysis. The method's accuracy was validated against traditional high-performance liquid chromatography (HPLC). Its applicability to both fast-growing Escherichia coli (investigated carbon sources: glucose, arabinose, sorbitol, xylose, and glycerol) and slow-growing Ustilago trichophora (glucose, glycerol, xylose, sorbitol, rhamnose, galacturonic acid, and lactic acid) was demonstrated. Additionally, it was successfully applied to the plant-based second-generation feedstock corn leaf hydrolysate, revealing the bioavailability of the included carbon sources (glucose, sucrose, arabinose, xylose, and galactose) and their order of metabolization by Ustilago maydis.

Hydrogen online monitoring based on thermal conductivity for anaerobic microorganisms

H₂ -producing microorganisms are a promising source of sustainable biohydrogen. However, most H₂ -producing microorganisms are anaerobes, which are difficult to cultivate and characterize. While several methods for measuring H₂ exist, common H₂ sensors often require oxygen, making them unsuitable for anaerobic processes. Other sensors can often not be operated at high gas humidity. Thus, we applied thermal conductivity (TC) sensors and developed a parallelized, online H₂ monitoring for time-efficient characterization of H₂ production by anaerobes. Since TC sensors are nonspecific for H₂ , the cross-sensitivity of the sensors was evaluated regarding temperature, gas humidity, and CO₂ concentrations. The systems' measurement range was validated with two anaerobes: a high H₂ -producer (Clostridium pasteurianum) and a low H₂ -producer (Phocaeicola vulgatus). Online monitoring of H₂ production in shake flask cultivations was demonstrated, and H₂ transfer rates were derived. Combined with online CO₂ and pressure measurements, molar gas balances of the cultivations were closed, and an anaerobic respiration quotient was calculated. Thus, insight into the effect of medium components and inhibitory cultivation conditions on H₂ production with the model anaerobes was gained. The presented online H₂ monitoring method can accelerate the characterization of anaerobes for biohydrogen production and reveal metabolic changes without expensive equipment and offline analysis.

Impact of different trace elements on metabolic routes during heterotrophic growth of C. ljungdahlii investigated through online measurement of the carbon dioxide transfer rate

Synthesis gas fermentation using acetogenic clostridia is a rapidly increasing research area. It offers the possibility to produce platform chemicals from sustainable C1 carbon sources. The Wood-Ljungdahl pathway (WLP), which allows acetogens to grow autotrophically, is also active during heterotrophic growth. It acts as an electron sink and allows for the utilization of a wide variety of soluble substrates and increases ATP yields during heterotrophic growth. While glycolysis leads to CO₂ evolution, WLP activity results in CO₂ fixation. Thus, a reduction of net CO₂ emissions during growth with sugars is an indicator of WLP activity. To study the effect of trace elements and ventilation rates on the interaction between glycolysis and the WLP, the model acetogen Clostridium ljungdahlii was cultivated in YTF medium, a complex medium generally employed for heterotrophic growth, with fructose as growth substrate. The recently reported anaRAMOS device was used for online measurement of metabolic activity, in form of CO₂ evolution. The addition of multiple trace elements (iron, cobalt, manganese, zinc, nickel, copper, selenium, and tungsten) was tested, to study the interaction between glycolysis and the Wood ljungdahl pathway. While the addition of iron(II) increased growth rates and ethanol production, added nickel(II) increased WLP activity and acetate formation, reducing net CO₂ production by 28%. Also, higher CO₂ availability through reduced volumetric gas flow resulted in 25% reduction of CO₂ evolution. These online metabolic data demonstrate that the anaRAMOS is a valuable tool in the investigation of metabolic responses i.e. to determine nutrient requirements that results in reduced CO₂ production. Thereby the media composition can be optimized depending on the specific goal. This article is protected by copyright. All rights reserved.

Online monitoring of gas transfer rates during CO and CO/H2 gas fermentation in quasi-continuously ventilated shake flasks

Syngas fermentation is a potential player for future emission reduction. The first demonstration and commercial plants have been successfully established. However, due to its novelty, development of syngas fermentation processes is still in its infancy, and the need to systematically unravel and understand further phenomena, such as substrate toxicity as well as gas transfer and uptake rates, still persists. This study describes a new online monitoring device based on the respiration activity monitoring system for cultivation of syngas fermenting microorganisms with gaseous substrates. The new device is designed to online monitor the carbon dioxide transfer rate (CO₂ TR) and the gross gas transfer rate during cultivation. Online measured data are used for the calculation of the carbon monoxide transfer rate (COTR) and hydrogen transfer rate (H₂ TR). In cultivation on pure CO and CO + H₂ , CO was continuously limiting, whereas hydrogen, when present, was sufficiently available. The maximum COTR measured was approximately 5 mmol/L/h for pure CO cultivation, and approximately 6 mmol/L/h for cultivation with additional H₂ in the gas supply. Additionally, calculation of the ratio of evolved carbon dioxide to consumed monoxide, similar to the respiratory quotient for aerobic fermentation, allows the prediction of whether acetate or ethanol is predominantly produced. Clostridium ljungdahlii, a model acetogen for syngas fermentation, was cultivated using only CO, and CO in combination with H₂ . Online monitoring of the mentioned parameters revealed a metabolic shift in fermentation with sole CO, depending on COTR. The device presented herein allows fast process development, because crucial parameters for scale-up can be measured online in small-scale gas fermentation.