Half-saturation Constant Research Articles

Isolation and degradation characterization of a 1, 4-dioxane-degrading bacterial strain

To address the potential pollution caused by the carcinogen 1, 4-dioxane in aquatic environments, we isolated a highly efficient 1, 4-dioxane-degrading bacterial strain, designated as DXTK-010, from the groundwater contaminated by 1, 4-dioxane. According to the morphological characteristics, the phylogenetic tree established based on the 16S rRNA gene sequence, and the whole genome sequence, we identified DXTK-010 as Aminobacter aminovorans. This strain demonstrated robust degradation capacity within a temperature range of 20 ℃ to 37 ℃ and a pH range of 5.0 to 8.0. Furthermore, single-factor experiments indicated the optimal degradation conditions at 30 ℃ and pH 7.5. Under the optimal conditions, the strain completely degraded 200 mg/L of 1, 4-dioxane within 24 h, achieving a maximum degradation rate of 9.367 mg/(L·h). The Monod equation was adopted to fit the degradation kinetics of 1, 4-dioxane at different initial concentrations, which revealed a maximum specific degradation rate of 0.224 mg 1, 4-dioxane/(mg protein·h), a half-saturation constant (Ks) of 41.350 mg/L, and a cell yield of 0.130 mg protein/(mg 1, 4-dioxane). Whole genome sequencing revealed a circular chromosome and three plasmids within DXTK-010. Functional gene annotation and analysis underscored the significance of the propane monooxygenase gene cluster and alcohol dehydrogenase gene in facilitating the efficient degradation of 1, 4-dioxane by this strain. DXTK-010 outperformed the existing degraders for 1, 4-dioxane, expanding the strain resources for the bioremediation of 1, 4-dioxane pollution. This study provides a theoretical basis for the practical application of DXTK-010 in the remediation of 1, 4-dioxane pollution.

Spatiotemporal dynamics of toxin producing phytoplankton–zooplankton interactions with Holling Type II functional responses

In this paper, we have studied the spatiotemporal dynamics of phytoplankton-zooplankton interactions using toxin-producing phytoplankton (TPP) with Holling type II functional responses. Toxin-producing phytoplankton (TPP) diffuses and reduces the grazing pressure on zooplankton. The grazing pressure of zooplankton is lessened by toxin-producing phytoplankton (TPP). The temporal system identifies all equilibrium points. Boundedness and local stability are established under specific parametric conditions. The conditions for the existence of a Hopf-bifurcation at the positive equilibrium by taking the half-saturation constant (b1), are also discussed. In a spatial system, we explored Turing instability conditions and patterns with an emphasis on the effect of diffusion variation. Furthermore, we obtained the time evaluation pattern formation of the spatial system. Moreover, the communities of toxin-producing phytoplankton (TPP) are essential to the marine ecosystem because they lessen the mortality of zooplankton caused by grazing pressure. Finally, the basic outcomes are mentioned along with numerical results to provide some support to the analytical findings.

Characterisation of Pseudomonas mosselii H6 for aerobic denitrification: stoichiometry and reaction kinetics

The application of aerobic denitrifying bacteria in biological nitrogen removal has been of great interest. However, the mechanism of nitrogen conversion by aerobic denitrifying bacteria is unclear due to the lack of stoichiometry and kinetic studies. Therefore, this study aimed to investigate the stoichiometry and kinetics of aerobic denitrification using Pseudomonas mosselii H6 as a model strain. The results showed that the stoichiometric equations of strain H6 under different nitrogen source conditions were obtained by nitrogen balance analysis; the concentrations of different limiting substrates were set and combined with the multi-substrate Monod kinetic equation to fit the half-saturation constants of COD, ammonia, nitrate and nitrite nitrogen for strain H6, which were 230.1008, 25.3435, 4.1009 and 23.4601 mg/L, respectively; based on the model of “two-step denitrification”, the numerical simulation of the growth and aerobic denitrification of strain H6 under different limiting substrate concentrations was carried out by using AQUASIM software to test the validity of stoichiometric coefficients and kinetic constants and perfect fitting results were obtained. These parameters will help to simulate the growth and denitrification performance of aerobic denitrifying bacteria and provide an experimental basis for further quantitatively elucidating the mechanism of action in the aerobic denitrification process.

Microbial community respiration kinetics and their dynamics in coastal seawater

Oxygen (O2) concentrations in coastal seawater have been declining for decades and models predict continued deoxygenation into the future. As O2 declines, metabolic energy use is progressively channelled from higher trophic levels into microbial community respiration, which in turn influences coastal ecology and biogeochemistry. Despite its critical role in deoxygenation and ecosystem functioning, the kinetics of microbial respiration at low O2 concentrations in coastal seawater remain uncertain and are mostly modeled based on parameters derived from laboratory cultures and a limited number of environmental observations. To explore microbial responses to declining O2, we measured respiration kinetics in coastal microbial communities in Hong Kong over the course of an entire year. We found the mean maximum respiration rate (Vmax) ranged between 560 ± 280 and 5930 ± 800 nmol O2 L−1 h−1, with apparent half-saturation constants (Km) for O2 uptake of between 50 ± 40 and 310 ± 260 nmol O2 L−1. These kinetic parameters vary seasonally in association with shifts in microbial community composition that were linked to nutrient availability, temperature, and biological productivity. In general, coastal communities in Hong Kong exhibited low affinities for O2, yet communities in the dry season had higher affinities, which may play a key role in shaping the relationship between community size, biomass, and O2 consumption rates through respiration. Overall, parameters derived from these experiments can be employed in models to predict the expansion of deoxygenated waters and associated effects on coastal ecology and biogeochemistry.

Modeling the differential functional responses and selectivity of a marine copepod to nano/microplastics in mixture

Nano- and microplastics (NMPs) pollution is widespread in the oceans, posing potential risks to marine species. This study examined the accumulation capacity and selectivity potentials of NMPs by a marine copepod Parvocalanus crassirostris under different food mixtures by modeling the combined biokinetic and functional response. We investigated two sizes of NMPs (200 nm and 5 µm) across a concentration gradient (0 - 5000 µg/L) and varying diatom abundances (0, 104, 105 cells/mL). Fluorescence imaging and quantification revealed that P. crassirostris actively ingested NMPs at low concentration. Accumulation increased with NMPs concentration but eventually saturated due to gut capacity limits, following a Holling type II functional response (i.e., hyperbolic curve). Our novel functional response model estimated the key parameters and demonstrated that the maximum accumulation reached 5.3 % of dry weight with averaged half-saturation constants of 229 µg/L. The size of NMPs did not significantly affect the total accumulation or satiety levels. The presence of diatoms influenced the feeding selectivity and decreased the microplastic accumulation by 73 % at 105 cells/mL, while facilitating nanoplastic accumulation by 81 % at 104 cells/mL. This study enhanced our understanding of NMPs bioavailability and environmental fate in marine ecosystems.

Microalgae in the Mediterranean area: A geographical survey outlining the diversity and technological potential

Microalgal diversity enables the possibility to employ them in technological applications, as widely shown by the modern literature. While there exists an extensive body of literature concerning the technological applications of microalgae, the scientific knowledge of microalgal species remains relatively limited. Therefore, there is still potential for unlocking new opportunities through the study of the microalgal biodiversity, particularly in the Mediterranean region, which is unique because of its sub regional diversity. While some studies have assessed microalgae distribution in the Mediterranean area, and others have focused on specific aspects of their technical exploitation, this review seeks to offer a comprehensive overview of isolated microalgal species and their technological applications. Microalgae from the Mediterranean area share common characteristics, such as low half-saturation constants and acclimation to high light intensity, making them ideal for specific technological applications. While the search for new microalgae for technological purposes can help in biodiversity conservation, numerous species still remain underexplored, offering potential for innovative applications. However, the key finding from the critical analysis of the literature is that the diversity of microalgae in the Mediterranean region is its true richness, allowing for their versatile applications across various processes. The work focuses on the Mediterranean area, i.e., having coastlines along the Mediterranean Sea and on aquatic microalgae, coming from water with different salinity levels. This review offers an intrinsic ecological and technological perspective and provides a fresh outlook on the microalgal sector, promoting its expansion in the Mediterranean area and the development of sustainable bio-industries.

Biodegradability and formulation of triclosan-degrading and plant-growth-promoting Pseudomonas sp. MS45

Abstract. Sipahutar MK. 2024. Biodegradability and formulation of triclosan-degrading and plant-growth-promoting Pseudomonas sp. MS45. Biodiversitas 25: 2980-2992. This study presents the isolation of Pseudomonas sp. MS45, a microorganism that can consume triclosan (TCS) as its only carbon and energy source, leading to its detoxification. The rarity of a toxicity study of TCS before and after biodegradation treatment is addressed in this study. Allium cepa's genotoxicity exposed TCS's cytotoxicity, generating multiple damaged chromosomes, in contrast to the insignificant consequences of its broken-down metabolites. The study also optimized the different kinetic parameters for TCS-specific biodegradation and Pseudomonas sp. MS45-specific growth. The biodegradation pattern followed the Michaelis-Menten model with a maximum specific biodegradation rate (Vmax) of 0.01 µmole h-1 mg cell protein-1 and a half saturation constant (Ks) of 21.98 µM. The growth pattern followed the Haldane model with a maximum specific growth rate (µmax) of 0.03 h-1, half saturation constant (Ks) of 4.56 µM, and TCS inhibition constant (Ki) of 32.81 µM. In addition to its TCS-degrading ability, Pseudomonas sp. MS45 also possesses various plant growth promotion traits, and it was incorporated into two solid carrier formulations: vermiculite and rice bran. The study found that vermiculite, especially with PEG, was the most consistent formulation for Pseudomonas sp. MS45, in terms of maintaining bacterial survival and TCS-degrading ability. This study marks the initial report on the biodegradation and formulation of TCS-destroying and Plant-Growth-Promoting (PGP) Pseudomonas sp. MS45, offering promising implications for environmental and agricultural applications.

A model of time-dependent macromolecular and elemental composition of phytoplankton

Phytoplankton Chl:C:N:P ratios are important from both an ecological and a biogeochemical perspective. We show that these elemental ratios can be represented by a phytoplankton physiological model of low complexity that includes major cellular macromolecular pools. In particular, our model resolves time-dependent intracellular pools of chlorophyll, proteins, nucleic acids, carbohydrates/lipids, and N and P storage. Batch culture data for two diatom and two prasinophyte species are used to constrain parameters that represent specific allocation traits and strategies. A key novelty is the simultaneous estimation of physiological parameters for two phytoplankton groups of such different sizes. The number of free parameters is reduced by assuming (i) allometric scaling for maximum uptake rates, (ii) shared half-saturation constants for synthesis of functional macromolecules, (iii) shared exudation rates of functional macromolecules across the species. The rationale behind this assumption is that across the different species, the same or similar processes, enzymes, and metabolites play a role in key physiological processes. For the turnover numbers of macromolecular synthesis and storage exudation rates, differences between diatoms and prasinophytes need to be taken into account to obtain a good fit. Our model fits suggest that the parameters related to storage dynamics dominate the differences in the C:N:P ratios between the different phytoplankton groups. Since descriptions of storage dynamics are still incomplete and imprecise, predictions of C:N:P ratios by phytoplankton models likely have a large uncertainty.

Evaluating the contribution of methanotrophy kinetics to uncertainty in the soil methane sink

The oxidation of atmospheric methane by soil microbes is an important natural sink for a potent greenhouse gas. However, estimates of the current and future soil methane sink are highly uncertain. Here we assessed the extent to which methanotrophy enzyme kinetics contribute to uncertainty in projections of the soil methane sink. We generated a comprehensive compilation of methanotrophy kinetic data from modern environments and assessed the patterns in kinetic parameters present in natural samples. Our compiled data enabled us to quantify the global soil methane sink through two idealized calculations comparing first-order and Michaelis–Menten models of kinetics. We show that these two kinetic models diverge only under high atmospheric CH4 scenarios, where first-order rate constants slightly overestimate the soil methane sink size, but produce similar predictions at modern atmospheric concentrations. Our compilation also shows that the kinetics of methanotrophy in natural soil samples is highly variable—both the V max (oxidation rate at saturation) and KM (half-saturation constant) in natural samples span over six orders of magnitude. However, accounting for the correlation we observe between V max and KM reduces the range of calculated uptake rates by as much as 96%. Additionally, our results indicate that variation in enzyme kinetics introduces a similar magnitude of variation in the calculated soil methane sink as temperature sensitivity. Systematic sampling of methanotroph kinetic parameters at multiple spatial scales should therefore be a key objective for closing the budget on the global soil methane sink.

Phosphomonoesterase and phosphodiesterase activities in the eastern Mediterranean in two contrasting seasonal situations

Abstract. Hydrolysis of dissolved organic phosphorus by marine planktonic microorganisms is a key process in the P cycle, particularly in P-depleted, oligotrophic environments. The present study assessed spatiotemporal variations in phosphomonoesterase (PME) and phosphodiesterase (PDE) activities using concentration kinetics in the eastern Mediterranean Sea in two contrasting situations: the end of winter (including a small bloom period) and autumn. The distribution and regulation of the maximum hydrolysis rate (Vm) and half-saturation constant (Km) of both ectoenzymes were assessed in relation to the vertical structure of the epipelagic layers. PME reached its maximum activities (Vm) after the addition of 1 µM MUF-P (4-methylumbelliferyl phosphate), whereas, for PDE, it was necessary to add up to 50 µM bis(4-methylumbelliferyl)phosphate (bis-MUF-P) to reach saturation state. On average, the Km of PDE was 33 ± 25 times higher than that of PME. The Vm of PME and Vm of PDE were linearly correlated. Conversely to the Km values, Vm values were on the same order of magnitude for both ectoenzymes, with their ratio (Vm PME : Vm PDE) ranging between 0.2 and 6.3. Dissolved organic phosphorus (DOP) and the phosphomonoesterase hydrolysable fraction of DOP explained most of the lack of variability in Vm PME and Vm PDE. On the contrary, Vm of both phosphohydrolase enzymes was inversely correlated to the concentration of dissolved inorganic phosphorus. The particular characteristics of concentration kinetics obtained for PDE (saturation at 50 µM, high Km, high turnover times) are discussed with respect to the possible unequal distribution of PDE and PME among the size continuum of organic material and accessibility of phosphodiesters.

Fructooligosaccharides: Production by recombinant fructosyltransferase from Festuca arundinacea in a continuous reactor and kinetic modeling profile

In the quest for maximum conversion of sucrose into 1-kestose using fructosyltransferases in a continuous reactor, consideration of the reversible reactions at competing concentrations of sucrose and 1-kestose is crucial. In this study, a kinetic model based on a combined approach of rapid equilibrium and steady-state kinetics was developed to predict the profile of FOS production using recombinant sucrose: sucrose 1-fructosyltransferase from Festuca arundinacea produced in Pichia pastoris. The purified enzyme selectively converted sucrose into 1-kestose with a final yield of 0.66 g/g sucrose consumed. The reaction rate laws adequately described the kinetic profile of the substrate and the products and accurately predicted the composition of the reaction mixture. The model was fitted to experimental data obtained at three enzyme dosages and three initial sucrose concentrations. The model could predict the effect of substrate concentration on initial rate kinetics, half-saturation constant, and transferase to hydrolase ratio. For the known values of initial sucrose and enzyme concentration, the optimum reaction time and the required dilution rate were predicted for the development of a continuous process to produce 1-kestose-enriched short-chain FOS. A volumetric productivity of ∼110 g/L/h of 1-kestose with 40 % carbohydrate composition in the permeate flux was demonstrated.

Machine learning approach for the effectual production of a novel esterase and its application in bioremediation of dairy effluent

The present work intends to maximize the production of an industrially important novel esterase from Trichoderma harzianum using both machine learning and statistical approaches and its application in the bioremediation of dairy effluent. Significant medium compounds, such as almond oil, casein, NaH2PO4, and trace elements, were screened from a pool of carbon, nitrogen, oil and surfactant sources along with other micronutrients using a first order statistical design. The optimum value of screened parameters was found using response surface methodology that resulted in the maximum esterase production of 2985.31 U/L. Artificial neural network trained with the experimental response of RSM experiments coupled with genetic algorithm optimization technique has resulted in the maximum esterase production of 3256.73 U/L, which is 150-folds higher as compared to the unoptimized condition and also found to be the higher than the outcome of statistical optimization. Dairy effluent was used for the production of esterase as well as for its bioremediation. At 50% load, the maximum reduction in chemical oxygen demand and the production of esterase were observed to be 46.84% and 34.12 U/L, respectively. The COD reduction of diary effluent was modelled using Michaelis-Menten kinetics and the kinetic parameters, half saturation constant (Km) and maximum reaction rate (rmax) value were evaluated as 286.88 mg/L and 26.04 mg/L/d, respectively.

Crucial impact of component Allee effect in predator-prey system

Allee effect in models in interacting species of predator prey system has great significance in ecological context. Allee effect plays crucial role in population dynamics in ecology, where it is the challenging fact that per capita population growth rate is positively dependent on the population density of a species. In this paper, we inspect the famous Hastings and Powell (HP) (Hastings and Powell 1991 Ecology 72 896–903) model incorporating component Allee effect on top predator’s reproduction. We analyse the updated model with the help of both analytical and numerical phenomena. Considering cost of Allee effect, half-saturation constant of prey as the key parameters, the Hopf bifurcations are also analysed. The directions of Hopf bifurcations at the critical values of Allee parameter and half-saturation constant of prey are studied theoretically by using normal form theory introduced by Hassard et al (1981 Theory and Applications of Hopf Bifurcation vol 41 (CUP Archive)). The formulated model indicates that the system demonstrates chaotic, periodic and stable dynamics in the variation of key parameters. The chaos can be controlled for proper application of the large values of parameter used as the cost of Allee effect and also for small values of the parameter used as the half saturation constant of prey population. The results of this study are applicable in the field of marine and wild ecosystem dynamics.

The superiority of hydrophilic polyurethane in comammox-dominant ammonia oxidation during low-strength wastewater treatment

Carriers have been extensively employed to enhance nitrification performance during low-strength wastewater treatment by retaining slow-growing ammonia oxidizing microorganisms (AOMs). Still, there is a dearth of systematic understanding of biofilm properties and microbial community structure formed on different carriers. In this study, hydrophilic polyurethane foam (PUF) carriers were prepared and compared with five widely used commercial carriers, namely Kaldness 3, Biochip, activated carbon, volcanic rock, and zeolite. The results indicated that the biofilms formed on carriers enhanced microbial ammonia oxidation activity. Additionally, the biofilm developed on the PUF demonstrated the most superior performance among all selected carriers, not only exhibiting the highest abundant and the most active AOMs, with amoA gene abundance of 1.41 × 1013 copies/m3 and specific ammonia oxidation rate of 9.84 g NH4+-N/(m3 × h), but also possessing a compact structure, with 3.41 kg VSS/m3 and 46.83 mg extracellular polymeric substances/g VSS. The high-throughput sequencing analysis revealed that the comammox (CMX) Nitrospira dominated on biofilm due to the intrinsically low apparent half-saturation constant for substrate. A unique ecological community structure was established on PUF, characterized by low species diversity and high homogeneity in alignment with community characteristics of CMX. The biofilms on PUF contributed to the proliferation of CMX Nitrospira dominated by Nitrospira nitrosa, achieving the highest proportion among colonial three AOMs at 86.58 %. The appropriate average pore size, superior hydrophilicity, and large specific surface area of PUF carriers provided a robust foundation for the exceptional ammonia oxidation performance of the formed biofilms.

Mathematical analysis of the role of host-to-host transmission of Maize Streak Virus Disease with Atangana-Baleanu derivative

In this article a deterministic model for Maize Streak Virus Disease (MSVD) using a fractional-order differential equation with the Atangana-Baleanu Caputo-type operator is developed. Focusing on the role of host-to-host transmission, the it is shown that the presence and stability of equilibria depends on maize field carrying capacity and half-saturation constant of susceptible maize. The MSVD-free equilibrium is globally asymptotically stable when the basic reproduction number is below unity. Local stability conditions for endemic equilibria are established using Lyapunov second technique and Routh-Hurwitz criteria. Matrix-based formulae are also presented for determining bifurcation and it is shown that the model exhibits forward bifurcation. Sensitivity analysis reveals the significant impact of the probability of infection between hosts on MSVD spread. A two-point Lagrange interpolation polynomial is developed for numerical solutions of the model, enabling exploration of theoretical findings and assessment of how epidemiological factors influence MSVD propagation. The paper contributes a comprehensive understanding of the disease's behavior under various conditions.

Utilization of native Chlorella strain in laboratory-scale raceway reactor for synthetic wastewater treatment: A study in batch and continuous modes with multi-substrate modeling

Despite of the possibility to include microalgae in civil wastewater treatment process, the practice is still not common due to the lack of available instruments to implement it. In this study, a straightforward comprehensive approach for dealing with microalgal wastewater treatment involving an original kinetic model is proposed. A first set of batch cultures of a native strain of Chlorella was firstly carried out to obtain the kinetic parameters: maximum growth factor (μmax) and half-saturation constant (Ks), for each limiting nutrient. Maximum growth factor values of 0.0279 for PO43−, 0.0319 h−1 for NH4+, 0.0352 h−1 for glucose, and 0.0263 h−1 for the overall medium were found. Regarding the Ks, values of 1.08 mg L−1, 27.70 mg L−1, 1.34 mg L−1, were found for PO43−, NH4+ and glucose respectively, and a value of 2.8 % of the total nutrients for the overall medium. These parameters were used to set a multi-substrate kinetic model able to predict the growth in batch and in continuous operation within a laboratory scale raceway reactor. The removal capabilities of the microalgae for each addressed pollutant were evaluated in a batch system and in a continuous system at dilution rates of 0.0025, 0.005, 0.01 and 0.015 h−1. This comprehensive approach represents a significant step towards addressing the continuous treatment of wastewater utilizing microalgae.

Examining the effects of elevated CO2 on the growth kinetics of two microalgae, Skeletonema dohrnii (Bacillariophyceae) and Heterosigma akashiwo (Raphidophyceae)

Carbon dioxide (CO2) serves as the primary substrate for the photosynthesis of phytoplankton, forming the foundation of marine food webs and mediating the biogeochemical cycling of C and N. We studied the effects of CO2 variation on the Michaelis-Menten equations and elemental composition of Skeletonema dohrnii and Heterosigma akashiwo. CO2 functional response curves were conducted from 100 to 2000 ppm. The growth of both phytoplankton was significantly affected by CO2, but in different trends. The growth rate of S. dohrnii increased as CO2 levels rose up to 400 ppm before reaching saturation. In contrast to S. dohrnii, the growth rate of H. akashiwo increased with CO2 increasing up to 1000 ppm, and then CO2 saturated. In addition, H. akashiwo showed a slower growth rate than S. dohrnii for all CO2 concentrations, aside from 1000 ppm, and the Michaelis-Menten equations revealed that the half-saturation constant of H. akashiwo was higher than S. dohrnii. An increase in CO2 concentration was seen to significantly affected the POC: Chl-a of both S. dohrnii and H. akashiwo, however, the effects on their elemental composition were minimal. Overall, our findings indicate that H. akashiwo had a more positive reaction to elevated CO2 than S. dohrnii, and with higher nutrient utilization efficiency, while S. dohrnii exhibited higher carbon fixation efficiency, which is in line with their respective carbon concentrating mechanisms. Consequently, elevated CO2, either alone or in combination with other limiting factors, may significantly alter the relative relationships between these two harmful algal blooms (HAB) species over the next century.

Phytoplankton Dynamics and Biogeochemistry: Model Studies

The seasonal dynamics of the NE Black Sea phytoplankton follow the following pattern: small diatoms (spring) → coccolithophorid Emiliania huxleyi (late spring–early summer) → large diatoms (summer). Our hypothesis states that nitrogen and phosphorus concentrations regulate the seasonal phytoplankton dynamics. A minimum number of parameters is enough to understand the mechanisms of dominant species change. Based on the concept of intracellular regulation, the following parameters were evaluated: the minimum nitrogen and phosphorus quotas; half-saturation constants for nitrogen and phosphorus uptake; the maximum specific growth rate of the dominant phytoplankton species. Computational experiments on the model show the following: (1) in spring, a species with a high maximum specific growth rate becomes dominant; (2) in late spring and early summer, a species with a low minimum nitrogen quota and a low half-saturation constant for nitrogen uptake is observed; (3) in summer, a low minimum phosphorus quota and a low half-saturation constant for phosphorus uptake allow the species to become dominant.

Functional asymmetry of the urea transporter UT-B in human red blood cells.

We studied urea, thiourea, and methylurea transport and interaction in human red blood cells (RBCs) under conditions of self-exchange (SE), net efflux (NE), and net influx (NI) at pH 7.2. We combined four methods, a four-centrifuge technique, the Millipore-Swinnex filtering technique, the continuous flow tube method, and a continuous pump method to measure the transport of the 14C-labeled compounds. Under SE conditions, both urea and thiourea show perfect Michaelis-Menten kinetics with half-saturation constants, K½,SE (mM), of ≈300 (urea) and ≈20 (thiourea). The solutes show no concentration-dependent saturation under NE conditions. Under NI conditions, transport displays saturation or self-inhibition kinetics with a K½,NI (mM) of ≈210 (urea) and ≈20 (thiourea). Urea, thiourea, and methylurea are competitive inhibitors of the transport of analog solutes. This study supports the hypothesis that the three compounds share the same urea transport system (UT-B). UT-B functions asymmetrically as it saturates from the outside only under SE and NI conditions, whereas it functions as a high-capacity channel-like transporter under NE conditions. When the red blood cell enters the urea-rich kidney tissue, self-inhibition reduces the urea uptake in the cell. When the cell leaves the kidney, the channel-like function of UT-B implies that intracellular urea rapidly equilibrates with external urea. The net result is that the cell during the passage in the kidney capillaries carries urea to the kidney to be excreted while the urea transfer from the kidney via the bloodstream is minimized.NEW & NOTEWORTHY The kinetics of urea transport in red blood cells was determined by means of a combination of four methods that ensures a high time resolution. In the present study, we disclose that the urea transporter UT-B functions highly asymmetric being channel-like with no saturation under conditions of net efflux and saturable under conditions of net influx and self-exchange in the concentration range 1-1,000 mM (pH 7.2 and 25-38 °C).

Why do lactic acid bacteria thrive in chain elongation microbiomes?

Efficient waste management is necessary to transition towards a more sustainable society. An emerging trend is to use mixed culture biotechnology to produce chemicals from organic waste. Insights into the metabolic interactions between community members and their growth characterization are needed to mediate knowledge-driven bioprocess development and optimization. Here, a granular sludge bioprocess for the production of caproic acid through sugar-based chain elongation metabolism was established. Lactic acid and chain-elongating bacteria were identified as the two main functional guilds in the granular community. The growth features of the main community representatives (isolate Limosilactobacillus musocae G03 for lactic acid bacteria and type strain Caproiciproducens lactatifermentans for chain-elongating bacteria) were characterized. The measured growth rates of lactic acid bacteria (0.051 ± 0.005h-1) were two times higher than those of chain-elongating bacteria (0.026 ± 0.004h-1), while the biomass yields of lactic acid bacteria (0.120 ± 0.005g biomass/g glucose) were two times lower than that of chain-elongating bacteria (0.239 ± 0.007g biomass/g glucose). This points towards differential growth strategies, with lactic acid bacteria resembling that of a r-strategist and chain-elongating bacteria resembling that of a K-strategist. Furthermore, the half-saturation constant of glucose for L. mucosae was determined to be 0.35 ± 0.05g/L of glucose. A linear trend of caproic acid inhibition on the growth of L. mucosae was observed, and the growth inhibitory caproic acid concentration was predicted to be 13.6 ± 0.5g/L, which is the highest reported so far. The pre-adjustment of L. mucosae to 4g/L of caproic acid did not improve the overall resistance to it, but did restore the growth rates at low caproic acid concentrations (1-4g/L) to the baseline values (i.e., growth rate at 0g/L of caproic acid). High resistance to caproic acid enables lactic acid bacteria to persist and thrive in the systems intended for caproic acid production. Here, insights into the growth of two main functional guilds of sugar-based chain elongation systems are provided which allows for a better understanding of their interactions and promotes future bioprocess design and optimization.