Controlling the membrane fluidity of yeasts during coupled thermal and osmotic treatments

A Closer Look at the Potential Mechanisms of Action of Protective Agents Used in the Drying of Microorganisms: A Review

Yeast, bacteria and sourdough are widely used in our daily lives, yet their drying and storage remains a significant challenge. A variety of techniques are used to improve the resistance of cells to thermal, dehydration, oxidative and osmotic stresses, which can occur at different stages of the process. The addition of protective agents prior to drying is a commonly used method, but the mechanisms that may lead to a change in viability following the addition of these agents, or more generally, the interaction between a protective agent and the drying process, are not yet fully understood. This review outlines seven main potential mechanisms, as highlighted in the literature, which can lead to internal or external modifications of the cells. The mechanisms in question are change of membrane fluidity, accumulation of compounds for osmoregulation, prior osmotic dehydration, prevention of oxidation, coating or encapsulation, enhancement in thermal resistance and change in drying kinetics. A comprehensive explanation of these mechanisms is provided. This review also highlights the connection between the mechanisms and the influence of the stresses occurring during drying and storage, which depend on the drying technique used and the operating conditions, the strains and the protective agents involved, on the importance of the different protection mechanisms. By gaining a deeper understanding of the mechanisms of action of protective agents, strategies to improve the quality of the microorganisms obtained after drying can be developed. One such strategy would be to combine several agents to achieve a synergistic effect.

Successful yeast microencapsulation of fluralaner and its potential as a larvicide for vector control

Fly control for any species is most effectively implemented in the immature stages when insects can be eliminated before emerging as adults capable of transmitting pathogens or becoming nuisance pests. Yet a limited number of insecticide classes are available for treating larval development sites for dipteran pest species. The most recently introduced class of insecticides in the United States (US) is the isoxazolines, including fluralaner. In the US, fluralaner is currently exclusively labeled for use against ectoparasites in companion animals. However, research has shown that it has a wider effective target range beyond ectoparasites and could be developed as an insecticide for vector control. Here we tested a novel, proprietary, yeast microencapsulated (YME) formulation of fluralaner against the larvae of three pest species: Musca domestica L. (Diptera: Muscidae), Aedes albopictus Skuse (Diptera: Culicidae), and Culicoides sonorensis Wirth and Jones (Diptera: Ceratopogonidae). These species all naturally consume microorganisms as larvae, including yeasts. Fluralaner was successfully microencapsulated in Saccharomyces cerevisiae yeast. YME fluralaner was reconstituted in water at concentrations of 0.00001-0.1 mg/mL (Aedes and Culicoides) or 1-50 mg/mL (Musca) for use in dose-response assays. For each species, the LC50 at 24 h was estimated using probit analyses. YME fluralaner was highly effective against all species tested (Ae. albopictus LC50 = 0.000077 mg/mL; C. sonorensis LC50 = 0.00067 mg/mL; M. domestica LC50 = 2.58 mg/mL). Additionally, laboratory assays were conducted to determine product reapplication rates using LC50 rates. Reapplication rates to maintain <50 % emergence were five weeks (Ae. albopictus) and greater than eight weeks (C. sonorensis). The results presented here indicate YME fluralaner is a promising candidate for controlling larval insects that naturally feed on detritus, thereby bypassing cuticular penetration barriers and safely delivering the active ingredient to the target species.

A Quantitative Re-Assessment of Microencapsulation in (Pre-Treated) Yeast

Most hydrophobes easily diffuse into yeast cells, where they experience reduced evaporation and protection from oxidation, thus allowing inherently biocompatible encapsulation processes. Despite a long-standing industrial interest, the effect of parameters such as how is yeast pre-treated (extraction with ethanol, plasmolysis with hypertonic NaCl, depletion to cell walls), the polarity of the hydrophobes and the process conditions are still not fully understood. Here, we have developed thorough analytical protocols to assess how the effects of the above on S. cerevisiae's morphology, permeability, and encapsulation efficiency, using three differently polar hydrophobes (linalool, 1,6-dihydrocarvone, limonene) and three separate processes (hydrophobes as pure 'oils', water dispersions, or acetone solutions). The harsher the pre-treatment (depleted > plasmolyzed/extracted > untreated cells), the easier the diffusion into yeast became, and the lower both encapsulation efficiency and protection from evaporation, possibly due to denaturation/removal of lipid-associated (membrane) proteins. More hydrophobic terpenes performed worst in encapsulation as pure 'oils' or in water dispersion, but much less of a difference existed in acetone. This indicates the specific advantage of solvents/dispersants for 'difficult' compounds, which was confirmed by principal component analysis; furthering this concept, we have used combinations of hydrophobes (e.g., linalool and α-tocopherol), with one acting as solvent/enhancer for the other. Our results thus indicate advantages in using untreated yeast and-if necessary-processes based on solvents/secondary hydrophobes.

Engineering acetyl-CoA metabolism to enhance stress tolerance of yeast by regulating membrane functionality

Zygosaccharomyces rouxii has excellent fermentation performance and good tolerance to osmotic stress. Acetyl-CoA is a crucial intermediate precursor in the central carbon metabolic pathway of yeast. This study investigated the effect of engineering acetyl-CoA metabolism on the membrane functionality and stress tolerance of yeast. Firstly, exogenous supplementation of acetyl-CoA improved the biomass and the ability of unsaturated fatty acid synthesis of Z. rouxii under salt stress. Q-PCR results suggested that the gene ACSS (coding acetyl-CoA synthetase) was significantly up-expressed. Subsequently, the gene ACSS from Z. rouxii was transformed and heterologously expressed in S. cerevisiae. The recombinant cells exhibited better multiple stress (salt, acid, heat, and cold) tolerance, higher fatty acid contents, membrane integrity, and fluidity. Our findings may provide a suitable means to enhance the stress tolerance and fermentation efficiency of yeast under harsh fermentation environments.

Lipid Composition Analysis Reveals Mechanisms of Ethanol Tolerance in the Model Yeast Saccharomyces cerevisiae

Saccharomyces cerevisiae is an important unicellular yeast species within the biotechnological and the food and beverage industries. A significant application of this species is the production of ethanol, where concentrations are limited by cellular toxicity, often at the level of the cell membrane. Here, we characterize 61 S. cerevisiae strains for ethanol tolerance and further analyze five representatives with various ethanol tolerances. The most tolerant strain, AJ4, was dominant in coculture at 0 and 10% ethanol. Unexpectedly, although it does not have the highest noninhibitory concentration or MIC, MY29 was the dominant strain in coculture at 6% ethanol, which may be linked to differences in its basal lipidome. Although relatively few lipidomic differences were observed between strains, a significantly higher phosphatidylethanolamine concentration was observed in the least tolerant strain, MY26, at 0 and 6% ethanol compared to the other strains that became more similar at 10%, indicating potential involvement of this lipid with ethanol sensitivity. Our findings reveal that AJ4 is best able to adapt its membrane to become more fluid in the presence of ethanol and that lipid extracts from AJ4 also form the most permeable membranes. Furthermore, MY26 is least able to modulate fluidity in response to ethanol, and membranes formed from extracted lipids are least leaky at physiological ethanol concentrations. Overall, these results reveal a potential mechanism of ethanol tolerance and suggest a limited set of membrane compositions that diverse yeast species use to achieve this. IMPORTANCE Many microbial processes are not implemented at the industrial level because the product yield is poorer and more expensive than can be achieved by chemical synthesis. It is well established that microbes show stress responses during bioprocessing, and one reason for poor product output from cell factories is production conditions that are ultimately toxic to the cells. During fermentative processes, yeast cells encounter culture media with a high sugar content, which is later transformed into high ethanol concentrations. Thus, ethanol toxicity is one of the major stresses in traditional and more recent biotechnological processes. We have performed a multilayer phenotypic and lipidomic characterization of a large number of industrial and environmental strains of Saccharomyces to identify key resistant and nonresistant isolates for future applications.

Yeast Cells in Microencapsulation. General Features and Controlling Factors of the Encapsulation Process

Besides their best-known uses in the food and fermentation industry, yeasts have also found application as microcapsules. In the encapsulation process, exogenous and most typically hydrophobic compounds diffuse and end up being passively entrapped in the cell body, and can be released upon application of appropriate stimuli. Yeast cells can be employed either living or dead, intact, permeabilized, or even emptied of all their original cytoplasmic contents. The main selling points of this set of encapsulation technologies, which to date has predominantly targeted food and-to a lesser extent-pharmaceutical applications, are the low cost, biodegradability and biocompatibility of the capsules, coupled to their sustainable origin (e.g., spent yeast from brewing). This review aims to provide a broad overview of the different kinds of yeast-based microcapsules and of the main physico-chemical characteristics that control the encapsulation process and its efficiency.

Effects of sodium and calcium chloride ionic stresses on model yeast membranes revealed by molecular dynamics simulation

As efforts to move a renewable economy grow, it will be necessary to make use of microbial conversion strategies for the production of novel materials or the upgrading of waste to high-value products. One critical technical challenge currently limiting waste upgrading remains the difficulty in obtaining single-pot conversion techniques where physical, chemical, and biological conversion are performed in a single step. To overcome this challenge, a detailed understanding of how different stresses impact microbial membrane stability will be necessary. Using all-atom molecular dynamics simulations, we examine the impacts of moderate concentrations of NaCl and CaCl₂on a model yeast plasma membrane. Weak, though statistically significant, changes in membrane morphology and dynamics functions are observed that are consistent with swelling and stiffening. Additionally, an examination of the ion-lipid contacts and the behavior of water at the water-membrane interface suggests that the impacts of these common salts may, in part, be mediated through changes to water-membrane hydrogen-bonding and hydration water dynamics.

Highlighting Protective Effect of Encapsulation on Yeast Cell Response to Dehydration Using Synchrotron Infrared Microspectroscopy at the Single-Cell Level

In the present paper, the Layer by Layer (LbL) method using β-lactoglobulin and sodium alginate was performed to individually encapsulate Saccharomyces cerevisiae cells in microorganized shells in order to protect them against stresses during dehydration. Higher survival (∼1 log) for encapsulated yeast cells was effectively observed after air dehydration at 45°C. For the first time, the potentiality of Synchrotron-Fourier Transform InfraRed microspectroscopy (S-FTIR) was used at the single-cell level in order to analyze the contribution of the biochemical composition of non-encapsulated vs. encapsulated cells in response to dehydration. The microspectroscopy measurements clearly differentiated between non-encapsulated and encapsulated yeast cells in the amide band region. In the spectral region specific to lipids, the S-FTIR results indicated probably the decrease in membrane fluidity of yeast after dehydration without significant distinction between the two samples. These data suggested minor apparent chemical changes in cell attributable to the LbL system upon dehydration. More insights are expected regarding the lower mortality among encapsulated cells. Indeed the hypothesis that the biopolymeric layers could induce less damage in cell by affecting the transfer kinetics during dehydration-rehydration cycle, should be verified in further work.

Biophysical Stress Responses of the Yeast Lachancea thermotolerans During Dehydration Using Synchrotron-FTIR Microspectroscopy

During industrial yeast production, cells are often subjected to deleterious hydric variations during dehydration, which reduces their viability and cellular activity. This study is focused on the yeast Lachancea thermotolerans, particularly sensitive to dehydration. The aim was to understand the modifications of single-cells biophysical profiles during different dehydration conditions. Infrared spectra of individual cells were acquired before and after dehydration kinetics using synchrotron radiation-based Fourier-transform infrared (S-FTIR) microspectroscopy. The cells were previously stained with fluorescent probes in order to measure only viable and active cells prior to dehydration. In parallel, cell viability was determined using flow cytometry under identical conditions. The S-FTIR analysis indicated that cells with the lowest viability showed signs of membrane rigidification and modifications in the amide I (α-helix and β-sheet) and amide II, which are indicators of secondary protein structure conformation and degradation or disorder. Shift of symmetric C–H stretching vibration of the CH₂ group upon a higher wavenumber correlated with better cell viability, suggesting a role of plasma membrane fluidity. This was the first time that the biophysical responses of L. thermotolerans single-cells to dehydration were explored with S-FTIR. These findings are important for clarifying the mechanisms of microbial resistance to stress in order to improve the viability of sensitive yeasts during dehydration.

Adaptations to High Salt in a Halophilic Protist: Differential Expression and Gene Acquisitions through Duplications and Gene Transfers

The capacity of halophiles to thrive in extreme hypersaline habitats derives partly from the tight regulation of ion homeostasis, the salt-dependent adjustment of plasma membrane fluidity, and the increased capability to manage oxidative stress. Halophilic bacteria, and archaea have been intensively studied, and substantial research has been conducted on halophilic fungi, and the green alga Dunaliella. By contrast, there have been very few investigations of halophiles that are phagotrophic protists, i.e., protozoa. To gather fundamental knowledge about salt adaptation in these organisms, we studied the transcriptome-level response of Halocafeteria seosinensis (Stramenopiles) grown under contrasting salinities. We provided further evolutionary context to our analysis by identifying genes that underwent recent duplications. Genes that were highly responsive to salinity variations were involved in stress response (e.g., chaperones), ion homeostasis (e.g., Na+/H+ transporter), metabolism and transport of lipids (e.g., sterol biosynthetic genes), carbohydrate metabolism (e.g., glycosidases), and signal transduction pathways (e.g., transcription factors). A significantly high proportion (43%) of duplicated genes were also differentially expressed, accentuating the importance of gene expansion in adaptation by H. seosinensis to high salt environments. Furthermore, we found two genes that were lateral acquisitions from bacteria, and were also highly up-regulated and highly expressed at high salt, suggesting that this evolutionary mechanism could also have facilitated adaptation to high salt. We propose that a transition toward high-salt adaptation in the ancestors of H. seosinensis required the acquisition of new genes via duplication, and some lateral gene transfers (LGTs), as well as the alteration of transcriptional programs, leading to increased stress resistance, proper establishment of ion gradients, and modification of cell structure properties like membrane fluidity.

Understanding the responses of Saccharomyces cerevisiae yeast strain during dehydration processes using synchrotron infrared spectroscopy

For the first time, synchrotron infrared spectroscopy was performed on yeast during dehydration processes in real time with simultaneously controlled relative humidity and temperature.

Characterization of the S. cerevisiae inp51 mutant links phosphatidylinositol 4,5-bisphosphate levels with lipid content, membrane fluidity and cold growth

Phosphatidylinositol 4,5-bisphosphate [PI(4,5)P2] and its derivatives diphosphoinositol phosphates (DPIPs) play key signaling and regulatory roles. However, a direct function of these molecules in lipid and membrane homeostasis remains obscure. Here, we have studied the cold tolerance phenotype of yeast cells lacking the Inp51-mediated phosphoinositide-5-phosphatase. Genetic and biochemical approaches showed that increased metabolism of PI(4,5)P2 reduces the activity of the Pho85 kinase by increasing the levels of the DPIP isomer 1-IP7. This effect was key in the cold tolerance phenotype. Indeed, pho85 mutant cells grew better than the wild-type at 15 °C, and lack of this kinase abolished the inp51-mediated cold phenotype. Remarkably, reduced Pho85 function by loss of Inp51 affected the activity of the Pho85-regulated target Pah1, the yeast phosphatidate phosphatase. Cells lacking Inp51 showed reduced Pah1 abundance, derepression of an INO1-lacZ reporter, decreased content of triacylglycerides and elevated levels of phosphatidate, hallmarks of the pah1 mutant. However, the inp51 phenotype was not associated to low Pah1 activity since deletion of PAH1 caused cold sensitivity. In addition, the inp51 mutant exhibited features not shared by pah1, including a 40%-reduction in total lipid content and decreased membrane fluidity. These changes may influence the activity of membrane-anchored and/or associated proteins since deletion of INP51 slows down the transit to the vacuole of the fluorescent dye FM4-64. In conclusion, our work supports a model in which changes in the PI(4,5)P2 pool affect the 1-IP7 levels modulating the activity of Pho85, Pah1 and likely additional Pho85-controlled targets, and regulate lipid composition and membrane properties.

Metabolic efficiency in yeast Saccharomyces cerevisiae in relation to temperature dependent growth and biomass yield

Canonized view on temperature effects on growth rate of microorganisms is based on assumption of protein denaturation, which is not confirmed experimentally so far. We develop an alternative concept, which is based on view that limits of thermal tolerance are based on imbalance of cellular energy allocation. Therefore, we investigated growth suppression of yeast Saccharomyces cerevisiae in the supraoptimal temperature range (30-40°C), i.e. above optimal temperature (Topt). The maximal specific growth rate (μmax) of biomass, its concentration and yield on glucose (Yx/glc) were measured across the whole thermal window (5-40°C) of the yeast in batch anaerobic growth on glucose. Specific rate of glucose consumption, specific rate of glucose consumption for maintenance (mglc), true biomass yield on glucose (Yx/glc(true)), fractional conservation of substrate carbon in product and ATP yield on glucose (Yatp/glc) were estimated from the experimental data. There was a negative linear relationship between ATP, ADP and AMP concentrations and specific growth rate at any growth conditions, whilst the energy charge was always high (~0.83). There were two temperature regions where mglc differed 12-fold, which points to the existence of a 'low' (within 5-31°C) and a 'high' (within 33-40°C) metabolic mode regarding maintenance requirements. The rise from the low to high mode occurred at 31-32°C in step-wise manner and it was accompanied with onset of suppression of μmax. High mglc at supraoptimal temperatures indicates a significant reduction of scope for growth, due to high maintenance cost. Analysis of temperature dependencies of product formation efficiency and Yatp/glc revealed that the efficiency of energy metabolism approaches its lower limit at 26-31°C. This limit is reflected in the predetermined combination of Yx/glc(true), elemental biomass composition and degree of reduction of the growth substrate. Approaching the limit implies a reduction of the safety margin of metabolic efficiency. We hypothesize that a temperature increase above Topt (e.g. >31°C) triggers both an increment in mglc and suppression of μmax, which together contribute to an upshift of Yatp/glc from the lower limit and thus compensate for the loss of the safety margin. This trade-off allows adding 10 more degrees to Topt and extends the thermal window up to 40°C, sustaining survival and reproduction in supraoptimal temperatures. Deeper understanding of the limits of thermal tolerance can be practically exploited in biotechnological applications.

Changes in SAM2 expression affect lactic acid tolerance and lactic acid production in Saccharomyces cerevisiae

Background

The great interest in the production of highly pure lactic acid enantiomers comes from the application of polylactic acid (PLA) for the production of biodegradable plastics. Yeasts can be considered as alternative cell factories to lactic acid bacteria for lactic acid production, despite not being natural producers, since they can better tolerate acidic environments. We have previously described metabolically engineered Saccharomyces cerevisiae strains producing high amounts of L-lactic acid (>60 g/L) at low pH. The high product concentration represents the major limiting step of the process, mainly because of its toxic effects. Therefore, our goal was the identification of novel targets for strain improvement possibly involved in the yeast response to lactic acid stress.

Results

The enzyme S-adenosylmethionine (SAM) synthetase catalyses the only known reaction leading to the biosynthesis of SAM, an important cellular cofactor. SAM is involved in phospholipid biosynthesis and hence in membrane remodelling during acid stress. Since only the enzyme isoform 2 seems to be responsive to membrane related signals (e.g. myo-inositol), Sam2p was tagged with GFP to analyse its abundance and cellular localization under different stress conditions. Western blot analyses showed that lactic acid exposure correlates with an increase in protein levels. The SAM2 gene was then overexpressed and deleted in laboratory strains. Remarkably, in the BY4741 strain its deletion conferred higher resistance to lactic acid, while its overexpression was detrimental. Therefore, SAM2 was deleted in a strain previously engineered and evolved for industrial lactic acid production and tolerance, resulting in higher production.

Conclusions

Here we demonstrated that the modulation of SAM2 can have different outcomes, from clear effects to no significant phenotypic responses, upon lactic acid stress in different genetic backgrounds, and that at least in one genetic background SAM2 deletion led to an industrially relevant increase in lactic acid production. Further work is needed to elucidate the molecular basis of these observations, which underline once more that strain robustness relies on complex cellular mechanisms, involving regulatory genes and proteins. Our data confirm cofactor engineering as an important tool for cell factory improvement.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-014-0147-7) contains supplementary material, which is available to authorized users.

Membrane fluidity of halophilic ectoine-secreting bacteria related to osmotic and thermal treatment

In response to sudden decrease in osmotic pressure, halophilic microorganisms secrete their accumulated osmolytes. This specific stress response, combined with physiochemical responses to the altered environment, influence the membrane properties and integrity of cells, with consequent effects on growth and yields in bioprocesses, such as bacterial milking. The aim of this study was to investigate changes in membrane fluidity and integrity induced by environmental stress in ectoine-secreting organisms. The halophilic ectoine-producing strains Alkalibacillus haloalkaliphilus and Chromohalobacter salexigens were treated hypo- and hyper-osmotically at several temperatures. The steady-state anisotropy of fluorescently labeled cells was measured, and membrane integrity assessed by flow cytometry and ectoine distribution. Strong osmotic downshocks slightly increased the fluidity of the bacterial membranes. As the temperature increased, the increasing membrane fluidity encouraged more ectoine release under the same osmotic shock conditions. On the other hand, combined shock treatments increased the number of disintegrated cells. From the ectoine release and membrane integrity measurements under coupled thermal and osmotic shock conditions, we could optimize the secretion conditions for both bacteria.

Evaluation of cell damage caused by cold sampling and quenching for metabolome analysis

Cell damage during sampling and quenching for metabolome analysis have been investigated at whole sample level using an OD-based method and ATP loss investigation, and at single cell level by means of flow cytometry. Escherichia coli was cultivated in shake flasks and sampled into several cold quenching solutions during exponential growth phase varying quenching solution composition and sampling temperature. For single cell analysis, the samples were incubated with selective propidium iodide dye and analysed via flow cytometry to differentiate between intact and damaged cells. It was found that every combination of quenching solution, temperature, or cooling rate tested influenced the E. coli cell membrane integrity indicating rupture which will not only let the dye in, but also intracellular ATP out of the cells, which is not desired in in vivo metabolome analysis.

Extremophilic yeasts: plasma-membrane fluidity as determinant of stress tolerance

Our aim was to investigate the response of selected yeasts and yeast-like fungi from extreme environments to various temperatures at the level of their plasma membranes, in order to elucidate the connections between their plasma-membrane fluidity (measured by electron paramagnetic resonance spectroscopy - EPR), growth temperature range, stress tolerance, and ecological distribution. Although all studied fungi can be considered mesophilic according to their growth temperature profiles, their plasma-membrane fluidity indicated otherwise. Arctic yeast Rhodosporidium diobovatum could be classified as psychrotolerant due to its higher average membrane fluidity. Extremely halotolerant black yeast-like fungus Hortaea werneckii isolated from solar salterns, on the other hand, is not adapted to low temperature, which is reflected in the higher average rigidity of its plasma membrane and as a consequence its inability to grow at temperatures lower than 10°C. The plasma membrane of Aureobasidium sp. isolated so far exclusively from an Arctic glacier with its intermediate fluidity and high fluidity variation at different temperatures may indicate the specialization of this yeast-like fungus to the specific glacial environment. Similar behaviour of plasma membrane was detected in the reference yeast, non-extremophilic Saccharomyces cerevisiae. Its membranes of intermediate fluidity and with high fluidity fluctuation at different temperatures may reflect the specialization of this yeast to mesophilic environments and prevent its colonization of extreme environments. Halotolerant Aureobasidium pullulans from salterns, and Arctic Cryptococcus liquefaciens and Rhodotorula mucilaginosa with moderately fluctuating plasma membranes of intermediate fluidity are representatives of globally distributed generalistic and stress-tolerant species that can thrive in a variety of environments. Keeping the membranes stable and flexible is one of the necessities for the microorganisms to survive changes in extreme habitats. Our data suggest that plasma-membrane fluidity can be used as an indicator of fitness for survival in the extreme environments. In addition to the average fluidity of plasma membrane, the fluctuation of fluidity is an important determinant of stress tolerance: high absolute fluidity fluctuation is tied to decreased survival. The fluidity and its variation therefore reflect survival strategy and fitness in extreme environments and are good indicators of the adaptability of microorganisms.

Water-forming NADH oxidase protects Torulopsis glabrata against hyperosmotic stress

A heterologous water-forming NADH oxidase was introduced into Torulopsis glabrata and the effect on cell growth under hyperosmotic conditions was investigated. Expression of the noxE gene from Lactococcus lactis NZ9000 in T. glabrata resulted in a marked decrease in the NADH : NAD+ ratio and higher activities of key enzymes in water-regenerating pathways, leading to an increase in intracellular water content. NaCl-induced reactive oxygen species production was also decreased by the introduction of NADH oxidase, resulting in a significant increase in the growth of T. glabrata under hyperosmotic stress conditions (3824 mOsmol/kg). The results indicated that the osmotolerance of cells can be enhanced by manipulating water-production pathways.