Pierre Barré

Stability of organic carbon in deep soil layers controlled by fresh carbon supply

The world’s soils store more carbon than is present in biomass and in the atmosphere. Little is known, however, about the factors controlling the stability of soil organic carbon stocks and the response of the soil carbon pool to climate change remains uncertain. We investigated the stability of carbon in deep soil layers in one soil profile by combining physical and chemical characterization of organic carbon, soil incubations and radiocarbon dating. Here we show that the supply of fresh plant-derived carbon to the subsoil (0.6–0.8 m depth) stimulated the microbial mineralization of 2,567 ± 226-year-old carbon. Our results support the previously suggested idea that in the absence of fresh organic carbon, an essential source of energy for soil microbes, the stability of organic carbon in deep soil layers is maintained. We propose that a lack of supply of fresh carbon may prevent the decomposition of the organic carbon pool in deep soil layers in response to future changes in temperature. Any change in land use and agricultural practice that increases the distribution of fresh carbon along the soil profile could however stimulate the loss of ancient buried carbon.

Automatic detection of assimilable nitrogen deficiencies during alcoholic fermentation in oenological conditions

Abstract To better describe the importance of initial assimilable nitrogen content on the kinetics of alcoholic fermentation and the effectiveness of ammonium additions, a study was done using an automatic device for fermentation monitoring. A good correlation was found between the maximum CO 2 production rate, obtained early during the fermentation, and the assimilable nitrogen content of the must. So it is possible to use this kinetic parameter to detect nitrogen deficiencies. For industrial applications, a threshold value of 1.3 gCO 2 / l ·h at 24°C (corresponding to about 140 mg N/ l ) is proposed. In the case of deficient must, a nitrogen addition during the fermentation rapidly increased the CO 2 production rate and reduced the fermentation duration. This reduction, which may be approximately predicted from (dCO 2 /d t ) max , is the same, provided that the addition is made before the halfway point of the fermentation.

Modulation of Glycerol and Ethanol Yields During Alcoholic Fermentation in Saccharomyces cerevisiae Strains Overexpressed or Disrupted for GPD1 Encoding Glycerol 3-Phosphate Dehydrogenase

The possibility of the diversion of carbon flux from ethanol towards glycerol in Saccharomyces cerevisiae during alcoholic fermentation was investigated. Variations in the glycerol 3‐phosphate dehydrogenase (GPDH) level and similar trends for alcohol dehydrogenase (ADH), pyruvate decarboxylase and glycerol‐3‐phosphatase were found when low and high glycerol‐forming wine yeast strains were compared. GPDH is thus a limiting enzyme for glycerol production. Wine yeast strains with modulated GPD1 (encoding one of the two GPDH isoenzymes) expression were constructed and characterized during fermentation on glucose‐rich medium. Engineered strains fermented glucose with a strongly modified [glycerol] : [ethanol] ratio. gpd1Δ mutants exhibited a 50% decrease in glycerol production and increased ethanol yield. Overexpression of GPD1 on synthetic must (200 g/l glucose) resulted in a substantial increase in glycerol production (×4) at the expense of ethanol. Acetaldehyde accumulated through the competitive regeneration of NADH via GPDH. Accumulation of by‐products such as pyruvate, acetate, acetoin, 2,3 butane‐diol and succinate was observed, with a marked increase in acetoin production.

Stationary-Phase Gene Expression in Saccharomyces cerevisiae During Wine Fermentation

Genetic engineering of wine yeast strains requires the identification of gene promoters specifically activated under wine processing conditions. In this study, transcriptional activation of specific genes was followed during the time course of wine fermentation by quantifying mRNA levels in a haploid wine strain of Saccharomyces cerevisiae grown on synthetic or natural winery musts. Northern analyses were performed using radioactive probes from 19 genes previously described as being expressed under laboratory growth conditions or on molasses in S. cerevisiae during the stationary phase and/or under nitrogen starvation. Nine genes, including members of the HSP family, showed a transition‐phase induction profile. For three of them, mRNA transcripts could be detected until the end of the fermentation. Expression of one of these genes, HSP30, was further studied using a HSP30::lacZ fusion on both multicopy and monocopy expression vectors. The production of β‐galactosidase by recombinant cells was measured during cell growth and fermentation on synthetic and natural winery musts. We showed that the HSP30 promoter can induce high gene expression during late stationary phase and remains active until the end of the wine fermentation process. Similar expression profiles were obtained on five natural winery musts.

Effectiveness of combined ammoniacal nitrogen and oxygen additions for completion of sluggish and stuck wine fermentations

Abstract The timing of combined additions of oxygen and assimilable nitrogen was optimized. Five mg/l oxygen and 300 mg/l (NH4)2HPO4 were added (i) before inoculation, (ii) at the end of the cell growth phase or (iii) at the halfway point of fermentation. There were marked differences between the nine combinations. In all cases, adding oxygen at the end of cell growth and nitrogen at the halfway point of fermentation appeared to be the most effective combination for completion of the fermentation whereas initial additions always had little favorable effect on this completion. The poor effect of initial oxygenation was partially explained by the large fraction of antimycin-sensitive oxygen consumption at the beginning of fermentation. The technological usefulness of optimized combined additions was shown by testing them during 10 different sluggish fermentations: the mean duration was almost 50% of the mean duration for control fermentations.

Historical and future perspectives of global soil carbon response to climate and land-use changes

In this paper, we attempt to analyse the respective influences of land-use and climate changes on the global and regional balances of soil organic carbon (SOC) stocks. Two time periods are analysed: the historical period 1901–2000 and the period 2000–2100. The historical period is analysed using a synthesis of published data as well as new global and regional model simulations, and the future is analysed using models only. Historical land cover changes have resulted globally in SOC release into the atmosphere. This human induced SOC decrease was nearly balanced by the net SOC increase due to higher CO2 and rainfall. Mechanization of agriculture after the 1950s has accelerated SOC losses in croplands, whereas development of carbon-sequestering practices over the past decades may have limited SOC loss from arable soils. In some regions (Europe, China and USA), croplands are currently estimated to be either a small C sink or a small source, but not a large source of CO2 to the atmosphere. In the future, according to terrestrial biosphere and climate models projections, both climate and land cover changes might cause a net SOC loss, particularly in tropical regions. The timing, magnitude, and regional distribution of future SOC changes are all highly uncertain. Reducing this uncertainty requires improving future anthropogenic CO2 emissions and land-use scenarios and better understanding of biogeochemical processes that control SOC turnover, for both managed and un-managed ecosystems.

Cloning, sequence and expression of the gene encoding the malolactic enzyme from Lactococcus lactis

Many lactic acid bacteria can carry out malolactic fermentation. This secondary fermentation is mediated by the NAD‐ and Mn2+‐dependent malolactic enzyme, which catalyses the decarboxylation of l‐malate to l‐lactate. The gene we call mleS, coding for malolactic enzyme, was isolated from Lactococcus lactis. The mleS gene consists of one open reading frame capable of coding for a protein with a calculated molecular mass of 59 kDa. The amino acid sequence of the predicted MleS gene product is homologous to the sequences of different malic enzymes. Bacterial and yeast cells expressing the malolactic gene convert l‐malate to l‐lactate.

Characterization of Schizosaccharomyces pombe Malate Permease by Expression in Saccharomyces cerevisiae

ABSTRACT In Saccharomyces cerevisiae, l-malic acid transport is not carrier mediated and is limited to slow, simple diffusion of the undissociated acid. Expression in S. cerevisiae of the MAE1 gene, encodingSchizosaccharomyces pombe malate permease, markedly increased l-malic acid uptake in this yeast. In this strain, at pH 3.5 (encountered in industrial processes),l-malic acid uptake involves Mae1p-mediated transport of the monoanionic form of the acid (apparent kinetic parameters:Vmax = 8.7 nmol/mg/min;Km = 1.6 mM) and some simple diffusion of the undissociated l-malic acid (Kd = 0.057 min−1). As total l-malic acid transport involved only low levels of diffusion, the Mae1p permease was further characterized in the recombinant strain. l-Malic acid transport was reversible and accumulative and depended on both the transmembrane gradient of the monoanionic acid form and the ΔpH component of the proton motive force. Dicarboxylic acids with stearic occupation closely related to l-malic acid, such as maleic, oxaloacetic, malonic, succinic and fumaric acids, inhibitedl-malic acid uptake, suggesting that these compounds use the same carrier. We found that increasing external pH directly inhibited malate uptake, resulting in a lower initial rate of uptake and a lower level of substrate accumulation. In S. pombe, proton movements, as shown by internal acidification, accompanied malate uptake, consistent with the proton/dicarboxylate mechanism previously proposed. Surprisingly, no proton fluxes were observed during Mae1p-mediated l-malic acid import inS. cerevisiae, and intracellular pH remained constant. This suggests that, in S. cerevisiae, either there is a proton counterflow or the Mae1p permease functions differently from a proton/dicarboxylate symport.

Clay minerals as a soil potassium reservoir: observation and quantification through X-ray diffraction

Potassium (K) is a major element for plant growth. The K+ ions fixed in soil 2:1 clay mineral interlayers contribute to plant K nutrition. Such clay minerals are most often the majority in temperate soils. Field and laboratory observations based on X-ray diffraction techniques suggest that 2:1 clay minerals behave as a K reservoir. The present work investigated this idea through data from a replicated long term fertilization experiment which allowed one to address the following questions: (1) Do fertilization treatments induce some modifications (as seen from X-ray diffraction measurements) on soil 2:1 clay mineralogy? (2) Are soil 2:1 clay mineral modifications related to soil K budget in the different plots? (3) Do fertilizer treatments modify clay Al, Si, Mg, Fe or K elemental content? (4) Are clay mineral modifications related to clay K content modifications? (5) Are clay mineral changes related to clay Al, Si, Mg or Fe content as well as those of K content? Our results showed that K fertilization treatments considered in the context of soil K budget are very significantly related to 2:1 soil clay mineralogy and clay K content. The 2:1 clay mineral modifications observed through X-ray measurements were quantitatively correlated with chemically analyzed clay K content. Clay K content modifications are independent from clay Al, Si, Mg or Fe contents. These results show that the soil chemical environment can modify interlayer site occupations (illite content) which suggests that high level accumulation of potassium can occur without any modification of the clay sheet structure. This study therefore validates the view of 2:1 clay minerals as a K reservoir easily quantifiable through X-ray observations.

Early thiamin assimilation by yeasts under enological conditions: Impact on alcoholic fermentation kinetics

The effect of early must thiamin depletion by wild yeast strains (i.e. Kloeckera and Saccharomyces species) on alcoholic fermentation kinetics was studied. Experimental conditions affecting thiamin assimilation by yeasts were first determined, using factorial designs. Then sequential and/or mixed cultures simulating wild yeast contamination, as observed during winemaking, were carried out in order to study the influence of early thiamin depletion on fermentation kinetics. The obtained results indicate that biological thiamin depletion leads to slow fermentation and may lead to sluggish or stuck fermentation. Moreover this phenomenon is amplified in media with high assimilable nitrogen contents. No release of thiamin from contaminant yeast and subsequent assimilation by fermentative yeast was observed, indicating that some enological practices (such as centrifugation of musts contaminated early) may lead to stuck fermentation.