Process Oscillations in Continuous Ethanol Fermentation with Saccharomyces cerevisiae

Based on ethanol fermentation kinetics and bioreactor engineering theory, a system composed of a continuously stirred tank reactor (CSTR) and three tubular bioreactors in series was established for continuous very high gravity (VHG) ethanol fermentation with Saccharomyces cerevisiae. Sustainable oscillations of residual glucose, ethanol, and biomass characterized by long oscillation periods and large oscillation amplitudes were observed when a VHG medium containing 280 g/L glucose was fed into the CSTR at a dilution rate of 0.027 h1. Mechanistic analysis indicated that the oscillations are due to ethanol inhibition and the lag response of yeast cells to ethanol inhibition. A high gravity (HG) medium containing 200 g/L glucose and a low gravity (LG) medium containing 120 g/L glucose were fed into the CSTR at the same dilution rate as that for the VHG medium, so that the impact of residual glucose and ethanol concentrations on the oscillations could be studied. The oscillations were not significantly affected when the HG medium was used, and residual glucose decreased significantly, but ethanol maintained at the same level, indicating that residual glucose was not the main factor triggering the oscillations. However, the oscillations disappeared after the LG medium was fed and ethanol concentration decreased to 58.2 g/L. Furthermore, when the LG medium was supplemented with 30 g/L ethanol to achieve the same level of ethanol in the fermentation system as that achieved under the HG condition, the steady state observed for the original LG medium was interrupted, and the oscillations observed under the HG condition occurred. The steady state was gradually restored after the original LG medium replaced the modified one. These experimental results confirmed that ethanol, whether produced by yeast cells during fermentation or externally added into a fermentation system, can trigger oscillations once its concentration approaches to a criterion. The impact of dilution rate on oscillations was also studied. It was found that oscillations occurred at certain dilution rate ranges for the two yeast strains. Since ethanol production is tightly coupled with yeast cell growth, it was speculated that the impact of the dilution rate on the oscillations is due to the synchronization of the mother and daughter cell growth rhythms. The difference in the oscillation profiles exhibited by the two yeast strains is due to their difference in ethanol tolerance. For more practical conditions, the behavior of continuous ethanol fermentation was studied using a self-flocculating industrial yeast strain and corn flour hydrolysate medium in a simulated tanks-in-series fermentation system. Amplified oscillations observed at the dilution rate of 0.12 h1 were postulated to be due to the synchronization of the two yeast cell populations generated by the continuous inoculation from the seed tank upstream of the fermentation system, which was partly validated by oscillation attenuation after the seed tank was removed from the fermentation system. The two populations consisted of the newly inoculated yeast cells and the yeast cells already adapted to the fermentation environment. Oscillations increased residual sugar at the end of the fermentation, and correspondingly, decreased the ethanol yield, indicating the need for attenuation strategies. When the tubular bioreactors were packed with ½” Intalox ceramic saddles, not only was their ethanol fermentation performance improved, but effective oscillation attenuation was also achieved. The oscillation attenuation was postulated to be due to the alleviation of backmixing in the packed tubular bioreactors as well as the yeast cell immobilization role of the packing. The residence time distribution analysis indicated that the mixing performance of the packed tubular bioreactors was close to a CSTR model for both residual glucose and ethanol, and the assumed backmixing alleviation could not be achieved. The impact of yeast cell immobilization was further studied using several different packing materials. Improvement in ethanol fermentation performance as well as oscillation attenuation was achieved for the wood chips, as well as the Intalox ceramic saddles, but not for the porous polyurethane particles, nor the steel Raschig rings. Analysis for the immobilized yeast cells indicated that high viability was the mechanistic reason for the improvement of the ethanol fermentation performance as well as the attenuation of the oscillations. A dynamic model was developed by incorporating the lag response of yeast cells to ethanol inhibition into the pseudo-steady state kinetic model, and dynamic simulation was performed, with good results. This not only provides a basis for developing process intervention strategies to minimize oscillations, but also theoretically support the mechanistic hypothesis for the oscillations

The global methane budget 2000–2017

Understanding and quantifying the global methane (CH4) budget is important for assessing realistic pathways to mitigate climate change. Atmospheric emissions and concentrations of CH4 continue to increase, making CH4 the second most important human-influenced greenhouse gas in terms of climate forcing, after carbon dioxide (CO2). The relative importance of CH4 compared to CO2 depends on its shorter atmospheric lifetime, stronger warming potential, and variations in atmospheric growth rate over the past decade, the causes of which are still debated. Two major challenges in reducing uncertainties in the atmospheric growth rate arise from the variety of geographically overlapping CH4 sources and from the destruction of CH4 by short-lived hydroxyl radicals (OH). To address these challenges, we have established a consortium of multidisciplinary scientists under the umbrella of the Global Carbon Project to synthesize and stimulate new research aimed at improving and regularly updating the global methane budget. Following Saunois et al. (2016), we present here the second version of the living review paper dedicated to the decadal methane budget, integrating results of top-down studies (atmospheric observations within an atmospheric inverse-modelling framework) and bottom-up estimates (including process-based models for estimating land surface emissions and atmospheric chemistry, inventories of anthropogenic emissions, and data-driven extrapolations). For the 2008–2017 decade, global methane emissions are estimated by atmospheric inversions (a top-down approach) to be 576 Tg CH4 yr−1 (range 550–594, corresponding to the minimum and maximum estimates of the model ensemble). Of this total, 359 Tg CH4 yr−1 or ∼ 60 % is attributed to anthropogenic sources, that is emissions caused by direct human activity (i.e. anthropogenic emissions; range 336–376 Tg CH4 yr−1 or 50 %–65 %). The mean annual total emission for the new decade (2008–2017) is 29 Tg CH4 yr−1 larger than our estimate for the previous decade (2000–2009), and 24 Tg CH4 yr−1 larger than the one reported in the previous budget for 2003–2012 (Saunois et al., 2016). Since 2012, global CH4 emissions have been tracking the warmest scenarios assessed by the Intergovernmental Panel on Climate Change. Bottom-up methods suggest almost 30 % larger global emissions (737 Tg CH4 yr−1, range 594–881) than top-down inversion methods. Indeed, bottom-up estimates for natural sources such as natural wetlands, other inland water systems, and geological sources are higher than top-down estimates. The atmospheric constraints on the top-down budget suggest that at least some of these bottom-up emissions are overestimated. The latitudinal distribution of atmospheric observation-based emissions indicates a predominance of tropical emissions (∼ 65 % of the global budget, < 30∘ N) compared to mid-latitudes (∼ 30 %, 30–60∘ N) and high northern latitudes (∼ 4 %, 60–90∘ N). The most important source of uncertainty in the methane budget is attributable to natural emissions, especially those from wetlands and other inland waters. Some of our global source estimates are smaller than those in previously published budgets (Saunois et al., 2016; Kirschke et al., 2013). In particular wetland emissions are about 35 Tg CH4 yr−1 lower due to improved partition wetlands and other inland waters. Emissions from geological sources and wild animals are also found to be smaller by 7 Tg CH4 yr−1 by 8 Tg CH4 yr−1, respectively. However, the overall discrepancy between bottom-up and top-down estimates has been reduced by only 5 % compared to Saunois et al. (2016), due to a higher estimate of emissions from inland waters, highlighting the need for more detailed research on emissions factors. Priorities for improving the methane budget include (i) a global, high-resolution map of water-saturated soils and inundated areas emitting methane based on a robust classification of different types of emitting habitats; (ii) further development of process-based models for inland-water emissions; (iii) intensification of methane observations at local scales (e.g., FLUXNET-CH4 measurements) and urban-scale monitoring to constrain bottom-up land surface models, and at regional scales (surface networks and satellites) to constrain atmospheric inversions; (iv) improvements of transport models and the representation of photochemical sinks in top-down inversions; and (v) development of a 3D variational inversion system using isotopic and/or co-emitted species such as ethane to improve source partitioning

Global Uptake of Atmospheric Methane by Soil From 1900 to 2100

Soil methanotrophy is the only biological process that removes methane (CH4) from the atmosphere. There is good agreement about the size of the global sink but great uncertainty about its interannual variability and regional responses to changes in key environmental drivers. We used the process-based methanotrophy model Methanotrophy Model (MeMo) v1.0 and output from global climate models to simulate regional and global changes in soil uptake of atmospheric CH4 from 1900 to 2100. The annual global uptake doubled from 17.1 ± 2.4 to 37.2 ± 3.3 Tg yr−1 from 1900-2015 and could increase further to 82.7 ± 4.4 Tg yr−1 by 2100 (RCP8.5), primarily due to enhanced diffusion of CH4 into soil as a result of increases in atmospheric CH4 mole fraction. We show that during the period 1980–2015 temperature became an important influence on the increasing rates of soil methanotrophy, particularly in the Northern Hemisphere. In RCP-forced simulations the relative influence of temperature on changes in the uptake continues to increase, enhancing the soil sink through higher rates of methanotrophic metabolic activity, increases in the global area of active soil methanotrophy and length of active season. During the late 21st century under RCP6.0, temperature is predicted to become the dominant driver of changes in global mean soil uptake rates for the first time. Regionally, in Europe and Asia, nitrogen inputs dominate changes in soil methanotrophy, while soil moisture is the most important influence in tropical South America. These findings highlight that the soil sink could change in response to drivers other than atmospheric CH4 mole fraction.SCOPUS: ar.jinfo:eu-repo/semantics/publishe

Polar amplification of Pliocene climate by elevated trace gas radiative forcing

International audienceWarm periods in Earth’s history offer opportunities to understand the dynamics of the Earth system under conditions that are similar to those expected in the near future. The Middle Pliocene warm period (MPWP), from 3.3 to 3.0 My B.P, is the most recent time when atmospheric CO2 levels were as high as today. However, climate model simulations of the Pliocene underestimate high-latitude warming that has been reconstructed from fossil pollen samples and other geological archives. One possible reason for this is that enhanced non-CO2 trace gas radiative forcing during the Pliocene, including from methane (CH4), has not been included in modeling. We use a suite of terrestrial biogeochemistry models forced with MPWP climate model simulations from four different climate models to produce a comprehensive reconstruction of the MPWP CH4 cycle, including uncertainty. We simulate an atmospheric CH4 mixing ratio of 1,000 to 1,200 ppbv, which in combination with estimates of radiative forcing from N2O and O3, contributes a non-CO2 radiative forcing of 0.9 W⋅m−2 (range 0.6 to 1.1), which is 43% (range 36 to 56%) of the CO2 radiative forcing used in MPWP climate simulations. This additional forcing would cause a global surface temperature increase of 0.6 to 1.0 °C, with amplified changes at high latitudes, improving agreement with geological evidence of Middle Pliocene climate. We conclude that natural trace gas feedbacks are critical for interpreting climate warmth during the Pliocene and potentially many other warm phases of the Cenezoic. These results also imply that using Pliocene CO2 and temperature reconstructions alone may lead to overestimates of the fast or Charney climate sensitivity

La prise en compte du méthane permet-elle de réduire le désaccord modèle-données dans les climats chauds du Phanérozoïque ? Le cas du Pliocène.

International audienc

La prise en compte du méthane permet-elle de réduire le désaccord modèle-données dans les climats chauds du Phanérozoïque ? Le cas du Pliocène.

International audienc

La prise en compte du méthane permet-elle de réduire le désaccord modèle-données dans les climats chauds du Phanérozoïque ? Le cas du Pliocène.

International audienc