Description

The "Global Climate Status 2023" report published by the World Meteorological Organization indicates that 2023 was the warmest year on record, with the global near-surface average temperature exceeding the pre-industrial baseline by 1.45°C. Furthermore, the decade spanning 2014-2023 has also been the warmest on record. This underscores the urgent red alert that the Earth is sending to the global community.

Surface air temperature anomaly for 2023 relative to the average for the 1991-2020 reference period. Data source: ERA5.

Fossil fuels—coal, oil, and natural gashave been the primary contributors to global climate change to date, accounting for over 75% of global greenhouse gas emissions and nearly 90% of all carbon dioxide emissions. The emitted greenhouse gases envelop the Earth, trapping the sun's heat and leading to global warming and climate change. Observations indicate that the concentrations of the three main greenhouse gases—carbon dioxide, methane, and nitrous oxide—reached record levels in 2022, and the concentrations will continue to rise.

Climate change is not merely reflected in temperature; it is also characterized by unprecedented ocean warming, gla-cier retreat, and Antarctic sea ice loss, which are particularly concerning. On a daily basis, nearly one-third of the global ocean experiences heatwaves, causing damage to vital ecosystems and food systems. By the end of 2023, over 90% of the ocean had experienced heatwave conditions at some point during the year. Concurrently, the Antarctic sea ice extent reached the lowest level on record, with the maximum extent at the end of winter being 1 million square kilometers less than the previous lowest record. As a result, the global average sea level in 2023 also reached its highest value since satellite records began (since 1993); the rate of global average sea level rise over the past decade is more than double that of the first decade of satellite records (1993-2002). Additionally, the escalating food insecurity, displacement of populations, and loss of biodiversity demonstrate the close interweaving of the climate crisis with the crisis of inequality. Data show that the number of people suffering from severe food insecurity has more than doubled, increasing from 149 million before the COVID-19 pandemic to 333 million in 2023.

Global surface air temperature increase relative to the average for 1850-1900, the designated pre-industrial reference period, based on several global temperature datasets shown as 5-year averages since 1850 (left) and as annual averages since 1967 (right). Data source: C3S/ECMWF.

Monthly global mean atmospheric CO2 (left) and CH4 (right) column-averaged concentration from satellites for 2003-2023 (grey curve) and 12-month average (red curve). Data source: C3S/Obs4MIPs (v4.5) consolidated (2003–2022) and CAMS preliminary near real-time data (2023) GOSAT (CH4) and GOSAT-2 (CO2) records. Spatial range: 60S - 60N over land. Credit: C3S/CAMS/ECMWF/University of Bremen/SRON.

In the face of such a dire situation, robust action is imperative to match the pace of climate change and to control the long-term temperature increase. It is essential for the global community to achieve net-zero carbon dioxide emissions by the middle of this century. Therefore it is urgent to expedite the energy structure transition, hastening the inevitable end of the fossil fuel era. From this view, the green and efficient utilization of renewable carbon resources plays a significant role in reducing carbon emissions and addressing climate change, which offers a glimmer of hope in combating climate change and become a hot topic of research worldwide.

Lignocellulose biomass is an important source of renewable carbon source and is the only non-fossil resource in na-ture that can provide aromatic compounds, accounting for approximately 30% of the Earth's total carbon resources.^[1] As a type of biomass resource, its global annual production is about 1.8 trillion tons. It is abundant, widely distributed, economically viable, and is a renewable energy variety that can fully replace fossil fuels.^[2] Biomass energy is compatible with existing fossil energy infrastructures, achieving zero carbon emissions and reaping substantial carbon reduction benefits. Moreover, it can also produce bio-based materials and chemicals, replacing petroleum-based materials, which holds significant application potential and value.^[3] Therefore, lignocellulose biofuels have important implications for environmental protection.

However, the structure of lignocellulose is extremely complex, featuring a highly cross-linked three-dimensional struc-ture that forms a stubborn barrier against degradation. In nature, lignocellulose degradation primarily relies on fungi as well as the combined effects of photodegradation and thermodegradation in the environment.^[4] But the growth conditions for fungi are quite demanding, greatly limiting their industrial applications. For instance, conditions commonly found in industrial fermentations, such as high pH values, anaerobic environments, and high substrate concentrations, significantly inhibit fungal growth. Moreover, the complex genetic systems of fungi restrict research on their metabolic engineering. Industrially, current lignocellulose conversion technologies mainly utilize efficient metal catalysts and rely on light and electrical assistance to achieve effective degradation and transformation.^[5] These methods often require harsh conditions of strong acids and bases, high temperatures and pressures, and the aid of costly metal catalysts, resulting in high costs and significant energy consumption.

In recent years, bacteria from various ecological environments have been found to possess the ability to metabolize lignocellulose. Compared to fungi, bacteria can grow over a broader range of pH values, temperatures, and oxygen levels, and are more amenable to genetic manipulation. Therefore, bacteria represent a breakthrough in the study of lignocellu-lose transformation and high-value utilization. Research has discovered that Pseudomonas putida (P. putida) has the capability to metabolize lignocellulose. Due to its strong carbon storage ability, environmental adaptability, and a well-understood genetic manipulation system, it has become a representative bacterial strain for studying lignocellulose metabolism within the Proteobacteria phylum.^[6-7]

Additionally, it is interesting to note that P. putida can mineralize Mn²⁺ in the surrounding environment through manga-nese oxidation, producing manganese minerals, which presents the ability to catalyze the oxidation of humic substances into low molecular weight organic materials.^[8-9] Therefore, the maganese minerals play a catalytic degradation role similar to that of lignocellulose-degrading enzyme, and are essentially a type of natural biological nanozyme.^[10-12] Interestingly, the secretion of various extracellular oxidases by bacteria, such as multicopper oxidases and manganese peroxidases, are key to achieving manganese oxidation. Therefore, the overexpression of these enzymes, coupled with the additional supplementation of Mn²⁺, can enhance the bacteria's manganese oxidation capacity, producing a large amount of highly reactive and high-valence manganese minerals, which in turn strengthens the degradation of lignocellulose.

The advancement of synthetic biology allows us to establish cell factories.^[14-15] NPU-China will introduce the ligno-cellulose degradation pathway with P. putida to build our cell factory. However, during the biological degradation of lignocellulose, the catalysis by extracellular lignocellulose-degrading enzymes is the first and most critical step. Their expression levels play a key role in degradation efficiency. However, the limited expression of ligninolytic enzymes results in low degradation efficiency far from satisfactory. How to effectively improve the expression level of ligninolytic enzymes from P. putida is the key problem we are facing.

To solve the above two problems, we turned our attention to promoters that control gene transcription.^[16] As the ini-tiation of gene transcription, promoters are key components of cell factory design and metabolic pathway modification. Due to the limited number of well-characterized promoters in yeast and their small dynamic ranges, it is often difficult to satisfy the fine regulation of genes, optimize metabolic flux, and improve the yield of the target product.^[17] Therefore, NPU-China wants to obtain high-strength promoters to increase the expression of ligninolytic enzyme through engineering modification of P. putida natural promoters, which will provide useful and efficient help for the improvement of cellulose degradation efficiency and the design of cell factory.