Review Article | Published:

Barriers and opportunities in bio-based production of hydrocarbons

Nature Energy (2018) | Download Citation

Abstract

Global climate change caused by the accumulation of greenhouse gases (GHGs) has caused concerns regarding the continued reliance on fossil fuels as our primary energy source. Hydrocarbons produced from biomass using microbial fermentation processes can serve as high-quality liquid transportation fuels and may contribute to a reduction in GHG emissions. Here, we discuss the barriers and opportunities for bio-based production of hydrocarbons to be used as diesel and jet fuels and review recent advances in engineering microbes for production of these chemicals. There are two main challenges associated with establishing bio-based hydrocarbon production from cheap feedstocks; lowering the cost of developing efficient and robust microbial cell factories and establishing more efficient routes for biomass hydrolysis to sugars for fermentation. We discuss how to develop novel systems and synthetic biology tools that can enable faster and cheaper construction of microbial cell factories and thereby address the first challenge, as well as recent advances in biomass processing that will likely lead to overcoming the second challenge in the near future.

Access optionsAccess options

Rent or Buy article

Get time limited or full article access on ReadCube.

from$8.99

All prices are NET prices.

Additional information

Publisher’s note: Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

References

  1. 1.

    Use of Energy in the United States Explained: Energy Use for Transportation (US Energy Information and Administration, 2016); https://go.nature.com/2sH2qsJ

  2. 2.

    Liao, J. C., Mi, L., Pontrelli, S. & Luo, S. Fuelling the future: microbial engineering for the production of sustainable biofuels. Nat. Rev. Microbiol. 14, 288–304 (2016).

  3. 3.

    Lynd, L. R. The grand challenge of cellulosic biofuels. Nat. Biotechnol. 35, 912–915 (2017).

  4. 4.

    ICAO Environmental Report 2016: Aviation and Environmental Outlook 38–65 (ICAO, 2016).

  5. 5.

    Moore, R. H. et al. Biofuel blending reduces particle emissions from aircraft engines at cruise conditions. Nature 543, 411–415 (2017).

  6. 6.

    Kocar, G. & Civas, N. An overview of biofuels from energy crops: Current status and future prospects. Renew. Sust. Energ. Rev. 28, 900–916 (2013).

  7. 7.

    Peralta-Yahya, P. P., Zhang, F., del Cardayre, S. B. & Keasling, J. D. Microbial engineering for the production of advanced biofuels. Nature 488, 320–328 (2012).

  8. 8.

    Lee, S. Y., Kim, H. M. & Cheon, S. Metabolic engineering for the production of hydrocarbon fuels. Curr. Opin. Biotechnol. 33, 15–22 (2015).

  9. 9.

    Fellet, M. Aviation industry hopes to cut emissions with jet biofuel. Chem. Eng. News 94, 16–18 (2016).

  10. 10.

    Gevo’s alcohol-to-jet fuel meets approved ASTM standard. Biomass Magazine http://biomassmagazine.com/articles/13078/gevoundefineds-alcohol-to-jet-fuel-meets-approved-astm-standard (2016).

  11. 11.

    Caspeta, L. & Nielsen, J. Economic and environmental impacts of microbial biodiesel. Nat. Biotechnol. 31, 789–793 (2013).

  12. 12.

    Technology Roadmap: Biofuels for Transport (International Energy Agency, 2011).

  13. 13.

    Nielsen, J. & Keasling, J. D. Engineering cellular metabolism. Cell 164, 1185–1197 (2016). This review comprehensively discusses the challenges and strategies in constructing microbial cell factories.

  14. 14.

    Whited, G. M. et al. Development of a gas-phase bioprocess for isoprene-monomer production using metabolic pathway engineering. Ind. Biotechnol. 6, 152–163 (2010).

  15. 15.

    Lv, X. M. et al. Dual regulation of cytoplasmic and mitochondrial acetyl-CoA utilization for improved isoprene production in Saccharomyces cerevisiae. Nat. Commun. 7, 12851 (2016).

  16. 16.

    Gao, X. et al. Engineering the methylerythritol phosphate pathway in cyanobacteria for photosynthetic isoprene production from CO2. Energy Environ. Sci. 9, 1400–1411 (2016).

  17. 17.

    Davies, F. K., Work, V. H., Beliaev, A. S. & Posewitz, M. C. Engineering limonene and bisabolene production in wild type and a glycogen-deficient mutant of Synechococcus sp. PCC 7002. Front. Bioeng. Biotechnol. 2, 21 (2014).

  18. 18.

    Zebec, Z. et al. Towards synthesis of monoterpenes and derivatives using synthetic biology. Curr. Opin. Chem. Biol. 34, 37–43 (2016).

  19. 19.

    Tashiro, M. et al. Bacterial production of pinene by a laboratory-evolved pinene-synthase. ACS Synth. Biol. 5, 1011–1120 (2016).

  20. 20.

    Zhang, H. et al. Microbial production of sabinene--a new terpene-based precursor of advanced biofuel. Microb. Cell Fact. 13, 20 (2014).

  21. 21.

    Alonso-Gutierrez, J. et al. Principal component analysis of proteomics (PCAP) as a tool to direct metabolic engineering. Metab. Eng. 28, 123–133 (2015).

  22. 22.

    Cao, X. et al. Metabolic engineering of oleaginous yeast Yarrowia lipolytica for limonene overproduction. Biotechnol. Biofuels 9, 214 (2016).

  23. 23.

    Ignea, C., Pontini, M., Maffei, M. E., Makris, A. M. & Kampranis, S. C. Engineering monoterpene production in yeast using a synthetic dominant negative geranyl diphosphate synthase. ACS Synth. Biol. 3, 298–306 (2014).

  24. 24.

    Meadows, A. L. et al. Rewriting yeast central carbon metabolism for industrial isoprenoid production. Nature 537, 694–697 (2016). This work rewired the yeast central metabolism by using four non-native metabolic reactions for improved cytosolic precursor acetyl-CoA supply, reduced ATP requirement, reduced CO 2 emissions and improved pathway redox balance, which had a 25% higher farnesene yield on glucose (130 g l –1 , 0.173 g per g glucose) and 75% less oxygen consumption.

  25. 25.

    Paddon, C. J. et al. High-level semi-synthetic production of the potent antimalarial artemisinin. Nature 496, 528–532 (2013).

  26. 26.

    Peralta-Yahya, P. P. et al. Identification and microbial production of a terpene-based advanced biofuel. Nat. Commun. 2, 483 (2011).

  27. 27.

    Pfleger, B. F., Gossing, M. & Nielsen, J. Metabolic engineering strategies for microbial synthesis of oleochemicals. Metab. Eng. 29, 1–11 (2015).

  28. 28.

    Schirmer, A., Rude, M. A., Li, X., Popova, E. & del Cardayre, S. B. Microbial biosynthesis of alkanes. Science 329, 559–562 (2010).

  29. 29.

    Bernard, A. et al. Reconstitution of plant alkane biosynthesis in yeast demonstrates that Arabidopsis ECERIFERUM1 and ECERIFERUM3 are core components of a very-long-chain alkane synthesis complex. Plant Cell 24, 3106–3118 (2012).

  30. 30.

    Rui, Z., Harris, N. C., Zhu, X. J., Huang, W. & Zhang, W. J. Discovery of a family of desaturase-like enzymes for 1-alkene biosynthesis. ACS Catal. 5, 7091–7094 (2015). This study discovered efficient membrane-bound desaturase-like enzymes for long-chain 1-alkene biosynthesis from fatty acids.

  31. 31.

    Rui, Z. et al. Microbial biosynthesis of medium-chain 1-alkenes by a nonheme iron oxidase. Proc. Natl Acad. Sci. USA 111, 18237–18242 (2014).

  32. 32.

    Rude, M. A. et al. Terminal olefin (1-alkene) biosynthesis by a novel P450 fatty acid decarboxylase from Jeotgalicoccus species. Appl. Environ. Microb. 77, 1718–1727 (2011).

  33. 33.

    Mendez-Perez, D., Begemann, M. B. & Pfleger, B. F. Modular synthase-encoding gene involved in alpha-olefin biosynthesis in Synechococcus sp. strain PCC 7002. Appl. Environ. Microb. 77, 4264–4267 (2011).

  34. 34.

    Choi, Y. J. & Lee, S. Y. Microbial production of short-chain alkanes. Nature 502, 571–574 (2013).

  35. 35.

    Kallio, P., Pasztor, A., Thiel, K., Akhtar, M. K. & Jones, P. R. An engineered pathway for the biosynthesis of renewable propane. Nat. Commun. 5, 4731 (2014).

  36. 36.

    Sheppard, M. J., Kunjapur, A. M. & Prather, K. L. Modular and selective biosynthesis of gasoline-range alkanes. Metab. Eng. 33, 28–40 (2016).

  37. 37.

    Andre, C., Kim, S. W., Yu, X. H. & Shanklin, J. Fusing catalase to an alkane-producing enzyme maintains enzymatic activity by converting the inhibitory byproduct H2O2 to the cosubstrate O2. Proc. Natl Acad. Sci. USA 110, 3191–3196 (2013).

  38. 38.

    Rodriguez, G. M. & Atsumi, S. Toward aldehyde and alkane production by removing aldehyde reductase activity in Escherichia coli. Metab. Eng. 25, 227–237 (2014).

  39. 39.

    Rahmana, Z. et al. Enhanced production of n-alkanes in Escherichia coli by spatial organization of biosynthetic pathway enzymes. J. Biotechnol. 192, 187–191 (2014).

  40. 40.

    Sachdeva, G., Garg, A., Godding, D., Way, J. C. & Silver, P. A. In vivo co-localization of enzymes on RNA scaffolds increases metabolic production in a geometrically dependent manner. Nucleic Acids Res. 42, 9493–9503 (2014).

  41. 41.

    Cao, Y. X. et al. Heterologous biosynthesis and manipulation of alkanes in Escherichia coli. Metab. Eng. 38, 19–28 (2016). This work used a multi-modular optimization approach to alkane production in E. coli by balancing the fatty aldehyde node and engineering fatty acid metabolism and electron transfer system, resulting in 1.31g l –1 alkane production in fed-batch fermentation by using glycerol as carbon source.

  42. 42.

    Liu, Y. et al. Hydrogen peroxide-independent production of alpha-alkenes by OleTJE P450 fatty acid decarboxylase. Biotechnol. Biofuels 7, 28 (2014).

  43. 43.

    Buijs, N. A., Zhou, Y. J., Siewers, V. & Nielsen, J. Long-chain alkane production by the yeast Saccharomyces cerevisiae. Biotechnol. Bioeng. 112, 1275–1279 (2015).

  44. 44.

    Zhou, Y. J. et al. Production of fatty acid-derived oleochemicals and biofuels by synthetic yeast cell factories. Nat. Commun. 7, 11709 (2016).

  45. 45.

    Zhou, Y. J. et al. Harnessing yeast peroxisomes for biosynthesis of fatty-acid-derived biofuels and chemicals with relieved side-pathway competition. J. Am. Chem. Soc. 138, 15368–15377 (2016).

  46. 46.

    Chen, B., Lee, D. Y. & Chang, M. W. Combinatorial metabolic engineering of Saccharomyces cerevisiae for terminal alkene production. Metab. Eng. 31, 53–61 (2015).

  47. 47.

    Zhou, Y. J., Hu, Y., Zhu, Z., Siewers, V. & Nielsen, J. Engineering 1-alkene biosynthesis and secretion by dynamic regulation in yeast. ACS Synth. Biol. 7, 584–590 (2018).

  48. 48.

    Zhu, Z. et al. Expanding the product portfolio of fungal type I fatty acid synthases. Nat. Chem. Biol. 13, 360–362 (2017).

  49. 49.

    Gajewski, J. et al. Engineering fatty acid synthases for directed polyketide production. Nat. Chem. Biol. 13, 363–365 (2017).

  50. 50.

    Zhu, Z. et al. Enabling the synthesis of medium chain alkanes and 1-alkenes in yeast. Metab. Eng. 44, 81–88 (2017).

  51. 51.

    Blazeck, J., Liu, L., Knight, R. & Alper, H. S. Heterologous production of pentane in the oleaginous yeast Yarrowia lipolytica. J. Biotechnol. 165, 184–194 (2013).

  52. 52.

    Xu, P., Qiao, K. J., Ahn, W. S. & Stephanopoulos, G. Engineering Yarrowia lipolytica as a platform for synthesis of drop-in transportation fuels and oleochemicals. Proc. Natl Acad. Sci. USA 113, 10848–10853 (2016).

  53. 53.

    Angermayr, S. A., Gorchs Rovira, A. & Hellingwerf, K. J. Metabolic engineering of cyanobacteria for the synthesis of commodity products. Trends Biotechnol. 33, 352–361 (2015).

  54. 54.

    Wang, W., Liu, X. & Lu, X. Engineering cyanobacteria to improve photosynthetic production of alka(e)nes. Biotechnol. Biofuels. 6, 69 (2013).

  55. 55.

    Liu, Q. et al. Engineering an iterative polyketide pathway in Escherichia coli results in single-form alkene and alkane overproduction. Metab. Eng. 28, 82–90 (2015). This paper shows that the polyketide pathway can be engineered for biosynthesis of alkenes.

  56. 56.

    Fortman, J. L., Katz, L., Steen, E. J. & Keasling, J. D. Producing alpha-olefins using polyketide synthases. US patent 2016/0068827 A1 (2016).

  57. 57.

    Yuzawa, S., Keasling, J. D. & Katz, L. Bio-based production of fuels and industrial chemicals by repurposing antibiotic-producing type I modular polyketide synthases: opportunities and challenges. J. Antibiot. 70, 378–385 (2017).

  58. 58.

    Sikkema, J., de Bont, J. A. & Poolman, B. Mechanisms of membrane toxicity of hydrocarbons. Microbiol. Rev. 59, 201–222 (1995).

  59. 59.

    Gong, Z., Nielsen, J. & Zhou, Y. J. Engineering robustness of microbial cell factories. Biotechnol. J. 12, 201700014 (2017).

  60. 60.

    Brennan, T. C. R., Turner, C. D., Kromer, J. O. & Nielsen, L. K. Alleviating monoterpene toxicity using a two-phase extractive fermentation for the bioproduction of jet fuel mixtures in Saccharomyces cerevisiae. Biotechnol. Bioeng. 109, 2513–2522 (2012).

  61. 61.

    Clomburg, J. M., Crumbley, A. M. & Gonzalez, R. Industrial biomanufacturing: The future of chemical production. Science 355, aag0804 (2017). This review compares the biomanufacturing and chemical process in regard to economies of unit number, investment scale and financial risk, and proposes that biomanufacturing could play an important role in conversion of single-carbon feedstocks to chemicals and biofuels with rapid adaptation to new and changing markets.

  62. 62.

    Schrader, J. et al. Methanol-based industrial biotechnology: current status and future perspectives of methylotrophic bacteria. Trends Biotechnol. 27, 107–115 (2009).

  63. 63.

    Shih, P. M., Zarzycki, J., Niyogi, K. K. & Kerfeld, C. A. Introduction of a synthetic CO2-fixing photorespiratory bypass into a cyanobacterium. J. Biol. Chem. 289, 9493–9500 (2014).

  64. 64.

    Lin, M. T., Occhialini, A., Andralojc, P. J., Parry, M. A. J. & Hanson, M. R. A faster Rubisco with potential to increase photosynthesis in crops. Nature 513, 547–550 (2014).

  65. 65.

    Gong, F. et al. Quantitative analysis of an engineered CO2-fixing Escherichia coli reveals great potential of heterotrophic CO2 fixation. Biotechnol. Biofuels 8, 86 (2015).

  66. 66.

    Guadalupe-Medina, V. et al. Carbon dioxide fixation by Calvin-Cycle enzymes improves ethanol yield in yeast. Biotechnol. Biofuels 6, 125 (2013).

  67. 67.

    Antonovsky, N. et al. Sugar Synthesis from CO2 in Escherichia coli. Cell 166, 115–125 (2016). A non-native Calvin–Benson–Bassham cycle was functionally constructed in E. coli and enabled biomass synthesis from CO 2 directly with the supply of ATP and reducing power.

  68. 68.

    Schwander, T., Schada von Borzyskowski, L., Burgener, S., Cortina, N. S. & Erb, T. J. A synthetic pathway for the fixation of carbon dioxide in vitro. Science 354, 900–904 (2016).

  69. 69.

    Sakimoto, K. K., Wong, A. B. & Yang, P. Self-photosensitization of nonphotosynthetic bacteria for solar-to-chemical production. Science 351, 74–77 (2016). A biological–inorganic hybrid, combining inorganic semiconductors and non-photosynthetic bacterium, was developed for highly efficient light harvesting and CO 2 fixation toward acetate production.

  70. 70.

    Li, H. et al. Integrated electromicrobial conversion of CO2 to higher alcohols. Science 335, 1596 (2012).

  71. 71.

    Romero, E., Novoderezhkin, V. I. & van Grondelle, R. Quantum design of photosynthesis for bio-inspired solar-energy conversion. Nature 543, 355–365 (2017).

  72. 72.

    Haynes, C. A. & Gonzalez, R. Rethinking biological activation of methane and conversion to liquid fuels. Nat. Chem. Biol. 10, 331–339 (2014).

  73. 73.

    Whitaker, W. B., Sandoval, N. R., Bennett, R. K., Fast, A. G. & Papoutsakis, E. T. Synthetic methylotrophy: engineering the production of biofuels and chemicals based on the biology of aerobic methanol utilization. Curr. Opin. Biotechnol. 33, 165–175 (2015).

  74. 74.

    Muller, J. E. et al. Engineering Escherichia coli for methanol conversion. Metab. Eng. 28, 190–201 (2015). A heterologous methanol utilization pathway was constructed in E. coli , which enabled up to 40% incorporation of methanol into central metabolites.

  75. 75.

    Whitaker, W. B. et al. Engineering the biological conversion of methanol to specialty chemicals in Escherichia coli. Metab. Eng. 39, 49–59 (2017).

  76. 76.

    Sonntag, F. et al. Engineering Methylobacterium extorquens for de novo synthesis of the sesquiterpenoid alpha-humulene from methanol. Metab. Eng. 32, 82–94 (2015).

  77. 77.

    Liang, W. F. et al. Biosensor-assisted transcriptional regulator engineering for Methylobacterium extorquens AM1 to improve mevalonate synthesis by increasing the acetyl-CoA supply. Metab. Eng. 39, 159–168 (2017).

  78. 78.

    Bhataya, A., Schmidt-Dannert, C. & Lee, P. C. Metabolic engineering of Pichia pastoris X-33 for lycopene production. Process Biochem. 44, 1095–1102 (2009).

  79. 79.

    Wriessnegger, T. et al. Production of the sesquiterpenoid (+)-nootkatone by metabolic engineering of Pichia pastoris. Metab. Eng. 24, 18–29 (2014).

  80. 80.

    Lawton, T. J. & Rosenzweig, A. C. Methane-oxidizing enzymes: An upstream problem in biological gas-to-liquids conversion. J. Am. Chem. Soc. 138, 9327–9340 (2016).

  81. 81.

    Conrado, R. J. & Gonzalez, R. Envisioning the bioconversion of methane to liquid fuels. Science 343, 621–623 (2014).

  82. 82.

    Kalyuzhnaya, M. G., Puri, A. W. & Lidstrom, M. E. Metabolic engineering in methanotrophic bacteria. Metab. Eng. 29, 142–152 (2015).

  83. 83.

    Coleman, W. J. et al. Biological conversion of multi-carbon compounds from methane. US patent 20160160243A1 (2016).

  84. 84.

    Intrexon’s industrial products division achieves bioconversion of methane to farnesene. Intrexon https://investors.dna.com/2014-06-30-Intrexons-Industrial-Products-Division-Achieves-Bioconversion-of-Methane-to-Farnesene (2014).

  85. 85.

    Soo, V. W. et al. Reversing methanogenesis to capture methane for liquid biofuel precursors. Microb. Cell Fact. 15, 11 (2016).

  86. 86.

    Balasubramanian, R. et al. Oxidation of methane by a biological dicopper centre. Nature 465, 115–119 (2010).

  87. 87.

    Hu, P., Rismani-Yazdi, H. & Stephanopoulos, G. Anaerobic CO2 fixation by the acetogenic bacterium Moorella thermoacetica. AICHE J. 59, 3176–3183 (2013).

  88. 88.

    Lynd, L. R. et al. How biotech can transform biofuels. Nat. Biotechnol. 26, 169–172 (2008).

  89. 89.

    Nielsen, J. Yeast cell factories on the horizon. Science 349, 1050–1051 (2015).

  90. 90.

    Krivoruchko, A., Zhang, Y., Siewers, V., Chen, Y. & Nielsen, J. Microbial acetyl-CoA metabolism and metabolic engineering. Metab. Eng. 28, 28–42 (2015).

  91. 91.

    Levering, J., Broddrick, J. & Zengler, K. Engineering of oleaginous organisms for lipid production. Curr. Opin. Biotechnol. 36, 32–39 (2015).

  92. 92.

    Lee, S. Y. & Kim, H. U. Systems strategies for developing industrial microbial strains. Nat. Biotechnol. 33, 1061–1072 (2015).

  93. 93.

    Kerkhoven, E. J., Lahtvee, P. J. & Nielsen, J. Applications of computational modeling in metabolic engineering of yeast. FEMS Yeast Res. 15, 1–13 (2015).

  94. 94.

    Brunk, E. et al. Characterizing strain variation in engineered E. coli using a multi-omics-based workflow. Cell Syst. 2, 335–346 (2016). A multi-omics workflow was applied to elucidate engineered cell factories and identify potential engineering targets for improved production of interesting metabolites.

  95. 95.

    Dragosits, M. & Mattanovich, D. Adaptive laboratory evolution - principles and applications for biotechnology. Microb. Cell Fact. 12, 64 (2013).

  96. 96.

    Caspeta, L. et al. Altered sterol composition renders yeast thermotolerant. Science 346, 75–78 (2014).

  97. 97.

    Fletcher, E. et al. Evolutionary engineering reveals divergent paths when yeast is adapted to different acidic environments. Metab. Eng. 39, 19–28 (2017).

  98. 98.

    Mi, L. et al. Efficient production of free fatty acids from ionic liquid-based acid- or enzyme-catalyzed bamboo hydrolysate. J. Ind. Microbiol. Biotechnol. 44, 419–430 (2017).

  99. 99.

    Agbor, V. B., Cicek, N., Sparling, R., Berlin, A. & Levin, D. B. Biomass pretreatment: fundamentals toward application. Biotechnol. Adv. 29, 675–685 (2011).

  100. 100.

    Anbarasan, P. et al. Integration of chemical catalysis with extractive fermentation to produce fuels. Nature 491, 235–239 (2012). This paper shows how chemical and biological processes work coordinatively for synthesizing hydrocarbon biofuels from renewable sources .

  101. 101.

    Sanchez, B. J. et al. Improving the phenotype predictions of a yeast genome-scale metabolic model by incorporating enzymatic constraints. Mol. Syst. Biol. 13, 935 (2017).

  102. 102.

    Wang, H. et al. RAVEN 2.0: a versatile platform for metabolic network reconstruction and a case study on Streptomyces coelicolor. Preprint at https://doi.org/10.1101/321067 (2018).

Download references

Acknowledgements

The authors acknowledge funding from National Natural Science Foundation of China (grant no. 31700082) and DMTO research grant from Dalian Institute of Chemicals Physics, CAS (grant no. DICP DMTO201701) (to Y.J.Z.); the Novo Nordisk Foundation, the Knut and Alice Wallenberg Foundation, the US Department of Energy, Office of Science, Office of Biological and Environmental Research, Genomic Science program (Award number DE-SC0008744) and Horizon2020 via the CHASSY project (grant no. 720824) (to J.N.) and Åforsk Foundation (to E.J.K.).

Author information

Affiliations

  1. Key Laboratory of Separation Science for Analytical Chemistry, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, China

    • Yongjin J. Zhou
  2. Division of Biotechnology, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, China

    • Yongjin J. Zhou
  3. Department of Biology and Biological Engineering, Chalmers University of Technology, Gothenburg, Sweden

    • Yongjin J. Zhou
    • , Eduard J. Kerkhoven
    •  & Jens Nielsen
  4. Novo Nordisk Foundation Center for Biosustainability, Technical University of Denmark, Lyngby, Denmark

    • Jens Nielsen
  5. Beijing Advanced Innovation Center for Soft Matter Science, Beijing University of Chemical Technology, Beijing, China

    • Jens Nielsen

Authors

  1. Search for Yongjin J. Zhou in:

  2. Search for Eduard J. Kerkhoven in:

  3. Search for Jens Nielsen in:

Competing interests

The authors declare no competing interests.

Corresponding author

Correspondence to Jens Nielsen.

About this article

Publication history

Received

Revised

Accepted

Published

DOI

https://doi.org/10.1038/s41560-018-0197-x