02/20/2019

Discovering an Internal Metabolic Switch in Algae

Discovering hexokinase as an algal regulator of lipids and high-value antioxidants will enable sustainable sources of biofuels and bioproducts.

The Science

Algae can accumulate lipids such as oils and waxes, as well as other useful chemicals. Scientists do not completely understand the genetic mechanisms that regulate how algae build up these chemicals. By analyzing algae’s genes, researchers discovered that an enzyme called hexokinase plays a key role in how algae accumulate lipids. It also plays an important role in how green algae build up large amounts of the antioxidant astaxanthin. This enzyme is also responsible for shutting off photosynthesis when sugars are present.

The Impact

Studying microalgae helps scientists better understand biological pathways that are also in many other species. It is then easier for scientists to manipulate these systems. This research expands what is known about how algae and plants regulate photosynthesis. It also reveals different ways species use energy under different metabolic conditions. This discovery could enable increased production of biofuels and bioproducts.

Summary

Photosynthesis and metabolism in plants and algae drive global carbon fixation. Algae also have the potential to contribute to a sustainable bioeconomy by delivering valuable chemicals with reduced environmental impacts. Unlocking the biology behind relevant phenotypes can reveal new opportunities for bioengineering and creating commercially viable sources of biofuels and bioproducts in a sustainable fashion.

The unicellular green alga Chromochloris zofingiensis accumulates high amounts of lipids in the form of triacylglycerols (TAGs), which are biodiesel precursors, and the high-value antioxidant astaxanthin. This study used forward genetics to reveal that the widely conserved glycolytic enzyme hexokinase (HXK1) is necessary for a photosynthetic and metabolic switch. Glucose represses photosynthesis both in plants and algae, but in C. zofingiensis, it also causes rapid accumulation of TAG and astaxanthin. Algae with mutations in HXK1 showed that this enzyme is necessary for shutting off photosynthesis and amassing bioproducts. C. zofingiensis is a promising candidate for bioproduction, and insights into its regulation of photosynthesis and metabolism will enable engineering of this organism to improve its commercial prospects. Nutrients such as glucose play essential regulatory roles in gene expression, metabolism, growth and aging in plants, animals, yeast, and bacteria. This study introduces C. zofingiensis as a simpler system to investigate HXK function, shedding light onto fundamental and evolutionarily conserved mechanisms of glucose signaling and regulation of photosynthesis at the base of the plant evolutionary tree.

Principal Investigator(s)

Melissa Roth
University of California, Berkeley
MRoth@berkeley.edu

Related Links

Funding

This work was supported by the Office of Biological and Environmental Research within the U.S. Department of Energy Office of Science; U.S. Department of Agriculture National Institute of Food and Agriculture, and the National Science Foundation.

References

Roth, M. S., D. J. Westcott, M. Iwai, and, K. K. Niyogi, “Hexokinase is necessary for glucose-mediated photosynthesis repression and lipid accumulation in a green alga.” Communications Biology 2, 347 (2019). DOI:10.1038/s42003-019-0577-1

Roth, M. S., S. D. Gallaher, D. J. Westcott, M. Iwai, K. B. Louie, M. Mueller, A. Walter, F. Foflonker;, B. P. Bowen, N. N. Ataii , J. Song, J.-H Chen, C. Blaby-Haas, C. Larabell, M. Auer, T. Northen, S. S. Merchant and, K. K. Niyogi. “Regulation of oxygenic photosynthesis during trophic transitions in the green alga Chromochloris zofingiensis.” The Plant Cell 31, 579-601 (2019). DOI:10.1105/tpc.18.00742