Iron Rescues Algal Photosynthesis During High Lipid Production
A study tracking the roles of iron during lipid accumulation in a single-celled alga reveals key features of photosynthesis and biofuel production.
Light microscopy images of the green alga Chromochloris zofingiensis responding to iron and/or glucose treatments, which induce several metabolic and structural changes. From left to right: cell 1, low iron, no glucose (photosynthetic); cell 2, high iron, no glucose (photosynthetic); cell 3, low iron, glucose (non-photosynthetic and lipid accumulating); cell 4, high iron, high glucose (photosynthetic and lipid accumulating). (Image courtesy of Tim Jeffers | UC Berkeley)
The Science
Photosynthesis is the process by which plants and algae convert sunlight and carbon dioxide into oxygen and biomass. However, it is not fully understood how photosynthesis responds to changes in an organism’s environment and nutritional state. A team of researchers from the University of California (UC), Berkeley, and the Environmental Molecular Sciences Laboratory (EMSL), a Department of Energy Office of Science user facility, found that iron nutrition determines whether the photosynthesis of the green alga Chromochloris zofingiensis switches off or is maintained in the presence of glucose. Without sufficient iron, glucose-treated cells turn off photosynthesis and accumulate storage lipids, which have the potential to be used as biofuels. However, if sufficient iron is added with glucose, cells maintain photosynthesis and still accumulate lipids. Because iron is a nutrient with essential roles in both lipid production and photosynthesis, an iron limitation with glucose causes cells to prioritize iron-dependent proteins of respiration and lipid synthesis over photosynthetic proteins and membranes. This research provides new insights into fundamental and evolutionarily conserved mechanisms of sugar and nutrient signaling and the regulation of photosynthesis in single-celled organisms that are at the base of the photosynthetic evolutionary tree.
The Impact
Most life on Earth depends on the oxygen and food produced from photosynthesis. Understanding which genes and proteins are responsible for controlling photosynthesis and its bioproducts provides researchers with targets for improving the production of crops and sustainable bioproducts. Toward these goals, this large-scale proteomic study unveiled the unique wiring of a green alga to precisely control various photosynthetic and lipid accumulation states. Moreover, data from this study provide a foundation for comparisons across large evolutionary scales. This research revealed the existence of proteins with unknown functions that are likely to have significant roles in photosynthesis and are conserved across algae and plants. Both regulatory and novel proteins such as these, which are associated with photosynthesis and lipid accumulation, could be manipulated in many organisms to enhance crop and biofuel production.
Summary
The single-cell green alga Chromochloris zofingiensis is an emerging model organism for photosynthesis and metabolism research. Previous research has shown that with the addition of glucose, Chromochloris zofingiensis shuts off photosynthesis, reroutes its metabolism, and accumulates high amounts of lipids (biofuel precursors) and astaxanthin (a high-value antioxidant). A team of scientists from the University of California, Berkeley, and EMSL has now shown that cells grown with an iron supplement and glucose could maintain photosynthesis and thylakoids, the membranes where photosynthetic energy transfer occurs, while still accumulating lipids. Proteomic samples analyzed by EMSL researchers revealed the metabolic wiring of cells associated with the loss of photosynthesis and lipid accumulation. Transmission electron microscopy conducted using EMSL capabilities allowed for the visualization of thylakoids, starch, and lipid accumulation. Together with physiological analyses, the team determined that glucose-mediated photosynthesis repression is associated with a reprioritization of iron resources toward respiratory instead of photosynthetic complexes, as well as a ferredoxin-dependent desaturase pathway that supported lipid storage over thylakoid lipid synthesis. Furthermore, the team conducted comparative evolutionary genomics on green algae and vascular plants to show how iron and metabolic trophic constraints can aid in gene discovery for photosynthesis and biofuel production. This study shows that at the base of the photosynthetic evolutionary tree, Chromochloris zofingiensis is a simple organism that can be used to study energy, nutrients, and photosynthesis, and that these results can provide a roadmap to enhance the production of bioenergy crops and bioproducts.
Contacts
Melissa Roth, UC Berkeley, mroth@berkeley.edu
Krishna Niyogi, UC Berkeley | Howard Hughes Medical Institute | Lawrence Berkeley National Laboratory, niyogi@berkeley.edu
Mary Lipton, EMSL, Mary.Lipton@pnnl.gov
Funding
This study was supported by the Department of Energy, Office of Science, Office of Biological and Environmental Research (BER) program. A portion of this research was performed on a Facilities Integrating Collaborations for User Science project award and used resources at the Joint Genome Institute and the Environmental Molecular Sciences Laboratory, which are both DOE Office of Science user facilities sponsored by BER. Additional support was provided by the Howard Hughes Medical Institute.
Publication
T.L. Jeffers, et al. “Iron rescues glucose-mediated photosynthesis repression during lipid accumulation in the green alga Chromochloris zofingiensis.” Nature Communications 15, 6046 (2024). [DOI: 10.1101/2023.07.31.551119]