The Science
Scientists have long been looking for new ways to make fuel, and algae that naturally produce and hold onto large amounts of fatty molecules within their bodies are a promising direction. For some algae species of interest, like Nannochloropsis oceanica and other Eustigmatophytes, questions remain regarding the basics of the organism’s life cycle — including a special structural feature, dubbed the ‘red body.’ Now, Fourier transform infrared spectroscopy has revealed an accumulation of antioxidant carotenoids, responsible for the red color, and large quantities of long-chain aliphatic lipids, a type of fatty molecule, within the globular structure. In the same study, ultra-performance liquid chromatography coupled with high-resolution mass spectrometry detected a C32 alkyl diol, a potential precursor of the material algaenan, which is a recalcitrant cell wall polymer produced by some green algae. Transmission electron microscope imaging and 3D cryo-tomography indicated the red body is a membrane-bound organelle that likely facilitates the transport of key molecules needed for cell wall construction, supporting the ability of N. oceanica to rapidly divide into two, four — or even eight — daughter cells every 24 hours. Without the organelle, N. oceanica could face challenges transporting the hydrophobic molecules, which include the C32 alkyl diol, through its water-based interior. The work represents a new biological link between molecular and large-scale processes in cellular systems.
The Impact
As the effects of climate change continue to grow, scientists face mounting calls to deliver alternative fuels with carbon-neutral emissions when burned. Eustigmatophytes, a group of single-celled algae found in freshwater, marine, and terrestrial environments, could offer an opportunity to produce new biofuels. However, a limited understanding of the life cycle and cell biology of the model species N. oceanica has restricted researchers’ ability to draw conclusions that could otherwise inform cultivation and genetic engineering directions of additional algae species considered to be good candidates for producing biofuels and/or sequestering carbon.
Summary
Because few algae species exhibit the ‘red body’ characteristic of Nannochloropsis oceanica, its role in the organism’s cellular life cycle has remained largely unknown. This pigmented organelle initially forms adjacent to the cell’s food-producing plastid, before ultimately being released outside the cell wall during autosporangial division — when a parent N. oceanica alga divides into daughter cells via spore production, in a process known as autospore release.
Scientists’ interest in Eustigmatophyte algae is two-fold: partly because of its rapid growth, and partly because of its ability to partition up to half its mass into valuable lipids. These hallmark features indicate the algae’s potential for use in biofuel applications. Once thought rare, today Eustigmatophytes have been found in a range of environments, including freshwater, marine, and terrestrial systems. Two genera of Eustigmatophyte, Nannochloropsis and Microchloropsis, have been established as model systems. In both cases, their cells are solitary, non-motile, and round, with diameters on the order of 2 to 4 microns. Species belonging to both genera reproduce on a diurnal basis via asexual fission, growing during the day and splitting each night. A range of reference genomes have been published, in association with growing interest in the potential application of gene editing tools to Eustigmatophyte algae, toward biofuel applications.
The presence of a red-orange globule outside the cell’s chloroplast during the day is a diagnostic characteristic of Eustigmatophytes. Whereas many have documented the globule’s widespread occurrence throughout the group, this work provided the first account of its formation and biological function, using a combination of techniques that included ultra-performance liquid chromatography coupled with high-resolution mass spectrometry, various laser and electron microscopy methods, and Fourier transform infrared spectroscopy. The intriguing autofluorescent, globular nature of the ‘red body’ — with its distinct compartmentalization and differentiation — led researchers to define it as a membrane-bound organelle.
Further study of the red body’s contents led researchers to hypothesize that it could be a delivery vessel for molecules used in cell wall construction. During the day, each N. oceanica grows rapidly. Then, at night, each large alga divides into multiple daughter autospores — and with four cells instead of just one, suddenly significantly more cell wall material is needed to fully encapsulate each daughter N. oceanica. The red body aids this process, researchers believe, by ensuring large amounts of new cell wall building materials — to make a specific part of the cell wall known as algaenan — are available when needed. Infrared spectroscopy analyses back up this hypothesis, revealing that red bodies discarded after autospore generation contain a range of precursor and intermediate products needed for cell wall formation, in addition to some fully polymerized algaenan matter.
Researchers see N. oceanica as a model organism for understanding the biosynthesis of “chemically recalcitrant lipidic biopolymers” via plastid-derived fatty molecules that must be transported through an aqueous inner-cell environment to the cell wall. Similar molecules are ubiquitous throughout plant lineages, because they play a key role in plant physiology by controlling the movement of water within and around plant bodies and the cells that constitute them. Until now, many of the details related to the transport of these molecules within the cell, and their final polymerization process during cell wall construction, remained unknown. As a result, this work contributes new insight into the biological link between molecular and large-scale processes at the cellular level.
The collaboration included researchers from the University of California, Berkeley, the Lawrence Berkeley National Laboratory (LBNL), and the University of Copenhagen in Denmark. LBNL is home to the Berkeley Synchrotron Infrared Structural Biology (BSISB) program, funded by DOE-BER, and the Advanced Light Source, a DOE Office of Science user facility.
Contacts
Hoi-Ying N. Holman
Molecular Biophysics and Bioimaging, Biosciences Division, Lawrence Berkeley National Laboratory
Krishna K. Niyogi
Howard Hughes Medical Institute and Department of Plant and Microbial Biology, University of California, Berkeley, and Molecular Biophysics and Bioimaging, Biosciences Division, Lawrence Berkeley National Laboratory
Funding Acknowledgements
This work was supported by the U.S. Department of Energy, Office of Science, Office of Biological and Environmental Research under Contract Nos. DEAC02-05CH11231 and by the Howard Hughes Medical Institute.
Related Links
References
Gee CW, Andersen-Ranberg J, Boynton E et al. Implicating the red body of Nannochloropsis in forming the recalcitrant cell wall polymer algaenan. Nature Communications 15, 5456 (2024). (https://doi.org/10.1038/s41467-024-49277-y)