Team:KU LEUVEN/Plant

Photosynthesis

Introduction Cyanobacteria in the biomass cycle Effect of our
project References

Photosynthesis lies at the basis of cyanobacteria use for durable solutions. This section takes a closer look at this process, its influence on global CO2 levels, and links photosynthesis to the choices made in our project [1].

Photosynthesis is the motor of the biosphere

Almost all life on planet earth, from the smallest possible bacterium to the largest mammal, is ultimately dependent on the sun. Indeed, through the process of photosynthesis, autotrophs directly convert the incoming solar beams to energy-rich molecules, such as glucose. Heterotrophs, in turn, consume autotrophic or other heterotrophic organisms, using their organic matter as an energy source. However, every case of heterotrophic energy uptake causes a loss in energy conversion as a result of the laws of thermodynamics. Therefore, it could be said that, of all life on this planet, autotrophs are the most efficient in using the energy provided by the sun [2].

Cyanobacteria contribute substantially to global biomass cycles

In ecosystems, energy transfers occur through biomass exchange. Particularly, in marine ecosystems, cyanobacteria amount to a significant fraction of primary biomass production through photosynthesis. For example, in certain areas, the marine cyanobacterium Prochlorococcus has been found to contribute up to 82% of oceanic primary production [1]. Similarly, the marine Synechococcus, a bacterium closely related to, is considered a major participant in global carbon cycles [2].

Although the contribution of cyanobacteria to global photosynthesis nowadays is significant, it used to be even higher. Indeed, primitive cyanobacteria were the reason that there is oxygen in the atmosphere at all [3]. These bacteria were the first species to develop photosynthesis, and any form of photosynthesis that takes place on earth today, be it in cyanobacteria or the chloroplasts of algae and plants, descends from these original pathways. Shortly put, chloroplasts are cyanobacteria that have been absorbed by other cells through phagocytosis; this theory is known as endosymbiosis. For a long time, the original cyanobacteria were the only photosynthetic organisms on earth, and through photosynthesis, they were the driving force that transformed the earth’s atmosphere to contain oxygen. This cyanobacteria-induced change in atmospheric concentrations is termed the ‘great oxidation event’ and was a crucial global change that paved the way for higher lifeform life [3].

Figure 1: Principle of photosynthesis. Cyanobacteria, as well as other photosynthetic organisms, produce energy-rich compounds and oxygen when irradiated with light while in the presence of CO2. This energy can then be transferred to higher organisms, either through the uptake of secreted molecules and proteins or by direct uptake when feeding on these organisms. However, the transfer of this energy is not efficient, losing almost 90% of energy with each transferring step. This makes immediate protein production with energy coming from the sun more sustainable than those produced with heterotrophic organisms.

The cyanobacteria in our project are a sustainable alternative to heterotrophic biosynthesis platforms

The issue, as mentioned above, about heterotrophic organisms and their limitedly efficient use of solar energy, is one that is also reflected in the biotechnological industry. Currently, heterotrophs such as yeast, E. Coli or B. Subtilis are often the workhorses used for production in biomanufacturing companies [4] [5]. The heterotrophic nature of this kind of production system implies the need for external energy and carbon supplies, which come in the form of the growth medium, often starch or glucose-based. Besides the associated cost, the production of this kind of medium requires vast amounts of arable land, which is a recourse of great scarcity in a world with a high population rate, driving the competition for arable land with other industries such as the food market. This issue adds up to the low solar energy-efficiency that characterizes these systems.

Addressing these problems, we propose cyanobacteria as an alternative for heterotrophic platforms. Their photosynthetic nature mitigates the need for arable land and provides more efficient use of solar rays that reach our planet. Moreover, cyanobacterial biomanufacturing has the additional advantage of being a carbon-negative technology helping tackle the global emissions problem. Our project, OCYANO, aims to tackle some of the sustainability issues that come with heterotrophic biomanufacturing, inspired by the first cyanobacteria that made our planet a suitable habitat for eukaryotic life and, in turn, human life.

References

1. H. Liu, H. A. Nolla, and L. Campbell, “Prochlorococcus growth rate and contribution to primary production in the equatorial and subtropical North Pacific Ocean.” Aquatic Microbial Ecology, vol. 12, no. 1. pp. 39-47, 1997.
2. C. S. Ting, G. Rocap, J. King, and S. W. Chisholm, “Cyanobacterial photosynthesis in the oceans: The origins and significance of divergent light-harvesting strategies,” Trends in Microbiology, vol. 10, no. 3. Elsevier Ltd, pp. 134–142, 01-Mar-2002.
3. B. E. Schirrmeister, M. Gugger, and P. C. J. Donoghue, “Cyanobacteria and the Great Oxidation Event: evidence from genes and fossils.,” Palaeontology, vol. 58, no. 5, pp. 769–785, Sep. 2015.
4. S. Mayfield and S. S. Golden, “Photosynthetic bio-manufacturing: Food, fuel, and medicine for the 21st century,” Photosynthesis Research, vol. 123, no. 3. Kluwer Academic Publishers, pp. 225–226, 2015.
5. A. W. Westbrook, X. Ren, M. Moo-Young, and C. P. Chou, “Metabolic engineering of Bacillus subtilis for l-valine overproduction.,” Biotechnol. Bioeng., vol. 115, no. 11, pp. 2778–2792, 2018.