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| Fig. 1: Hill Reaction - the process by which an electron aceptor is photoreduced by the hydrogen found in water, resulting in the evolution of oxygen. (Source: Wikimedia Commons) |
Photosynthesis is the reaction performed by many organisms on earth - including plants and cyanobacteria, among others - which evolves oxygen and enables the survival of humans within the terrestrial atmosphere. The full photosynthetic reaction involves taking 6 parts carbon dioxide and 12 parts water, converting it into glucose, 6 parts oxygen, and 6 parts water. Chlorophyll-a is the main pigment involved in this process, capturing energy from light to carry out the photosynthetic reaction. One particularly interesting process which provides insights into the process of photosynthesis is the Hill Reaction (Fig. 1), named after Robert Hill, who discovered it in 1937. [1] Hill showed that isolated chloroplasts are capable of evolving oxygen, even in the absence of carbon dioxide, implicating water as the source of the free electrons in the photosynthetic process. The critical aspect of photosynthesis - with or without carbon dioxide - is the electron transport chain, which occurs within the chloroplast in a suborganelle known as the thylakoid membrane. The electrons freed from water, as previously mentioned, help to establish a proton gradient powering the production of ATP and the storage of energy in reduced NADPH. This energy, finally, is used to produce glucose as well as other carbohydrates, the energy source of the organisms which rely on the process to produce their own "food".
The enhancement of photosynthesis could prove critical in the slowing and reversal of excessive greenhouse gas buildup, also known as global warming. What exactly the "enhancement of photosynthesis" means has, however, been interpreted differently by different groups of scientists. This report shall detail a number of these efforts, ranging from optimizing existing pathways in carbon fixation, to creation of synthetic pathways for carbon fixation, to improved energy collection efficiency in the existing photosynthetic process. [2-4] Finally, it will touch on a novel approach in increasing energy absorption. [5]
Rubisco is the primary enzyme in the process of fixing carbon dioxide into oxygen in the process of photosynthesis. Rubisco, unfortunately, is slow and inefficient. Furthermore, carbon dioxide must diffuse through multiple layers to reach its final destination to be fixed in a photosynthetic organism. Stomata can be thought of as gates through which carbon dioxide must pass in order to be fixed. For the purposes of water maintenance, these stomata open and close, meaning CO2 diffusing is not always optimized. However, affecting these important regulators could mess with other life support systems in an organism, and thus it is not ideal to modify their activity. Rubisco, meanwhile, serves the sole purpose of fixing CO2 and should be optimized, but is not. Increasing the specificity for CO2 would be the primary means of doing this, as Rubisco struggles to differentiate CO2 from O2, resulting in photorespiration instead of photosynthesis. Transplantation of Rubisco enzymes from C4 plants - those with superior CO2 turnover rates - to C3 species is the method of choice for carrying out this move toward optimal. [2] Another optimization technique is to bypass this metabolic process altogether in favor of an artificial process. The synthetic process, rather than relying on a few inefficient enzymes, includes 17 "designer" enzymes. Several different phases of the process were individually crafted for compatibility and optimality. The result - the CETCH cycle - includes enzymes from a variety of biological origins. Problems with compatibility were resolved by forcing alternative reaction sequences and through enzyme engineering. The process not only functions in vivo but is also superior in terms of efficiency to the naturally-evolved chemical process involving Rubisco. [3] The final method for enhancing photosynthesis detailed here involves increasing the speed of light adaptation in photosynthesis to reduce the amount of wasted incident energy. Plants protect themselves in high sunlight environments by dissipating some of the light back as heat energy, but this process is slow, resulting in suboptimal performance during times of shifting light intensity. Calculations by scientists have shown that this can cost certain crop plants as much as 20% of their potential yield and CO2 fixation capacity. Through the bioengineering of the xanthophyll cycle - more specifically, accelerating the conversion of violaxanthin to zeaxanthin and the reverse process - carbon uptake by these crop plants was increased by 15% during conditions of fluctuating light intensity.
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| Fig. 2: Sunlight Spectrum - the wavelengths and quantities of light incident upon the surface of the earth from the sun. (Source: Wikimedia Commons) |
While chlorophyll-a and chlorophyll-b are the primary photosynthetic pigments involved in absorbing light for the photosynthetic process in most plants, there are a number of auxiliary pigments - including carotenoids and phycobilins - which aid in absorbing parts of the incident sunlight spectrum (Fig. 2) which the chlorophyll pigments cannot. However, there are vast swaths of sunlight that are left untapped even by these pigments which are found in photosynthetic organisms such as cyanobacteria. To get around this limitation, scientists bioengineering biohybrid photosynthetic "antenna" systems which were analogous to photosynthetic light-harvesting devices naturally found in organisms, but were capable of absorbing light in different parts of the spectrum. Highly rapid and efficient energy transfer processes were observed from these antenna systems to the photosynthetic machinery, with high energy yields for the accessory chromophore absorption systems also measured. By relying on natural light-harvesting structures, the scientists were able to engineer systems which reliably replaced but augmented the abilities of their natural counterparts. Further work on engineering super-absorptive light-harvesting systems is underway at the Photosynthetic Antenna Research Center at Washington University in St. Louis.
Photosynthesis is the most important process in the battle against climate change. By increasing the efficiency of the process through which autotrophs create their food through the fixation of CO2, scientists actively reduce the amount of carbon pollution affecting the atmosphere. This research also has the potential to increase crop yields by increasing how much photosynthesis can be performed - and thus, how much carbon uptake can be undergone - with the same amount of incident sunlight. The energy from the sun is far from being tapped in the pursuit of optimal carbon fixation, but steps improving this process are crucial for feeding a growing population and keeping the environment suitable for human life on earth. Energy transformation efficiency plays a key role in many aspects of human life, yet perhaps none greater than in the process of photosynthesis fixing carbon and creating food from the energy of the sun.
The question of whether enhanced photosynthesis is an economical approach to fighting climate change is an interesting and important one. Some groups have estimated that implementation of the Renewable Portfolio Standards (RPS) in the United States would result in cost reductions due in health and climate of $1.1 trillion. The social benefit is estimated to be quite large as well. Furthermore, with current increases in population, the world will need to produce 70% more food to feed everyone in 2050, a growth which our current rate of progress will not meet. Therefore, the economic and social benefits to enhanced photosynthesis go beyond health and climate. The issue with funding solutions to climate change is that it is a societal problem that does not "affect" any individual yet requires individuals to pay for solutions and act in a way that is environmentally-friendly. This misalignment of incentives makes the issue especially perilous.
© Wyatt Pontius. The author warrants that the work is the author's own and that Stanford University provided no input other than typesetting and referencing guidelines. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
[1] R. Hill, "Oxygen Evolved by Isolated Chloroplasts," Nature 139, 882 (1937).
[2] S. von Caemmerer and J. R. Evans, "Enhancing C3 Photosynthesis," Plant Physiol. 154, 592 (2010).
[3] T. Schwander et al, "A Synthetic Pathway For The Fixation Of Carbon Dioxide In Vitro," Science 354, 6314 (2016).
[4] J. Kromdijk et al, "Improving Photosynthesis And Crop Productivity By Accelerating Recovery From Photoprotection," Science 354, 6314 (2016).
[5] J. W. Springer et al, "Biohybrid Photosynthetic Antenna Complexes For Enhanced Light-Harvesting," J. Am. Chem. Soc. 134, 10 (2012).
