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This is the published version of a paper published in Physiologia Plantarum: An International Journal for Plant Biology.
Citation for the original published paper (version of record):
Calderon, R H. (2020)
More than just a pair of blue genes: how cyanobacteria adapt to changes in their light environment
Physiologia Plantarum: An International Journal for Plant Biology, 170(1): 7-9 https://doi.org/10.1111/ppl.13178
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IN THE SPOTLIGHT
More than just a pair of blue genes: how cyanobacteria adapt to changes in their light environment
Robert H. Calderon*
Department of Plant Physiology, Umeå University, Umeå 90736, Sweden
Linked article: This spotlight refers to Luimstra et al. (2020). To view this article, please visit https://doi.org/10.1111/ppl.13086
Cyanobacteria require light to perform photosynthesis, but not all colors of light are equally useable for them. In par- ticular, blue light-grown cyanobacterial strains, including the well-studied model organismSynechocystis sp. PCC 6803 (Synechocystis), have been observed to exhibit slower growth rates than white or red light-grown cells. In this issue of Physiologia Plantarum, Luimstra et al. (2020) have attempted to understand why cyanobacterial cells suffer under blue light. They measured the molecular and genetic responses ofSynechocystis cells to being shifted from white light to blue light. They found that blue light-grown cells make changes that lead to a redistribution of energy flow between the two photosystems that power photosynthesis. These findings could help researchers identify ave- nues for optimizing photosynthesis in cyanobacterial species, a group of organisms which show great promise as potential solar-powered factories for the production of biofuels and other high-value products.
Human beings have a relationship with cyanobacteria not unlike our relationship with sharks. While we can sometimes see Spirulina being sold as a nutritional sup- plement or a friendly (and vegetarian) Great White Shark in animatedfilms, humans generally regard sharks and cyanobacteria as dangers to human health. That sharks have sharp teeth or that some cyanobacteria cause toxic algal blooms is indisputable, but these two groups alsofill important ecological roles that deserve our appreciation. Cyanobacteria are the closest living relatives of the microbes responsible for oxygenating our atmosphere over 2 billion years ago and widely dis- tributed on the planet to this day. They use photosynthe- sis to harvest sunlight,fix carbon dioxide and produce oxygen. For this reason, they are of great interest as potential solar-powered producers of a wide array of products ranging from biofuels to cosmetics and more (Abed et al. 2009).
For cyanobacteria to conduct photosynthesis effi- ciently, they must closely coordinate the activities of pho- tosystem II (PSII) and photosystem I (PSI). If one of the two photosystems become damaged or its activity slows down, the activity of the other photosystem must adjust accordingly. Not doing so could lead to imbalances in the cell that reduce its chances of growing or even surviv- ing. One of the factors that can lead to an imbalance between the two photosystems is the quality (color) of light. Specifically, blue light has been hypothesized to cause overexcitation of PSI at the expense of PSII (Luimstra et al. 2018).
In this month’s issue of Physiologia Plantarum, Luim- stra et al. have looked at the transcriptional response of Synechocystis sp. PCC 6803 (hereafter Synechocystis) after exposure to blue light. It has been previously observed that cyanobacteria have lower growth rates under blue light than under standard white light condi- tions (Wilde et al. 1997). Because blue light is dispropor- tionately used by PSI at the expense of PSII, Luimstra et al.
hypothesized that cells would compensate for this by adjusting the PSI:PSII ratio in response to blue light.
The authors examined two possible ways thatSynecho- cystis might adjust its PSI:PSII ratio, but first they confirmed that blue light-grownSynechocystis do grow at a slower rate than red or orange light-grown cells (Fig. 1A). They then looked at whether or notSynechocystis redistributes cap- tured light energy between the two photosystems. Most cya- nobacterial species use specialized light-harvesting antennae called phycobilisomes (PCBs) that trap light energy and funnel it to both PSI and PSII (Fig. 1B). In response to blue light, the PCBs inSynechocystis cells remain attached to PSII but detach from PSI (Fig. 1C). This redistributes incom- ing light energy to PSII at the expense of PSI.
At the same time, they found that Synechocystis cells were specifically increasing the transcript levels of genes encoding subunits of PSII (Fig. 1C). These changes were detectable 4 h after exposure to blue light and persisted even 8 days later. The cells also appeared to be preparing for a reduction in photosynthesis by decreasing transcript levels of genes associated with ATP production, carbon fixation and protein translation. Interestingly, they
Physiologia Plantarum 170: 7–9. 2020 ISSN 0031-9317
© 2020 The Author. Physiologia Plantarum published by John Wiley & Sons Ltd on behalf of Scandinavian Plant Physiology Society.
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massively increased the transcription of a set of genes associated with stress.
So, what is the implication of the work by Luimstra and colleagues? For starters, it lends further support to the hypothesis that blue light is preferentially absorbed by PSI over PSII in cyanobacteria. This can be seen in the detachment of light-harvesting antennae from PSI and the increased transcription of PSII genes.
On a larger scale, their work also highlights one of the potential challenges in using bioengineered cyanobacterial strains to make high-value products for humans. Efforts
have already been made to expand the spectrum of light that cyanobacteria can use for photosynthesis. This includes bioengineering a strain of cyanobacteria to be able to absorb and use far-red light for photosynthesis (Ho et al. 2016). There have been some attempts at improv- ing the usage of blue light for photosynthesis in cyanobac- teria. These approaches have focused on expressing plant light-harvesting proteins in cyanobacteria, a method that has unfortunately not yet been successful. This is likely due to the difficulty of ensuring proper folding of these proteins in a different organism (Jensen and
Physiol. Plant. 170, 2020 8
Leister 2014). Because the results of Luimstra and co- workers have focused on the native response of cyano- bacteria to blue light, their work could lead to new approaches for better using blue light that bypass this problem altogether. Manipulating the expression of some of the genes identified in this study, for example, could lead to strains of cyanobacteria that are better suited for the artificial growth environments required for the production of biofuels. Perhaps the apprecia- tion for these biofuel-producing strains willfinally lead to cyanobacteria getting the respect that they deserve.
References
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Hellingwerf KJ, Huisman J, Matthijs HCP (2018) Blue light reduces photosynthetic efficiency of cyanobacteria through an imbalance between photosystems I and II.
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Luimstra VM, Schuurmans JM, Hellingwerf KJ, Matthijs HCP, Huisman J (2020) Blue light induces major changes in the gene expression profile of the cyanobacterium
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