Photosynthesis for fresh water

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Published on: February 10, 2014

Annegret Honsbein is a post-doc in Anna Amtmann‘s lab at the University of Glasgow. As she explains in this guest post, she is working on an EPSRC project that hopes to harness the power of photosynthesis to desalinate sea water. 

Water covers more than 70% of Earth’s surface but less than 2% of it is available as freshwater. Many of the driest regions of our planet are close to the sea but irrigating fields with seawater – even if diluted – leads to build-up of salt in the soil to levels toxic to all common food crops. Current desalination technologies, such as membrane-based reverse osmosis, are successfully used in large-scale desalination plants, but they are expensive and energy inefficient.

desalinationOur multi-disciplinary EPSRC-funded project takes a synthetic biology approach to the development of an innovative desalination technology based on biological processes. We are a team of biologists and engineers from the Universities of Glasgow, Sheffield, Newcastle, Robert Gordon University at Aberdeen and Imperial College London, led by Dr. Anna Amtmann from Glasgow University.

Our idea is solar energy-fuelled desalination – with a twist. Instead of using solar panels we intend to let photosynthetic microorganisms desalinate the sea water. Cyanobacteria are ideal candidates, and we are currently working with two strains that are naturally able to adapt to a wide range of salt concentrations from fresh to sea water.

In principle, salt is toxic to all living cells, which is why most living systems have developed means to actively export sodium. In some cyanobacteria species that grow to very high densities, this ability means they actually form a low-salt reservoir within their saline environment.

We intend to use this low-salt reservoir as ion exchanger to extract the salt from the surrounding seawater. We aim to engineer cyanobacteria so we can switch off the endogenous salt export mechanism towards the end of their growth cycle, and activate a synthetic intracellular sodium accumulation unit. This synthetic unit will be assembled from membrane transport proteins evolved by different organisms to import sodium and chloride ions.

Our team’s engineers are developing techniques to manipulate the surface properties of the cyanobacteria and effectively separate the ‘salty’ cells from the desalinated water before they die, preventing release of the accumulated salt back into the ‘fresh’ water.

The final stage of the project will be to build a model version of the actual plant that could house our photosynthesis-driven bio-desalination process.

This work is published in: Jaime M. Amezaga, Anna Amtmann, Catherine A. Biggs, Tom Bond, Catherine J. Gandy, Annegret Honsbein, Esther Karunakaran, Linda Lawton, Mary Ann Madsen, Konstantinos Minas and Michael R. Templeton (2014) Biodesalination: A Case Study for Applications of Photosynthetic Bacteria in Water Treatment. Plant Physiology 164: 1661-1676; doi: http:/​/​dx.​doi.​org/​10.​1104/​pp.​113.​233973.

Image c/o Annegret Honsbein.

Investigating photosynthesis

In today’s highlighted article, the authors use traditional and far more modern biochemistry to uncover why photosynthesis is inhibited by Streptomyces spp., and characterise a previously unknown step in cyclic electron flow. This is also a good opportunity to point out these great photosynthesis outreach and education resources from Science and Plants for Schools. Admittedly, they don’t have anything on AA-sensitive CEF, because it’s unlikely anyone without a plant biochemistry PhD needs to know about that! But they have brilliant basic photosynthesis teaching resources, including these amazing algae-jelly-balls.

Photosynthesis background: Broadly speaking, photosynthesis is the process by which light energy from the sun is absorbed by Photosystems I and II (PSI and PSII), where it is channelled into electron transport chains and stored in ATP and NADPH. One electron carrier is ferredoxin (Fd).

There are two types of electron flow, cyclic and linear (CEF and LEF), which generate ATP. Though they are different processes, both CEF and LEF require PSI and PSII, two other thylakoid proteins, PGR5 and PGRL1, and electron carriers Fd, plastoquinone (PQ) and plastocyanin.

In CEF there are two processes by which electrons are transferred from Fd to PQ. One is a characterised NADH dehydrogenase-like compex dependent pathway, and all that is known about the other is that it is sensitive to antimycin A (AA), a product of Streptomyces spp. which inhibits CEF.

For background on AA-inhibition and the divergent electron transfer pathways in CEF, see Joët et al. (2001; Plant Phys. 24:1919). For more information on PGR5 and PGRL1, see DalCorso et al. (2008; Cell 132:273).

Gaps in knowledge of CEF:

  • What makes that electron transfer process from Fd to PQ sensitive to AA?
  • How do the electrons from photoreduced Fd get transferred to PQ and back into the electron transport chain?
  • What do functions do PGR5 and PGRL1 perform?

New from Hertle et al.: Titration and Western blotting experiments showed that PGR5 and PGRL1 dimerize to each other. Six cysteine residues were conserved in all PGRL1 proteins. Hertle et al. made mutant PGRL1 proteins in which one or more cysteine residues were substituted for serine, and tested the varients for their capacity to bind PGR5, iron, and to promote AA-sensitive CEF. They worked out that all six cysteines were essential for AA-sensitive CEF, while specific cysteines were involved in binding iron and PGR5.

In vitro assays demonstrated that PGRL1 is capable of transferring electrons between Fd and PQ analogue DMBQ in the presence of PGR5, and this reaction was inhibited in the presence of AA. Hertle et al. show clearly that PGRL1 is physically a fit for a ferredoxin-plastoquinone reductase and make a strong case for it being the mediator between Fd and PQ in CEF, and the AA-sensitive step in the cyclic electron flow.

Highlighted article: Alexander P. Hertle, Thomas Blunder, Tobias Wunder, Paolo Pesaresi, Mathias Pribil, Ute Armbruster, Dario Leister (2013) PGRL1 Is the Elusive Ferredoxin-Plastoquinone Reductase in Photosynthetic Cyclic Electron Flow. Molecular Cell – 03 January 2013, 10.1016/j.molcel.2012.11.030

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