Objectives Partners Publications

Programme ERASYNBIO1-062, ANR financing for a duration of 36 months, 2014-2017.
Coordinator: Thomas Happe (Ruhr University, Bochum, Germany)

Objectives

One of the greatest challenges of the 21st century is the sustainable supply of energy and chemicals from renewable resources. Driven by solar energy, chloroplasts function in nature as the most efficient minimal cell factories for generating chemical energy through the oxidation of water, but they are naturally tuned towards the fixation of carbon for building-up cellular components.

Our long-term goal is to design a synthetic chloroplast in the “green yeast” Chlamydomonas reinhardtii that can be used as a chassis for the sustainable production of biofuels and chemicals. This will be achieved by developing microalgal BioBricks to allow the plug-in of protein and metabolic circuits in the chloroplast. Specifically, we aim to re-direct photosynthetic electron transport through the engineering of key players in the photosynthetic chain.

State-of-the-art protein film electrochemistry of the engineered biocatalysts will guide the design processes. As a proof-of-principle, we will then use the chloroplast of optimised strains for assembling the BioBricks with the engineered photosynthetic chain players to produce bio-hydrogen and alkanes as by-products from light and water.

This project is to be considered as a proof of principle and will step-up the development of novel biotechnology concepts that will establish “solar-cell chloroplast factories”. The design and construction of a chloroplast chassis following synthetic biology principles will allow the sustainable production of biofuels and valuable chemicals, paving the way for a carbon-neutral bio-economy that can supply our society with an increasing energy demand, while mitigating the damaging effects of climate change.

Why microalgae?

Microalgae are attractive systems for the production of biofuels, recombinant proteins and fine-chemicals. They are cheap and easy to cultivate, can be grown on large scales, and have been classified as GRAS (generally regarded as safe) as they do not contain any toxic compounds or human/animal pathogens. Microalgae can be grown on non-arable land year-round, using saline or waste water sources, under close to optimal conditions. This eliminates competition with food crops, reduces fresh water usage and allows for the continual harvesting of biomass. Most importantly, these organisms are able to use solar energy to generate energy-rich metabolites through the process of photosynthesis, providing us with the ability to tap into the vast, renewable power source of the sun. They can also absorb atmospheric carbon dioxide (CO2), making all microalgal products both renewable and carbon-neutral.

Hydrogen

Hydrogen (H2) is a particularly promising renewable fuel. The main attraction of H2 is that the only by-product of its combustion is water (H2 + 1⁄2 O2 → H2O), meaning that it can be carbon neutral, or even carbon negative when produced renewably. Hydrogen also has a higher energy density compared to hydrocarbon fuels . Currently, H2 is produced via expensive energy-intensive fossil-fuel based methods such as steam reformation of natural gas, industrial oil and naphtha reforming, coalgasification and fossil fuel driven water electrolysis. However, it can also be produced under less energy intensive conditions in cyanobacterial or microalgal systems, which use sunlight as their energy source

In green microalgae, H2 can be produced by a few different pathways. The pathways of most interest to us are the light-dependent pathways (Figure 1). In the direct pathway, HYDA1 receives electrons generated by the splitting of water (2H2O → 4H+ + 4e- + O2) at a protein complex known as Photosystem II (PSII) via the photosynthetic electron transfer (PET) pathway. This involves the light-dependent excitation of electrons by another protein complex, Photosystem I (PSI), the reduction of the ferredoxin PETF and the subsequent transfer of electrons from PETF to the hydrogenase (HYDA1), which combines them with protons to produce molecular H2 (2H+ + 2e- → H2) . In the indirect pathway, which is independent of PSII, electrons are also received via PETF, but are derived from the catabolism of starch. In this pathway, the plastoquinone pool is reduced by NAD(P)H in a reaction mediated by the type II NADH deyhdrogenase NDA2 . However, in this pathway, H2 production is ~10x lower than in the direct pathway


Figure 1 Overview of light-dependent H2 production in green microaglae. In the direct pathway, electrons are derived from the water splitting reaction at photosystem II (PSII), while in the indirect pathway electrons are derived from starch catabolism. Both pathways involve the capture of light, either by both light harvesting complex I and II (LHCI and LHCII, respectively) or by LHCI, and the transfer of electrons to the hydrogenase HYDA1 through the electron transport chain via the plastoquinone pool (PQ), cytochrome b6f (Cytb6f), plastocyanin (PC), PSI and the ferredoxin PETF. The indirect pathway additionally involves the NADPH-dehydrogenase NDA2.


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