Modulation of Intracellular O2 Concentration in Escherichia coli Strains Using Oxygen Consuming Devices

Outlook on Synthetic Biology-Driven Hydrogen Production: Lessons from Algal Photosynthesis Applied to Cyanobacteria

Photobiological hydrogen production offers a sustainable route to clean energy by harnessing solar energy through photosynthetic microorganisms. The pioneering sulfur-deprivation technique developed by Melis and colleagues in the green alga Chlamydomonas reinhardtii successfully enabled sustained hydrogen production by downregulating photosystem II (PSII) activity to reduce oxygen evolution, creating anaerobic conditions necessary for hydrogenase activity. Inspired by this approach, we present the project of the European consortium PhotoSynH2, which builds on these biological insights and employs synthetic biology to replicate and enhance this strategy in cyanobacteria, specifically, Synechocystis sp. PCC 6803. By genetically engineering precise downregulation of PSII, we aim to reduce oxygen evolution without the unintended effects associated with nutrient deprivation, enabling efficient hydrogen production. Additionally, re-engineering endogenous respiration to continuously replenish glycogen consumed during respiration allows matching oxygen production with consumption, maintaining anaerobic conditions conducive to hydrogen production. This review discusses how focusing on molecular-level processes and leveraging advanced genetic tools can lead to a new methodology that potentially offers improved results over traditional approaches. By redirecting electron flow and optimizing redox pathways, we seek to enhance hydrogen production efficiency in cyanobacteria. Our approach demonstrates how harnessing photosynthesis through synthetic biology can contribute to scalable and sustainable hydrogen production, addressing the growing demand for renewable energy and advancing toward a carbon-neutral future.

Novel concepts and engineering strategies for heterologous expression of efficient hydrogenases in photosynthetic microorganisms

Hydrogen is considered one of the key enablers of the transition towards a sustainable and net-zero carbon economy. When produced from renewable sources, hydrogen can be used as a clean and carbon-free energy carrier, as well as improve the sustainability of a wide range of industrial processes. Photobiological hydrogen production is considered one of the most promising technologies, avoiding the need for renewable electricity and rare earth metal elements, the demands for which are greatly increasing due to the current simultaneous electrification and decarbonization goals. Photobiological hydrogen production employs photosynthetic microorganisms to harvest solar energy and split water into molecular oxygen and hydrogen gas, unlocking the long-pursued target of solar energy storage. However, photobiological hydrogen production has to-date been constrained by several limitations. This review aims to discuss the current state-of-the art regarding hydrogenase-driven photobiological hydrogen production. Emphasis is placed on engineering strategies for the expression of improved, non-native, hydrogenases or photosynthesis re-engineering, as well as their combination as one of the most promising pathways to develop viable large-scale hydrogen green cell factories. Herein we provide an overview of the current knowledge and technological gaps curbing the development of photobiological hydrogenase-driven hydrogen production, as well as summarizing the recent advances and future prospects regarding the expression of non-native hydrogenases in cyanobacteria and green algae with an emphasis on [FeFe] hydrogenases.