The photobiological production of H₂ gas, using water as the only electron donor, is a property of two types of photosynthetic microorganisms: green algae and cyanobacteria. In these organisms, photosynthetic water splitting is functionally linked to H₂ production by the activity of hydrogenase enzymes. Interestingly, each of these organisms contains only one of two major types of hydrogenases, [FeFe] or [NiFe] enzymes, which are phylogenetically distinct but perform the same catalytic reaction, suggesting convergent evolution. This idea is supported by the observation that each of the two classes of hydrogenases has a different metallo-cluster, is encoded by entirely different sets of genes (apparently under the control of different promoter elements), and exhibits different maturation pathways. The genetics, biosynthesis, structure, function, and O₂ sensitivity of these enzymes have been the focus of extensive research in recent years. Some of this effort is clearly driven by the potential for using these enzymes in future biological or biohybrid systems to produce renewable fuel or in fuel cell applications.

Hydrogen photoproduction by micro-organisms combines the photosynthetic properties of oxygenic and non-oxygenic microbes with the activity of H₂-producing enzymes in nature: hydrogenases and nitrogenases. The overall efficiency of the process depends on the separate efficiencies of photosynthesis and enzymatic catalysis. This tutorial review discusses the biochemical pathways for H₂ production in different organisms, barriers to be overcome, and possible suggestions for integrating photobiological H₂ production with fermentative, anaerobic systems for a potentially more efficient process.

Ethylene is the most widely produced petrochemical feedstock globally. It is currently produced exclusively from fossil fuels, and its production is the largest CO₂-emitting process in the chemical industry. In this study, we report on a photobiological process for sustained production of ethylene from CO₂. The efe gene encoding an ethylene-forming enzyme from Pseudomonas syringae pv. Phaseolicola was previously expressed in cyanobacterial strains, but was not stable. We modified the gene sequence to enhance its stability, and expressed it in Synechocystis sp. PCC 6803, leading to continuous ethylene production. The same ethylene production rate was sustained across four successive sub-cultures without apparent loss of ethylene-forming ability. Up to 5.5% of the fixed carbon was directed to ethylene synthesis, surpassing the published carbon-partition rate into the TCA cycle. Nitrogen- and phosphorus-enriched seawater can support both growth and ethylene production. Factors limiting ethylene production, including efe expression levels, light intensity and nutrient status, were identified and alleviated, resulting in a peak production rate of 5650 μL L⁻¹ h⁻¹ (7125 μg L⁻¹ h⁻¹, 252 μmol L⁻¹ h⁻¹, or 171 mg L⁻¹ day⁻¹), which is higher than that reported for other algae biofuels and chemicals. This study suggests that Synechocystis, expressing the modified efe gene, has potential to be an efficient biological catalyst for the uptake and conversion of CO₂ to ethylene.

Understanding how living cells manage high-energy metabolites such as ATP and NADPH is essential for understanding energy transformations in the biosphere. Using light as the energy input, we find that energy charge (ratio of ATP over ADP+ATP) in the cyanobacterium Synechocystis sp. PCC 6803 varies in different growth stages, with a peak upon entry into the rapid growth phase, as well as a positive correlation with light intensity. In contrast, a mutant that can no longer synthesize the main carbon storage compound glycogen showed higher energy charge. The overflow of organic acids in this mutant under nitrogen depletion could also be triggered under high light in nitrogen-replete conditions, with an energy input level dependency. These findings suggest that energy charge in cyanobacteria is tightly linked to growth and carbon partition and that energy management is of key significance for their application as photosynthetic carbon dioxide-assimilating cell factories.

Worldwide, ethylene is the most produced organic compound. It serves as a building block for a wide variety of plastics, textiles, and chemicals, and a process has been developed for its conversion into liquid transportation fuels. Currently, commercial ethylene production involves steam cracking of fossil fuels, and is the highest CO2-emitting process in the chemical industry. Therefore, there is great interest in developing technology for ethylene production from renewable resources including CO2 and biomass. Ethylene is produced naturally by plants and some microbes that live with plants. One of the metabolic pathways used by microbes is via an ethylene-forming enzyme (EFE), which uses α-ketoglutarate and arginine as substrates. EFE is a promising biotechnology target because the expression of a single gene is sufficient for ethylene production in the absence of toxic intermediates. Here we present the first comprehensive review and analysis of EFE, including its discovery, sequence diversity, reaction mechanism, predicted involvement in diverse metabolic modes, heterologous expression, and requirements for harvesting of bioethylene. A number of knowledge gaps and factors that limit ethylene productivity are identified, as well as strategies that could guide future research directions.