Powerful fermentative hydrogen evolution of photosynthate in the cyanobacterium Lyngbya aestuarii BL J mediated by a bidirectional hydrogenase

Rewiring of Cyanobacterial Metabolism for Hydrogen Production: Synthetic Biology Approaches and Challenges

With the demand for renewable energy growing, hydrogen (H₂) is becoming an attractive energy carrier. Developing H₂ production technologies with near-net zero carbon emissions is a major challenge for the "H₂ economy." Certain cyanobacteria inherently possess enzymes, nitrogenases, and bidirectional hydrogenases that are capable of H₂ evolution using sunlight, making them ideal cell factories for photocatalytic conversion of water to H₂. With the advances in synthetic biology, cyanobacteria are currently being developed as a "plug and play" chassis to produce H₂. This chapter describes the metabolic pathways involved and the theoretical limits to cyanobacterial H₂ production and summarizes the metabolic engineering technologies pursued.

Anionic metabolite biosynthesis enhanced by potassium under dark, anaerobic conditions in cyanobacteria

Potassium (K(+)) is an essential macronutrient for all living organisms including cyanobacteria. Cyanobacteria are a group of bacteria performing oxygenic photosynthesis, widely studied in basic and applied sciences. The primary metabolism of the unicellular cyanobacterium Synechocystis sp. PCC 6803 is altered by environmental conditions, and it excretes organic acids and hydrogen under dark, anaerobic conditions. Here we demonstrated that K(+) widely changes the primary carbon metabolism of this cyanobacterium. Succinate and lactate excretion from the cells incubated under dark, anaerobic conditions was enhanced in the presence of K(+), while hydrogen production was repressed. The addition of K(+) and the genetic manipulation of acetate kinase AckA and an RNA polymerase sigma factor SigE additively increased succinate and lactate production to 141.0 and 217.6 mg/L, which are 11 and 46 times, compared to the wild-type strain without K(+), respectively. Intracellular levels of 2-oxoglutarate, succinate, fumarate, and malate increased by K(+) under dark, anaerobic conditions. This study provides the evidence of the considerable effect of K(+) on the biosynthesis of anionic metabolites in a unicellular cyanobacterium.

Microsensor measurements of hydrogen gas dynamics in cyanobacterial microbial mats

We used a novel amperometric microsensor for measuring hydrogen gas production and consumption at high spatio-temporal resolution in cyanobacterial biofilms and mats dominated by non-heterocystous filamentous cyanobacteria (Microcoleus chtonoplastes and Oscillatoria sp.). The new microsensor is based on the use of an organic electrolyte and a stable internal reference system and can be equipped with a chemical sulfide trap in the measuring tip; it exhibits very stable and sulfide-insensitive measuring signals and a high sensitivity (1.5–5 pA per μmol L^-1 H₂). Hydrogen gas measurements were done in combination with microsensor measurements of scalar irradiance, O₂, pH, and H₂S and showed a pronounced H₂ accumulation (of up to 8–10% H₂ saturation) within the upper mm of cyanobacterial mats after onset of darkness and O₂ depletion. The peak concentration of H₂ increased with the irradiance level prior to darkening. After an initial build-up over the first 1–2 h in darkness, H₂ was depleted over several hours due to efflux to the overlaying water, and due to biogeochemical processes in the uppermost oxic layers and the anoxic layers of the mats. Depletion could be prevented by addition of molybdate pointing to sulfate reduction as a major sink for H₂. Immediately after onset of illumination, a short burst of presumably photo-produced H₂ due to direct biophotolysis was observed in the illuminated but anoxic mat layers. As soon as O₂ from photosynthesis started to accumulate, the H₂ was consumed rapidly and production ceased. Our data give detailed insights into the microscale distribution and dynamics of H₂ in cyanobacterial biofilms and mats, and further support that cyanobacterial H₂ production can play a significant role in fueling anaerobic processes like e.g., sulfate reduction or anoxygenic photosynthesis in microbial mats.

Draft Genome Sequence of Filamentous Marine Cyanobacterium Lyngbya confervoides Strain BDU141951

Lyngbya confervoides strain BDU141951 is a fast-growing, unicellular, marine, nonheterocystous cyanobacterium forming long unbranched filaments inside sheaths. Here, we report the draft genome assembly of Lyngbya confervoides BDU141951 for the first time. The genome size is 8,799,693 bp and has 6,093 putative protein-coding genes assembled into 298 scaffolds.