No abstract

Summary Pyridine nucleotide transhydrogenase (PntAB) is an integral membrane protein complex participating in the regulation of NAD(P)+:NAD(P)H redox homeostasis in various prokaryotic and eukaryotic organisms. In the present study we addressed the function and biological role of PntAB in oxygenic photosynthetic cyanobacteria capable of both autotrophic and heterotrophic growth, with support from structural three‐dimensional (3D)‐modeling. The pntA gene encoding the α subunit of heteromultimeric PntAB in Synechocystis sp. PCC 6803 was inactivated, followed by phenotypic and biophysical characterization of the ΔpntA mutant under autotrophic and mixotrophic conditions. Disruption of pntA resulted in phenotypic growth defects observed under low light intensities in the presence of glucose, whereas under autotrophic conditions the mutant did not differ from the wild‐type strain. Biophysical characterization and protein‐level analysis of the ΔpntA mutant revealed that the phenotypic defects were accompanied by significant malfunction and damage of the photosynthetic machinery. Our observations link the activity of PntAB in Synechocystis directly to mixotrophic growth, implicating that under these conditions PntAB functions to balance the NADH: NADPH equilibrium specifically in the direction of NADPH. The results also emphasize the importance of NAD(P)+:NAD(P)H redox homeostasis and associated ATP:ADP equilibrium for maintaining the integrity of the photosynthetic apparatus under low‐light glycolytic metabolism.

The ability to produce medium chain length aliphatic hydrocarbons is strictly conserved in all photosynthetic cyanobacteria, but the molecular function and biological significance of these compounds still remain poorly understood. This study gives a detailed view to the changes in intracellular hydrocarbon chain saturation in response to different growth temperatures and osmotic stress, and the associated physiological effects in the model cyanobacterium Synechocystis sp. PCC 6803. We show that the ratio between the representative hydrocarbons, saturated heptadecane and desaturated heptadecene, is reduced upon transition from 38°C toward 15°C, while the total content is not much altered. In parallel, it appears that in the hydrocarbon-deficient ∆ado (aldehyde deformylating oxygenase) mutant, phenotypic and metabolic changes become more evident under suboptimal temperatures. These include hindered growth, accumulation of polyhydroxybutyrate, altered pigment profile, restricted phycobilisome movement, and ultimately reduced CO2 uptake and oxygen evolution in the ∆ado strain as compared to Synechocystis wild type. The hydrocarbons are present in relatively low amounts and expected to interact with other nonpolar cellular components, including the hydrophobic part of the membrane lipids. We hypothesize that the function of the aliphatic chains is specifically associated with local fluidity effects of the thylakoid membrane, which may be required for the optimal movement of the integral components of the photosynthetic machinery. The findings support earlier studies and expand our understanding of the biological role of aliphatic hydrocarbons in acclimation to low temperature in cyanobacteria and link the proposed role in the thylakoid membrane to changes in photosynthetic performance, central carbon metabolism, and cell growth, which need to be effectively fine-tuned under alternating conditions in nature.

Cyanobacteria are photosynthetic prokaryotes that have been extensively studied as potential autotrophic biotechnological hosts for the production of different carbon-based end-products directly from atmospheric CO. While commercially competitive applications do not yet exist, the production of ethanol in cyanobacteria is the most mature technology, endorsed by relatively high production yields and established status of ethanol in the global biofuel market. Within this concept, the aim here was to systematically compare ethanol tolerance of different commonly used cyanobacterial strains and substrains, in order to assess their relative potential for biotechnological production platforms. The comparison revealed clear strain-specific differences in ethanol toxicity, with growth inhibition GI values ranging between 3 g L (0.4% V/V) and 28 g L (3.5% V/V). The most tolerant wild-type strains were Synechocystis sp. PCC 6803 (substrain A) and Synechococcus sp. PCC 7002, which did not show any apparent effect in growth below ethanol concentrations 9.2 g L (1.2% V/V). In comparison to typical biotechnological yeast strains used for ethanol fermentation, these values are clearly lower but still around the same order of magnitude. The results also underlined the challenges in direct number-based comparison between cyanobacterial strains and culture conditions due to inconsistencies in respect to chlorophyll content, cell morphology and optical properties.