Nitric oxide represses photosystem II and NDH-1 in the cyanobacterium Synechocystis sp. PCC 6803

Glycogen synthesis prevents metabolic imbalance and disruption of photosynthetic electron transport from photosystem II during transition to photomixotrophy in Synechocystis sp. PCC 6803

Some cyanobacteria can grow photoautotrophically or photomixotrophically by using simultaneously CO2 and glucose. The switch between these trophic modes and the role of glycogen, their main carbon storage macromolecule, was investigated. We analysed the effect of glucose addition on the physiology, metabolic and photosynthetic state of Synechocystis sp. PCC 6803 and mutants lacking phosphoglucomutase and ADP-glucose pyrophosphorylase, with limitations in glycogen synthesis. Glycogen acted as a metabolic buffer: glucose addition increased growth and glycogen reserves in the wild-type (WT), but arrested growth in the glycogen synthesis mutants. Already 30 min after glucose addition, metabolites from the Calvin-Benson-Bassham cycle and the oxidative pentose phosphate shunt increased threefold more in the glycogen synthesis mutants than the WT. These alterations substantially affected the photosynthetic performance of the glycogen synthesis mutants, as O2 evolution and CO2 uptake were both impaired. We conclude that glycogen synthesis is essential during transitions to photomixotrophy to avoid metabolic imbalance that induces inhibition of electron transfer from PSII and subsequently accumulation of reactive oxygen species, loss of PSII core proteins, and cell death. Our study lays foundations for optimising photomixotrophy-based biotechnologies through understanding the coordination of the crosstalk between photosynthetic electron transport and metabolism.

Transcriptomic and photosynthetic analyses of Synechocystis sp. PCC6803 and Chlorogloeopsis fritschii sp. PCC6912 exposed to an M-dwarf spectrum under an anoxic atmosphere

Introduction

Cyanobacteria appeared in the anoxic Archean Earth, evolving for the first time oxygenic photosynthesis and deeply changing the atmosphere by introducing oxygen. Starting possibly from UV-protected environments, characterized by low visible and far-red enriched light spectra, cyanobacteria spread everywhere on Earth thanks to their adaptation capabilities in light harvesting. In the last decade, few cyanobacteria species which can acclimate to far-red light through Far-Red Light Photoacclimation (FaRLiP) have been isolated. FaRLiP cyanobacteria were thus proposed as model organisms to study the origin of oxygenic photosynthesis as well as its possible functionality around stars with high far-red emission, the M-dwarfs. These stars are astrobiological targets, as their longevity could sustain life evolution and they demonstrated to host rocky terrestrial-like exoplanets within their Habitable Zone.

Methods

We studied the acclimation responses of the FaRLiP strain Chlorogloeopsis fritschii sp. PCC6912 and the non-FaRLiP strain Synechocystis sp. PCC6803 to the combination of three simulated light spectra (M-dwarf, solar and far-red) and two atmospheric compositions (oxic, anoxic). We first checked their growth, O2 production and pigment composition, then we studied their transcriptional responses by RNA sequencing under each combination of light spectrum and atmosphere conditions.

Results and discussion

PCC6803 did not show relevant differences in gene expression when comparing the responses to M-dwarf and solar-simulated lights, while far-red caused a variation in the transcriptional level of many genes. PCC6912 showed, on the contrary, different transcriptional responses to each light condition and activated the FaRLiP response under the M-dwarf simulated light. Surprisingly, the anoxic atmosphere did not impact the transcriptional profile of the 2 strains significantly. Results show that both cyanobacteria seem inherently prepared for anoxia and to harvest the photons emitted by a simulated M-dwarf star, whether they are only visible (PCC6803) or also far-red photons (PCC6912). They also show that visible photons in the simulated M-dwarf are sufficient to keep a similar metabolism with respect to solar-simulated light.

Conclusion

Results prove the adaptability of the cyanobacterial metabolism and enhance the plausibility of finding oxygenic biospheres on exoplanets orbiting M-dwarf stars.

Flv3A facilitates O2 photoreduction and affects H2 photoproduction independently of Flv1A in diazotrophic Anabaena filaments

The model heterocyst-forming filamentous cyanobacterium Anabaena sp. PCC 7120 (Anabaena) is a typical example of a multicellular organism capable of simultaneously performing oxygenic photosynthesis in vegetative cells and O₂ -sensitive N₂ -fixation inside heterocysts. The flavodiiron proteins have been shown to participate in photoprotection of photosynthesis by driving excess electrons to O₂ (a Mehler-like reaction). Here, we performed a phenotypic and biophysical characterization of Anabaena mutants impaired in vegetative-specific Flv1A and Flv3A in order to address their physiological relevance in the bioenergetic processes occurring in diazotrophic Anabaena under variable CO₂ conditions. We demonstrate that both Flv1A and Flv3A are required for proper induction of the Mehler-like reaction upon a sudden increase in light intensity, which is likely important for the activation of carbon-concentrating mechanisms and CO₂ fixation. Under ambient CO₂ diazotrophic conditions, Flv3A is responsible for moderate O₂ photoreduction, independently of Flv1A, but only in the presence of Flv2 and Flv4. Strikingly, the lack of Flv3A resulted in strong downregulation of the heterocyst-specific uptake hydrogenase, which led to enhanced H₂ photoproduction under both oxic and micro-oxic conditions. These results reveal a novel regulatory network between the Mehler-like reaction and the diazotrophic metabolism, which is of great interest for future biotechnological applications.