Compensatory Transcriptional Response of Fischerella thermalis to Thermal Damage of the Photosynthetic Electron Transfer Chain.

Key organisms in the environment, such as oxygenic photosynthetic primary producers (photosynthetic eukaryotes and cyanobacteria), are responsible for fixing most of the carbon globally. However, they are affected by environmental conditions, such as temperature, which in turn affect their distribution. Globally, the cyanobacterium Fischerella thermalis is one of the main primary producers in terrestrial hot springs with thermal gradients up to 60 °C, but the mechanisms by which F. thermalis maintains its photosynthetic activity at these high temperatures are not known. In this study, we used molecular approaches and bioinformatics, in addition to photophysiological analyses, to determine the genetic activity associated with the energy metabolism of F. thermalis both in situ and in high-temperature (40 °C to 65 °C) cultures. Our results show that photosynthesis of F. thermalis decays with temperature, while increased transcriptional activity of genes encoding photosystem II reaction center proteins, such as PsbA (D1), could help overcome thermal damage at up to 60 °C. We observed that F. thermalis tends to lose copies of the standard G4 D1 isoform while maintaining the recently described D1INT isoform, suggesting a preference for photoresistant isoforms in response to the thermal gradient. The transcriptional activity and metabolic characteristics of F. thermalis, as measured by metatranscriptomics, further suggest that carbon metabolism occurs in parallel with photosynthesis, thereby assisting in energy acquisition under high temperatures at which other photosynthetic organisms cannot survive. This study reveals that, to cope with the harsh conditions of hot springs, F. thermalis has several compensatory adaptations, and provides emerging evidence for mixotrophic metabolism as being potentially relevant to the thermotolerance of this species. Ultimately, this work increases our knowledge about thermal adaptation strategies of cyanobacteria.

Characterization of chlorophyll f synthase heterologously produced in Synechococcus sp. PCC 7002.

In diverse terrestrial cyanobacteria, Far-Red Light Photoacclimation (FaRLiP) promotes extensive remodeling of the photosynthetic apparatus, including photosystems (PS)I and PSII and the cores of phycobilisomes, and is accompanied by the concomitant biosynthesis of chlorophyll (Chl) d and Chl f. Chl f synthase, encoded by chlF, is a highly divergent paralog of psbA; heterologous expression of chlF from Chlorogloeopsis fritscii PCC 9212 led to the light-dependent production of Chl f in Synechococcus sp. PCC 7002 (Ho et al., Science 353, aaf9178 (2016)). In the studies reported here, expression of the chlF gene from Fischerella thermalis PCC 7521 in the heterologous system led to enhanced synthesis of Chl f. N-terminally [His]10-tagged ChlF7521 was purified and identified by immunoblotting and tryptic-peptide mass fingerprinting. As predicted from its sequence similarity to PsbA, ChlF bound Chl a and pheophytin a at a ratio of ~ 3-4:1, bound ß-carotene and zeaxanthin, and was inhibited in vivo by 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Cross-linking studies and the absence of copurifying proteins indicated that ChlF forms homodimers. Flash photolysis of ChlF produced a Chl a triplet that decayed with a lifetime (1/e) of ~ 817 µs and that could be attributed to intersystem crossing by EPR spectroscopy at 90 K. When the chlF7521 gene was expressed in a strain in which the psbD1 and psbD2 genes had been deleted, significantly more Chl f was produced, and Chl f levels could be further enhanced by specific growth-light conditions. Chl f synthesized in Synechococcus sp. PCC 7002 was inserted into trimeric PSI complexes.