Electron balancing under different sink conditions reveals positive effects on photon efficiency and metabolic activity of Synechocystis sp. PCC 6803

Marcel Grund,Andreas Schmid,Torsten Jakob,Bruno Bühler,Christian Wilhelm

doi:10.1186/s13068-019-1378-y

Marcel Grund, Andreas Schmid + Show 3 more

Open Access

https://doi.org/10.1186/s13068-019-1378-y

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Abstract

BackgroundCyanobacteria are ideal model organisms to exploit photosynthetically derived electrons or fixed carbon for the biotechnological synthesis of high value compounds and energy carriers. Much effort is spent on the rational design of heterologous pathways to produce value-added chemicals. Much less focus is drawn on the basic physiological responses and potentials of phototrophs to deal with natural or artificial electron and carbon sinks. However, an understanding of how electron sinks influence or regulate cellular physiology is essential for the efficient application of phototrophic organisms in an industrial setting, i.e., to achieve high productivities and product yields.ResultsThe physiological responses of the cyanobacterium Synechocystis sp. PCC 6803 to electron sink variation were investigated in a systematic and quantitative manner. A variation in electron demand was achieved by providing two N sources with different degrees of reduction. By additionally varying light and CO2 availabilities, steady state conditions with strongly differing source–sink ratios were established. Balancing absorbed photons and electrons used for different metabolic processes revealed physiological responses to sink/source ratio variation. Surprisingly, an additional electron sink under light and thus energy limitation was found not to hamper growth, but was compensated by improved photosynthetic efficiency and activity. In the absence of carbon and light limitation, an increase in electron demand even stimulated carbon assimilation and growth.ConclusionThe metabolism of Synechocystis sp. PCC 6803 is highly flexible regarding the compensation of additional electron demands. Under light limitation, photosynthesis obviously does not necessarily run at its maximal capacity, possibly for the sake of robustness. Increased electron demands can even boost photosynthetic activity and growth.

Highlights

Cyanobacteria are ideal model organisms to exploit photosynthetically derived electrons or fixed carbon for the biotechnological synthesis of high value compounds and energy carriers
A photon/electronbalancing approach was applied. This approach was used previously to investigate the physiological response and the metabolic electron partitioning in unicellular algae in response to changing environmental conditions [42,43,44,45]
The assimilation of nitrate was employed as additional electron sink and compared with cultures grown on ammonium as N-source

Summary

Introduction

Cyanobacteria are ideal model organisms to exploit photosynthetically derived electrons or fixed carbon for the biotechnological synthesis of high value compounds and energy carriers. Any absorbed light energy exceeding the metabolic sink demands or sink utilization capacity has to be dissipated to prevent cellular damage and photoinhibition. Phototrophic microorganisms continuously balance the cellular source/sink ratio of energy assimilation (light harvesting and photosynthetic electron flux) and the metabolic energy demand (mainly for nutrient assimilation and biomass formation) under changing environmental conditions. Cyanobacteria possess a variety of alternative electron sink pathways mediated by, e.g., flavodiiron proteins [11, 12] This indicates that cyanobacteria strongly rely on alternative electron sinks such as O2 to cope with unbalanced and suboptimal growth conditions as described above. Such dissipation of photosynthetic energy restricts the quantum use efficiency of photosynthetic biomass formation

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