Bi-directional electron transfer between H2 and NADPH mitigates the response to light fluctuations in green algae

Abstract

The metabolism of green algae has been the focus of much research over the last century. These photosynthetic organisms can thrive under various conditions and adapt quickly to changing environments by concomitant usage of several metabolic apparatuses. The main electron coordinator in their chloroplasts, nicotinamide adenine dinucleotide phosphate (NADPH), participates in many enzymatic activities and is also responsible for interorganelle communication. Under anaerobic conditions, green algae also accumulate molecular hydrogen (H2), a promising alternative for fossil fuels. However, in order to scale-up its accumulation, a firm understanding of its integration in the photosynthetic apparatus is still lacking. While it is generally accepted that NADPH metabolism correlates to H2 accumulation, the mechanism of this collaboration is still vague and rely on indirect measurements. Here, we investigated this connection using simultaneous measurements of both dissolved gases concentration, NADPH fluorescence and electrochromic shifts at 520-546 nm. Our results indicate that energy transfer between H2 and NADPH is bi-directional and crucial for the maintenance of redox balance under light fluctuations. At light onset, NADPH consumption is initially eventuated in H2 evolution, which initiate the photosynthetic electron flow. Later on, as illumination continues the majority of NADPH is recycled by Nda2 rather than consumed by terminal sinks such as CBB cycle and H2 production. Dark onset triggers re-assimilation of H2, which produces NADPH and so, enables initiation of dark fermentative metabolism. One sentence summaryEnergy transfer between H2 and NADPH is bi-directional and crucial for the maintenance of redox balance under light fluctuations.