Bi-directional electron transfer between H2 and NADPH mitigates light fluctuation responses 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 inter-organellar communication. Under anaerobic conditions, green algae also accumulate molecular hydrogen (H2), a promising alternative for fossil fuels. However, to scale-up its accumulation, a firm understanding of its integration in the photosynthetic apparatus is still required. While it is generally accepted that NADPH metabolism correlates to H2 accumulation, the mechanism of this collaboration is still vague and relies on indirect measurements. Here, we investigated this connection in Chlamydomonas reinhardtii 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 initially eventuates in H2 evolution, which initiates the photosynthetic electron flow. Later on, as illumination continues the majority of NADPH is diverted to the Calvin-Benson-Bassham cycle. Dark onset triggers re-assimilation of H2, which produces NADPH and so, enables initiation of dark fermentative metabolism.

A novel PSII photosynthetic control is activated in anoxic cultures of green algae

“…5b). The results show that the rate of H2 accumulation is impaired in all light fluctuations, following a short burst of H2 evolution at light onset, as was previously described (Milrad et al, 2021). In addition, we observed no significant changes in-between fluctuations.…”

“…2d). It was previously shown that under anoxia, at light onset, NADPH levels decline due to its oxidation via Type-II NADPH dehydrogenase (Nda2) and/or shuttling via the Malate dehydrogenase (MDH) (Milrad et al, 2021). Our results here show that the rapid decline of NADPH concentration reaches steady state much faster in the initial exposure, approximately 10 seconds in the initial light exposure versus a minute in the successive fluctuations.…”

“…CO2 concentrations were also monitored, and observed to rapidly accumulate at light onset in both initial and successive light exposures. Such evolution has been the subject of much research, and is attributed to either a reverse activity of carbonic anhydrase, which is part of the carbon concentrating mechanism (CCM) (Kaplan and Reinhold, 1999;Milrad et al, 2021), and/or the activity of PSII itself, as was recently proposed (Brinkert et al, 2016;Koroidov et al, 2014;Kosourov et al, 2020;Shevela et al, 2020). To compare the kinetics of CO2 evolution/fixation between initial and successive light fluctuations, we calculated the rates of CO2 concentration change (Figure .…”

Regulatory electron transport pathways of photosynthesis in cyanobacteria and microalgae: Recent advances and biotechnological prospects

“…Therefore, NDA2‐mediated CET affects the NADPH/ATP ratio but is less effective in generating pmf than the cyanobacterial NDH‐1 complex. The non‐photochemical PQ‐reduction via NDA2 is the point of entry for electrons that derive from glycolysis and other breakdown processes to PETC, and can subsequently feed alternative electron sinks such as hydrogenases (Mignolet et al, 2012; Milrad et al, 2021) or PTOX (Saroussi et al, 2016).…”

Photosynthetic hydrogen production: Novel protocols, promising engineering approaches and application of semi‐synthetic hydrogenases

“…These data thus indicate another unknown terminal acceptor for H 2 oxidation. Recently,Milrad et al (2021) presented evidence that H 2 uptake in the darkness is accompanied by restoration of the NADPH pool. The latter suggests that H 2 may serve as an electron donor to NADP + via [FeFe]-hydrogenase.…”