Nitrogen deprivation results in photosynthetic hydrogen production in Chlamydomonas reinhardtii

Abstract

The unicellular green alga Chlamydomonas reinhardtii is able to use photosynthetically provided electrons for the production of molecular hydrogen by an [FeFe]-hydrogenase HYD1 accepting electrons from ferredoxin PetF. Despite the severe sensitivity of HYD1 towards oxygen, a sustained and relatively high photosynthetic hydrogen evolution capacity is established in C. reinhardtii cultures when deprived of sulfur. One of the major electron sources for proton reduction under this condition is the oxidation of starch and subsequent non-photochemical transfer of electrons to the plastoquinone pool. Here we report on the induction of photosynthetic hydrogen production by Chlamydomonas upon nitrogen starvation, a nutritional condition known to trigger the accumulation of large deposits of starch and lipids in the green alga. Photochemistry of photosystem II initially remained on a higher level in nitrogen-starved cells, resulting in a 2-day delay of the onset of hydrogen production compared with sulfur-deprived cells. Furthermore, though nitrogen-depleted cells accumulated large amounts of starch, both hydrogen yields and the extent of starch degradation were significantly lower than upon sulfur deficiency. Starch breakdown rates in nitrogen or sulfur-starved cultures transferred to darkness were comparable in both nutritional conditions. Methyl viologen treatment of illuminated cells significantly enhanced the efficiency of photosystem II photochemistry in sulfur-depleted cells, but had a minor effect on nitrogen-starved algae. Both the degradation of the cytochrome b₆ f complex which occurs in C. reinhardtii upon nitrogen starvation and lower ferredoxin amounts might create a bottleneck impeding the conversion of carbohydrate reserves into hydrogen evolution.

The Regulation of Photosynthetic Structure and Function during Nitrogen Deprivation in Chlamydomonas reinhardtii

“…Photosynthetic yields decrease during N deprivation, even under phototrophic conditions where cells are entirely dependent upon photosynthesis (Philipps et al, 2012;Simionato et al, 2013). Chlorophyll (Chl) fluorescence, which is sensitive to environmental changes and stress conditions that induce alterations in photosynthetic components (Iwai et al, 2008), can monitor the efficiency of linear electron flow (Baker et al, 2007) and has indicated that in N. oceanica (Simionato et al, 2013) and other algae, including C. reinhardtii (Berges et al, 1996;Li et al, 2010;Blaby et al, 2013), photosynthetic efficiency falls after N deprivation.…”

“…The most abundant classes are the neutral lipids monogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) and the anionic lipids sulfoquinovosyldiacylglycerol (SQGD) and phosphatidylglycerol (PG; Frentzen, 2004;Shimojima et al, 2010;Mizusawa and Wada, 2012). Stress, nutrient deprivation, or changing environmental conditions cause changes in lipid composition, leading to effects on photosynthetic yield and efficiency (Li et al, 2012;Philipps et al, 2012). Cellular Chl and carotenoid concentrations provide additional measures of photosynthetic potential.…”

“…In C. reinhardtii, decreasing Chl content is associated with down-regulation of synthesis genes and potentially also degradation and follows a decrease in photosynthetic yield during N deprivation Philipps et al, 2012).…”

“…Until recently, studies of nutrient limitation in microalgae have focused on the effects seen one to several days after deprivation (Wykoff et al, 1998;Moseley et al, 2002;Urzica et al, 2013) when oil accumulation rates are high Li et al, 2010;Miller et al, 2010;Philipps et al, 2012;Simionato et al, 2013;Yang et al, 2013) and photosynthetic functions are already substantially diminished. Here, we focus on the induction phase to investigate the transition into a nutrient-deprived, oil-accumulating state and the regulation of photosynthesis during that transition.…”

Screening for hydrogen-producing strains of green microalgae in phosphorus or sulphur deprived medium under nitrogen limitation

“…S-deprivation reduced the photosynthetic O 2 -evolving machineries in the algal cells leading to intracellular anaerobiosis which would favour H 2 production. N-and P-deprivation that can also stimulate H 2 production, but with lower H 2 yields and longer induction time to reach an anoxic state 4,5 . Currently, green algae such as C. reinhardtii, Tetraspora sp.…”