“…This result indicates an increased PSII-independent reduction of plastoquinones in the presence of acetate which oxidation causes release of protons, as reported in the case of C . reinhardtii 10 , 13 . Figure 2 Photosynthetic properties of C .…”
Section: Resultsmentioning
confidence: 99%
“…The interaction between photosynthesis and acetate metabolism is further complicated by the reciprocal influence of mitochondria and chloroplasts redox state: for instance, in C . reinhardtii alteration of mitochondrial respiration and different NADH availability changed the NADPH content in the chloroplast, inducing different reduction state of plastoquinones 4 , 13 . To investigate the influence of mixotrophic growth in presence of acetate in C .…”
In this work, we investigated the molecular basis of autotrophic vs. mixotrophic growth of Chlorella sorokiniana, one of the most productive microalgae species with high potential to produce biofuels, food and high value compounds. To increase biomass accumulation, photosynthetic microalgae are commonly cultivated in mixotrophic conditions, adding reduced carbon sources to the growth media. In the case of C. sorokiniana, the presence of acetate enhanced biomass, proteins, lipids and starch productivity when compared to autotrophic conditions. Despite decreased chlorophyll content, photosynthetic properties were essentially unaffected while differential gene expression profile revealed transcriptional regulation of several genes mainly involved in control of carbon flux. Interestingly, acetate assimilation caused upregulation of phosphoenolpyruvate carboxylase enzyme, enabling potential recovery of carbon atoms lost by acetate oxidation. The obtained results allowed to associate the increased productivity observed in mixotrophy in C. sorokiniana with a different gene regulation leading to a fine regulation of cell metabolism.
“…This result indicates an increased PSII-independent reduction of plastoquinones in the presence of acetate which oxidation causes release of protons, as reported in the case of C . reinhardtii 10 , 13 . Figure 2 Photosynthetic properties of C .…”
Section: Resultsmentioning
confidence: 99%
“…The interaction between photosynthesis and acetate metabolism is further complicated by the reciprocal influence of mitochondria and chloroplasts redox state: for instance, in C . reinhardtii alteration of mitochondrial respiration and different NADH availability changed the NADPH content in the chloroplast, inducing different reduction state of plastoquinones 4 , 13 . To investigate the influence of mixotrophic growth in presence of acetate in C .…”
In this work, we investigated the molecular basis of autotrophic vs. mixotrophic growth of Chlorella sorokiniana, one of the most productive microalgae species with high potential to produce biofuels, food and high value compounds. To increase biomass accumulation, photosynthetic microalgae are commonly cultivated in mixotrophic conditions, adding reduced carbon sources to the growth media. In the case of C. sorokiniana, the presence of acetate enhanced biomass, proteins, lipids and starch productivity when compared to autotrophic conditions. Despite decreased chlorophyll content, photosynthetic properties were essentially unaffected while differential gene expression profile revealed transcriptional regulation of several genes mainly involved in control of carbon flux. Interestingly, acetate assimilation caused upregulation of phosphoenolpyruvate carboxylase enzyme, enabling potential recovery of carbon atoms lost by acetate oxidation. The obtained results allowed to associate the increased productivity observed in mixotrophy in C. sorokiniana with a different gene regulation leading to a fine regulation of cell metabolism.
“…A number of studies have shown that C. reinhardtii cells impaired in mitochondrial activity exhibit reduced fitness and aberrant photosynthetic activity when they experience highly reducing conditions (14,17,43,46,57). Furthermore, mitochondrial CrAOX1 seems to diminish ROS accumulation when cells are exposed to high light (25).…”
Section: Impact Of Aox Loss On Fitness Under Hl Exposurementioning
confidence: 99%
“…As noted above, some studies have demonstrated that impaired mitochondrial activity perturbs C. reinhardtii's ability to efficiently acclimate to excess light and sustain functional photosynthetic activity under highly reducing conditions (14,17,43,46,57). CrAOX proteins were shown to be important for controlling stress responses and ROS levels (33), with CrAOX1 knockdown strains exhibiting hyper-reduction of the respiratory chain, which could trigger elevated ROS production.…”
Edited by Ursula JakobPhotosynthetic organisms often experience extreme light conditions that can cause hyper-reduction of the chloroplast electron transport chain, resulting in oxidative damage. Accumulating evidence suggests that mitochondrial respiration and chloroplast photosynthesis are coupled when cells are absorbing high levels of excitation energy. This coupling helps protect the cells from hyper-reduction of photosynthetic electron carriers and diminishes the production of reactive oxygen species (ROS). To examine this cooperative protection, here we characterized Chlamydomonas reinhardtii mutants lacking the mitochondrial alternative terminal respiratory oxidases, CrAOX1 and CrAOX2. Using fluorescent fusion proteins, we experimentally demonstrated that both enzymes localize to mitochondria. We also observed that the mutant strains were more sensitive than WT cells to high light under mixotrophic and photoautotrophic conditions, with the aox1 strain being more sensitive than aox2. Additionally, the lack of CrAOX1 increased ROS accumulation, especially in very high light, and damaged the photosynthetic machinery, ultimately resulting in cell death. These findings indicate that the Chlamydomonas AOX proteins can participate in acclimation of C. reinhardtii cells to excess absorbed light energy. They suggest that when photosynthetic electron carriers are highly reduced, a chloroplast-mitochondria coupling allows safe dissipation of photosynthetically derived electrons via the reduction of O 2 through AOX (especially AOX1)-dependent mitochondrial respiration.
“…Although a massive decline in water-splitting activity is a prerequisite for the establishment of anaerobic conditions, which enable hydrogen production via the oxygen-sensitive hydrogenase enzyme, several studies clearly demonstrated that residual PSII activity and linear electron transport toward the hydrogenase are indispensable for efficient hydrogen production in C. reinhardtii ( Antal et al, 2003 ; Volgusheva et al, 2013 ; Baltz et al, 2014 ; Steinbeck et al, 2015 ). The C. reinhardtii mutant stm6 ( Schönfeld et al, 2004 ) displays an enhanced hydrogen production capacity ( Kruse et al, 2005 ) and its increased rate of mitochondrial oxygen consumption ( Uhmeyer et al, 2017 ), was proposed to protect PSII during sulfur deprivation by accelerating the establishment of anaerobic conditions ( Volgusheva et al, 2013 ), where irreversible, oxygen-dependent photoinhibition ( Vass et al, 1992 ) cannot occur. Besides the PSII-dependent pathway of hydrogen production, starch degradation and subsequent glycolysis can provide NADH, which can be used to feed electrons into the photosynthetic electron transport chain without the need for water-splitting at PSII ( Chochois et al, 2009 ; Baltz et al, 2014 ).…”
The protein superfamily of short-chain dehydrogenases/reductases (SDR), including members of the atypical type (aSDR), covers a huge range of catalyzed reactions and in vivo substrates. This superfamily also comprises isoflavone reductase-like (IRL) proteins, which are aSDRs highly homologous to isoflavone reductases from leguminous plants. The molecular function of IRLs in non-leguminous plants and green microalgae has not been identified as yet, but several lines of evidence point at their implication in reactive oxygen species homeostasis. The Chlamydomonas reinhardtii IRL protein IFR1 was identified in a previous study, analyzing the transcriptomic changes occurring during the acclimation to sulfur deprivation and anaerobiosis, a condition that triggers photobiological hydrogen production in this microalgae. Accumulation of the cytosolic IFR1 protein is induced by sulfur limitation as well as by the exposure of C. reinhardtii cells to reactive electrophile species (RES) such as reactive carbonyls. The latter has not been described for IRL proteins before. Over-accumulation of IFR1 in the singlet oxygen response 1 (sor1) mutant together with the presence of an electrophile response element, known to be required for SOR1-dependent gene activation as a response to RES, in the promoter of IFR1, indicate that IFR1 expression is controlled by the SOR1-dependent pathway. An implication of IFR1 into RES homeostasis, is further implied by a knock-down of IFR1, which results in a diminished tolerance toward RES. Intriguingly, IFR1 knock-down has a positive effect on photosystem II (PSII) stability under sulfur-deprived conditions used to trigger photobiological hydrogen production, by reducing PSII-dependent oxygen evolution, in C. reinhardtii. Reduced PSII photoinhibition in IFR1 knock-down strains prolongs the hydrogen production phase resulting in an almost doubled final hydrogen yield compared to the parental strain. Finally, IFR1 knock-down could be successfully used to further increase hydrogen yields of the high hydrogen-producing mutant stm6, demonstrating that IFR1 is a promising target for genetic engineering approaches aiming at an increased hydrogen production capacity of C. reinhardtii cells.
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