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Carbon Dioxide Sequestration by Microalgae
Published in Gokare A. Ravishankar, Ranga Rao Ambati, Handbook of Algal Technologies and Phytochemicals, 2019
G.V. Swarnalatha, Ajam Shekh, P.V. Sijil, C.K. Madhubalaji, Vikas Singh Chauhan, Ravi Sarada
The supplementation of CO2 and its assimilation in microalgal cells are dependent on the photosynthetic CO2 fixation, known as the Calvin cycle. It has been observed that the CO2 supplementation up-regulated the genes encoding the major enzymes in the Calvin cycle. Phosphoglycerate kinase (PGK) and glyceraldehyde-3-phosphate dehydrogenase (GAPDH) are up-regulated. It catalyzes the phosphorylation and reduction of 3-carbon intermediates, respectively, in the presence of ATP and NADPH to generate glyceraldehyde-3-phosphate. This up-regulation of the Calvin cycle by CO2 supplementation shows higher CO2 fixation by microalgae resulting in increased biomass production. Even so, it is reported that the higher CO2 concentration inhibited the photosynthetic efficiency of microalgae. According to Winck et al. (2016), at 10% CO2 supplementation sucrose was reduced, and the xylose was accumulated, which is a clear indication of inhibition of photosynthesis. These results also suggest that photorespiration or an alternative pathway with similar substrates and products may be modulated in cells at a high CO2 concentration. It has been observed that the gene encoding the ferredoxin was up-regulated by CO2 supplementation which is suggested to enhance the Calvin cycle and carbohydrate synthesis (Peng et al. 2016; Zhu et al. 2017).
Ascorbic Acid
Published in Ruth G. Alscher, John L. Hess, Antioxidants in Higher Plants, 2017
Oxygen has several functions in photosynthesis. It has a protective role, since it is involved in the processes that avoid over-reduction of the stroma and electron transport chain. Excess energy, in terms of reducing power, can be lost through the processes of photorespiration and pseudocyclic electron flow. In the latter process, oxygen serves as an electron acceptor and produces superoxide (Figure 7). The capacity for oxygen reduction may be higher than 20 μmol h-1 mg-1 chlorophyll.87,88 Electron transport to oxygen poises the electron carriers in terms of preventing over-reduction, and is a coupled process leading to ATP synthesis.87,88 In addition, it is possible that pseudocyclic electron flow enables the electron transport system to produce an ATP-2e_ ratio of 1:5. This is required for optimal photosynthetic CO2 assimilation. Noncyclic electron flow alone (Figure 7) is believed to provide an ATP-2e_ ratio of only 1:33. Thus, ATP synthesis driven by pseudocyclic electron flow could make up the balance by producing ATP, but not NADPH.
Anatomy, Biochemistry and Physiology
Published in Massimo Maffei, Vetiveria, 2002
Cinzia M. Bertea, Wanda Camusso
The dissipative effects of photorespiration are avoided in C4 plants by the mechanism that concentrate CO2 at the carboxylation sites in the bundle sheath chloroplasts. By expressing the photosynthetic rates as a function of CO2 concentrations it is possible to calculate the compensation point (the CO2 concentration at which CO2 assimilation is zero). The remarkable differences between the photosynthetic responses of C3 and C4plants to CO2 became apparent in this type of analysis. In plants with CO2 concentrating mechanisms, including C4 plants, the CO2 concentrations at the carboxylation sites are often saturating. Plants with C4 metabolism have a CO2 compensation point of zero or nearly zero, reflecting their very low levels of photorespiration (Maffei, 1999).
Still challenging: the ecological function of the cyanobacterial toxin microcystin – What we know so far
Published in Toxin Reviews, 2018
Azam Omidi, Maranda Esterhuizen-Londt, Stephan Pflugmacher
Proteomics studies revealed the potential role of MCs in protection against oxidative stress as MC bound covalently to the cysteine residues of certain proteins via its N-methyl-dehydroalanine moiety (Dziallas & Grossart, 2011; Kaplan et al., 2012; Zilliges et al., 2011). These proteins include phycobiliproteins, CpcB and ApcA, RuBisCo, glutathione reductase, and various hypothetical proteins that were expressed differentially in the wild-type and mutant strain (Zilliges et al., 2011). Under oxidative stress due to the iron depletion, MCs showed a greater tendency to the binding sites in thioredoxin-regulated proteins (Alexova et al., 2016). On one hand, in M. aeruginosa PCC 7806 wild--type grown under high light and iron deficiency or exposed to 10 μM hydrogen peroxide, MC-protein formation was stimulated. On the other hand, in cultures treated with a protease such as subtilisin under high light (51 800 lm m−2), the large subunit of RuBisCo was more stable in the wild type. It was assumed that MC attachment to proteins avoid the dimerization of cysteines and consequently caused a delay in conformational changes of proteins and enzymes inactivation (Zilliges et al., 2011). Thus, the increased protein stability led to more adaptation to the various stresses (Kaplan et al., 2012; Zilliges et al., 2011). Moreover, under high light, the decreased oxygenase function of RuBisCo protected the cells against photorespiration (Gerbersdorf, 2006). On the contrary, current findings indicated MCs as additional radical scavengers which might protect the cells against oxidative stress damage (Martin-Luna et al., 2006a, Zilliges et al., 2011). The ability of MCs to bind to metals such as zinc and cadmium also point to the possible role of the toxin in metal detoxification in metal-induced oxidative stresses (Dziallas & Grossart, 2011).