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Microalgae II: Cell Structure, Nutrition and Metabolism
Published in Arun Kumar, Jay Shankar Singh, Microalgae in Waste Water Remediation, 2021
For the fixation of one molecule of CO2, two molecules of NADPH2 and three molecules of ATP are required. It is also reported that a minimum 10 quanta of absorbed light are required for each molecule of CO2 fixed or O2 evolved; this is the quantum efficiency of CO2 fixation. In 1940, Calvin and Benson discovered the reaction mechanism of carbon fixation using 14C radio-labeling technique and were awarded the Nobel Prize in 1961. The conversion of CO2 to sugar (or other compounds), known as Calvin–Benson cycle, involves four phases: Carboxylation phase: The CO2 molecule combines with the ribulosebisphosphate (RuBP) to form two molecules of 3-phosphoglycerate (3PGA); by the help of the enzyme ribulosebisphospate carboxylase/oxygenase (RuBisCo).Reduction phase: 3-Phosphoglycerate (3PGA) is reduced to intermediate 1,3-bisphosphoglyceric acid (1,3-BPGA) and finally in to Glyceraldehydes-3-Phosphate (G3P) and the energy is used either as ATP in conversion of 3PGA TO 1,3BPGA or as NADPH2 in conversion of 1,3BPGA to G3P.Regeneration phase: Most of the G3P molecules are used to regenerate Ribulose phosphate (RuPB) molecules for further CO2 fixation. The enzymes transketolase and aldolase are required in the conversion of 6-C and 3-C sugars in to a 5-C compound.Production phase: Some G3P molecules turn in to sugars which are considered primary end products of the Calvin cycle, but some G3P molecules are also used in the synthesis of fatty acids, amino acids and organic acids through various mechanisms.
Enhancement for the synthesis of bio-energy molecules (carbohydrates and lipids) in Desmodesmus subspicatus: experiments and optimization techniques
Published in Preparative Biochemistry & Biotechnology, 2023
Sreya Sarkar, Tridib Kumar Bhowmick, Kalyan Gayen
In the isolated microalgae strain, SN and DHP were demonstrated to be the most significant nutrients for the accumulation of carbohydrates. An increased SN concentration adversely affects carbohydrate productivity (Figure 3a). SN concentration at level 1 had resulted in the highest carbohydrate productivity. Then, with increasing SN concentration, carbohydrate productivity decreased consequently. Ho et al., reported that nitrogen limitation increases carbohydrate productivity.[42] Phosphate limitation in the media also influences carbohydrate productivity. Nitrogen and phosphorus scarcity hindered photosynthesis, protein and chlorophyll production, and accumulation of carbohydrates.[43] Thus, lower phosphate concentration increases carbohydrate productivity than the higher phosphate concentration in the culture media (Figure 3b). The addition of bicarbonate in the growth medium initially increased the production of carbohydrates in the isolated microalgae. According to Peng et al., inorganic carbon in the media helps to increase the activity of Ribulose-1, 5-bisphosphate carboxylase/oxygenase (RuBisCO) which is essential for converting 3-phosphoglycerate, a substrate for carbohydrate synthesis in microalgae[44]
The enigma of environmental organoarsenicals: Insights and implications
Published in Critical Reviews in Environmental Science and Technology, 2022
Xi-Mei Xue, Chan Xiong, Masafumi Yoshinaga, Barry Rosen, Yong-Guan Zhu
Little is known about the biological or ecological functions of organoarsenicals, except arsenic methylation that is demonstrated to serve as a mechanism of antibiotic production, as a detoxification process or as a step in the biosynthesis of more complex organoarsenicals such as arsenosugars. Environmental arsenic enters cells adventitiously via transport systems for essential nutrients that are unable to distinguish arsenic from their actual substrates. As(V) competes with phosphate, inhibiting alkylation, acylation or phosphorylation reactions, leading to futile cycles of synthesis and breakdown of extremely unstable compounds such as organoarsenical 1-arseno-3-phosphoglycerate, which has a half-life of only a few seconds (Chen et al., 2016; Rosen et al., 2011). As(III) has high affinity for thiol groups in proteins and small molecules (Shen et al., 2013). Arsenic-sulfur bond formation inhibits a number of biochemical reactions. Pentavalent MAs(V), DMAs(V) and trimethylarsine oxide are less toxic than As(III). In addition, the uptake efficiency of organisms for pentavalent methylated arsenic is generally very low, although mechanisms for uptake of methylated arsenicals have been identified (Chen et al., 2016).
Organoarsenical compounds: Occurrence, toxicology and biotransformation
Published in Critical Reviews in Environmental Science and Technology, 2020
Jian Chen, Luis D. Garbinski, Barry Rosen, Jun Zhang, Ping Xiang, Lena Q. Ma
Much less is known about Aso compounds compared to Asi. The main reason for this is the lack of analytical techniques and tools to detect various Aso compounds in the environment. Unknown Aso compounds may also have not yet been identified due to their low concentrations or unstable state. For example, the degradation product of glyceraldehyde-3-phosphate dehydrogenase (G3PDH), found in bacterial ars operons, 1-arseno-3-phosphoglycerate (1As3PGA), is unstable as it immediately hydrolyzes to AsV (Chen, Yoshinaga, Garbinski, & Rosen, 2016). Its chemical structure is analogous to that of 1,3-bisphosphoglycerate (1’3 PGA), which is a metabolite in glycolysis and always present during glucose metabolism. However, 1As3PGA is involved in a novel AsV detoxification pathway.