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Microalgae II: Cell Structure, Nutrition and Metabolism
Published in Arun Kumar, Jay Shankar Singh, Microalgae in Waste Water Remediation, 2021
Photorespiration is a similar process like respiration in other organisms, in which fixed organic carbon is consumed for their growth and CO2 is generated as a byproduct in place of O2 in photosynthesis. This process starts with the reaction of O2 with ribulosebisphosphate to form phosphoglycolate by the help of the enzyme RuBisCo (which functions as an oxygenase). Then phosphoglycolate phosphorylates in to glycolate, which is later transported in to peroxisome to convert to intermediate compound glyoxylate to glycine; and again conveyed to mitochondrion to carry out the final conversion in serine, ammonia and CO2. In this process, no metabolic gain is achieved. The photorespiration process primarily depends on the relative ratio of O2 and CO2 concentrations; it has been observed that a high O2/CO2 ratio (i.e., high concentration of O2 and low concentration of CO2) encourages this process, while a low O2/CO2 ratio prefers carboxylation. Due to low half saturation value i.e., km (which is roughly equal to the level of CO2 in air), RuBisCo shows low affinity to CO2, which leads to shifting of the reaction equilibrium to photorespiration, on facing high irradiance, high oxygen level and reduced CO2 conditions.
Mechanisms for Carbon Assimilation and Utilization in Microalgae and Their Metabolites for Value-Added Products
Published in Ashok Kumar, Swati Sharma, 2 Utilization, 2020
Varsha S.S. Vuppaladadiyam, Zenab T. Baig, Abdul F. Soomro, Arun K. Vuppaladadiyam
The CB cycle or the reductive pentose phosphate cycle consists of an irreversible reaction catalysed by RuBisCo, where one molecule of ribulose 1,5-bisphosphate (RuBP), water, and CO2 combine to produce two molecules of 3-phospoglycerate (3-PGA). Five out of six molecules of 3-PGA, which are produced from three molecules of CO2 and RuBP, are used to bring back RuBP, and the excess molecule of 3-PGA is consumed to build cell material. The RuBP is then regenerated. Together with the light-independent reaction, through photorespiration, the oxygenation of RuBP may be catalysed by the RuBisCo enzyme. This uses the energy from photosynthesis to produce phosphoglycolate that cannot be used in the CB cycle and leads to a reduction in the efficiency of carbon fixation via photosynthesis by 20%–30% (Sayre 2010, Zhu, Long, and Ort 2008). A few eukaryotic algae and cyanobacteria possess carbon-concentrating mechanisms (CCMs), which minimizes this problem (Hügler and Sievert 2011). For instance, in cyanobacteria, bicarbonates are initially accumulated and then transported to carboxysome, where they are converted into CO2 by CA to create a microenvironment rich in CO2 (Moroney et al. 2013).
Algal Photosynthesis and Physiology
Published in Stephen P. Slocombe, John R. Benemann, Microalgal Production, 2017
John A. Raven and John Beardall
The oxygenase activity of RuBisCO leads to additional energy costs. All oxygenic photosynthetic organisms express photorespiratory carbon oxidation cycles, which convert two 2-phosphoglycolate into one 3-phosphoglycerate (and hence triose phosphates) with the loss of one carbon dioxide and an input of ATP and NADPH for the metabolism of glycolate, as well as the ATP and NADPH used in the Benson–Calvin cycle. Diffusive entry of carbon dioxide from the medium to a RuBisCO, with the kinetics giving the smallest known ratio of oxygenase to carboxylase activities in an air-equilibrated solution, leads to a minimum photon cost of photosynthesis of 9.92–9.96, depending on the photorespiratory pathway used, and most RuBisCOs have kinetics that incur higher photon costs.
Arthrospira sp. mediated bioremediation of gray water in ceramic membrane based photobioreactor: process optimization by response surface methodology
Published in International Journal of Phytoremediation, 2022
Shritama Mukhopadhyay, Animesh Jana, Sourja Ghosh, Swachchha Majumdar, Tapan Kumar Ghosh
High concentrations of DO hamper biomass productivity, thereby inhibiting photosynthesis. Marquez et al. (1995) observed a decrease in microalgal growth rate at higher DO concentration due to a decline in photosynthetic activity which, in turn, activated different enzymes involved in antioxidant defense mechanisms like superoxide dismutase and ascorbate peroxidase to combat the oxidative stress. Reduced growth and biomass yield due to elevated DO concentration in photobioreactor may also be linked to increased oxygenase activity of RuBisCo and the resulting process of photorespiration (Kliphuis et al.2011). RuBisCo, a bifunctional enzyme, catalyzes both the carboxylation (at high CO2 and low O2 concentration) and the oxygenation (at high O2 and low CO2 concentration) of RuBP (Ribulose 1,5-bisphosphate) during the Calvin cycle and Glycolate pathway, respectively. However, it cannot be utilized in the Calvin cycle during photorespiration as a result of which 3-phosphoglycerate and 2-phosphoglycolate are formed as by-products instead of only 3-phosphoglycerate (Cox and Nelson 2008). As recorded by Kazbar et al. (2019), DO concentration above 30 g/m3 in the photobioreactor reduced biomass production of Chlorella vulgaris by 30%. Hence, in the current study, DO concentration was properly maintained by supplying CO2 in the photobioreactor at regular intervals. Figure 2d represents the change in DO concentration with time, from which it can be clearly seen that the DO level increases each 24 h due to the increased photosynthetic activity of the microalgal biomass cultured in the photobioreactor and then decreases after CO2 purging. The removal of DO in the photobioreactor was also facilitated by the hydrophobic membranes having pores filled with gases that diminished the gas-liquid mass transfer resistance (Jana et al.2017). The maximum DO concentration was 10.1 mg/L on the 7th day which was finally reduced to 7.7 mg/L at the end of 10 days of culture. A relatively same pattern of DO profile, accompanied by an increase in every 24 h and decrease on the following day after CO2 saturation, was detected by Jana et al. (2017) during the growth of Spirulina sp. in membrane photobioreactor.