Shimakawa 2017 Sci Rep

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Shimakawa G, Matsuda Y, Nakajima K, Tamoi M, Shigeoka S, Miyake C (2017) Diverse strategies of O2 usage for preventing photo-oxidative damage under CO2 limitation during algal photosynthesis. Sci Rep 7:41022. doi: 10.1038/srep41022

» PMID: 28106164 Open Access

Shimakawa G, Matsuda Y, Nakajima K, Tamoi M, Shigeoka S, Miyake C (2017) Sci Rep

Abstract: Photosynthesis produces chemical energy from photon energy in the photosynthetic electron transport and assimilates CO2 using the chemical energy. Thus, CO2 limitation causes an accumulation of excess energy, resulting in reactive oxygen species (ROS) which can cause oxidative damage to cells. O2 can be used as an alternative energy sink when oxygenic phototrophs are exposed to high light. Here, we examined the responses to CO2 limitation and O2 dependency of two secondary algae, Euglena gracilis and Phaeodactylum tricornutum. In E. gracilis, approximately half of the relative electron transport rate (ETR) of CO2-saturated photosynthesis was maintained and was uncoupled from photosynthesis under CO2 limitation. The ETR showed biphasic dependencies on O2 at high and low O2 concentrations. Conversely, in P. tricornutum, most relative ETR decreased in parallel with the photosynthetic O2 evolution rate in response to CO2 limitation. Instead, non-photochemical quenching was strongly activated under CO2 limitation in P. tricornutum. The results indicate that these secondary algae adopt different strategies to acclimatize to CO2 limitation, and that both strategies differ from those utilized by cyanobacteria and green algae. We summarize the diversity of strategies for prevention of photo-oxidative damage under CO2 limitation in cyanobacterial and algal photosynthesis.

Bioblast editor: Gnaiger E

Selected quotes

  • Oxygenic photosynthesis uses photon energy to produce sugar from CO2 and H2O, and releases O2 as a waste product. Two photosystems, PSI and PSII, play central roles in this process, which involves an electron transport system located on thylakoid membranes. The reaction centers, P700 and P680, are photo-oxidized via light-harvesting pigments such as chlorophyll (Chl). The oxidized P700 in PSI accepts electrons from PSII via plastoquinone, the cytochrome b6/f complex, and plastocyanin (or cytochrome c6). This electron transport is accompanied by the generation of a proton gradient across the membranes (ΔpH), allowing the production of ATP by ATP synthase2. At the acceptor side of PSI, NADP+ is reduced to NADPH by accepting electrons from P700 through ferredoxin and ferredoxin-NADP+ reductase. O2 is produced via the oxidation of H2O by oxidized P680 in the luminal side of PSII. Together, these reactions are termed ‘photosynthetic electron transport’, and are the source of chemical energy in the forms NADPH and ATP, which are used for CO2 assimilation in the Calvin-Benson cycle3.
  • The production and consumption of energy by photosynthetic electron transport and the Calvin-Benson cycle becomes unbalanced without sufficient CO2 (CO2-limited photosynthesis; Fig. 1). Excess photon energy causes the production of reactive oxygen species (ROS), which trigger oxidative damage to PSII and PSI, so-called photoinhibition4–7.

Labels: MiParea: Respiration 

Organism: Plants, Eubacteria 

Regulation: Oxygen kinetics  Coupling state: ROUTINE 

Algae, AOX, Photosynthesis