Regulation of electron transport in microalgae

Abstract

Unicellular algae are characterized by an extreme flexibility with respect to their responses to environmental constraints. This flexibility probably explains why microalgae show a very high biomass yield, constitute one of the major contributors to primary productivity in the oceans and are considered a promising choice for biotechnological applications. Flexibility results from a combination of several factors including fast changes in the light-harvesting apparatus and a strong interaction between different metabolic processes (e.g. respiration and photosynthesis), which all take place within the same cell. Microalgae are also capable of modifying their photosynthetic electron flow capacity, by changing its maximum rate and/or by diverting photogenerated electrons towards different sinks depending on their growth status. In this review, we will focus on the occurrence and regulation of alternative electron flows in unicellular algae and compare data obtained in these systems with those available in vascular plants. This article is part of a Special Issue entitled: Regulation of Electron Transport in Chloroplasts.

Inactivation of genes coding for mitochondrial Nd7 and Nd9 complex I subunits in Chlamydomonas reinhardtii. Impact of complex I loss on respiration and energetic metabolism

“…Lower capacity of complex I-deficient mutant cells to reoxidize cellular NADH must limit the rate of catabolic pathways (Krebs cycle, glycolysis and acetate assimilation). In photosynthetic organisms, reducing equivalents generated by photosynthetic electron transfer chain can in part be consumed by the mitochondrial respiratory chain, owing to metabolic exchanges between the two organelles (reviewed in Cardol et al, 2011;Noctor et al, 2007). This process can be mediated by the activity of the malate-aspartate shuttle (Noguchi and Yoshida, 2008), whose genes are present in Chlamydomonas (Merchant et al, 2007).…”

Diurnal variation in the coupling of photosynthetic electron transport and carbon fixation in iron-limited phytoplankton in the NE subarctic Pacific

“…The dissipation of excess excitation energy as thermal energy before reaching RCII, estimated as NPQ NSV , prevents excess electron transport and over-reduction of the ETC. After the initial charge separation in RCII, excess electron transport and over-reduction of the ETC can be alleviated by a number of alternative electron pathways; the upregulation of which will increase K c /n PSII (e.g., Bailey et al, 2008;Cardol et al, 2011;Laureau et al, 2013;Mackey et al, 2008;McDonald et al, 2011;Niyogi, 2000;Streb et al, 2005;Vass, 2011;Zehr and Kudela, 2009). Thus, both NPQ NSV and K c /n PSII respond strongly to excess excitation pressure, providing a possible mechanistic interpretation for their correlation.…”

Induction of Photosynthetic Carbon Fixation in Anoxia Relies on Hydrogenase Activity and Proton-Gradient Regulation-Like1-Mediated Cyclic Electron Flow in Chlamydomonas reinhardtii

“…On the other side, ability of microalgae to perform photosynthetic carbon fixation when transferred from dark to light in the absence of oxygen might also be critical for adaptation to their environment. In such conditions, not only the linear electron flow (LEF) to Rubisco, but also alternative electron flow (AEF) toward oxygen (chlororespiration, Mehler reaction, and mitochondrial respiration; for review, see Miyake, 2010;Peltier et al, 2010;Cardol et al, 2011) is impaired. Thus, cells need to circumvent a paradoxical situation: the activity of the CBB cycle requires the restoration of the cellular ATP, but the chloroplastic F 1 F O ATP synthase activity is compromised by the impairment of most of the photosynthetic electron flows that usually generate the proton motive force in oxic conditions.…”