LHCSR Expression under HSP70/RBCS2 Promoter as a Strategy to Increase Productivity in Microalgae

Abstract

Microalgae are unicellular photosynthetic organisms considered as potential alternative sources for biomass, biofuels or high value products. However, limited biomass productivity is commonly experienced in their cultivating system despite their high potential. One of the reasons for this limitation is the high thermal dissipation of the light absorbed by the outer layers of the cultures exposed to high light caused by the activation of a photoprotective mechanism called non-photochemical quenching (NPQ). In the model organism for green algae Chlamydomonas reinhardtii, NPQ is triggered by pigment binding proteins called light-harvesting-complexes-stress-related (LHCSRs), which are over-accumulated in high light. It was recently reported that biomass productivity can be increased both in microalgae and higher plants by properly tuning NPQ induction. In this work increased light use efficiency is reported by introducing in C. reinhardtii a LHCSR3 gene under the control of Heat Shock Protein 70/RUBISCO small chain 2 promoter in a npq4 lhcsr1 background, a mutant strain knockout for all LHCSR genes. This complementation strategy leads to a low expression of LHCSR3, causing a strong reduction of NPQ induction but is still capable of protecting from photodamage at high irradiance, resulting in an improved photosynthetic efficiency and higher biomass accumulation.

Improved lipid productivity in Nannochloropsis gaditana in nitrogen-replete conditions by selection of pale green mutants

“…PSII antenna size is inversely proportional to the time required for reaching 2/3 of the maximum fluorescence emission [33]. Oxygen evolution curves were performed as described [63]. Net oxygen production was calculated subtracting the oxygen consumption in the dark after each measurement at the different actinic lights.…”

Chlamydomonas reinhardtii LHCSR1 and LHCSR3 proteins involved in photoprotective non-photochemical quenching have different quenching efficiency and different carotenoid affinity

“…Previous works demonstrated that C. reinhardtii npq4 mutant, which lacks both genes encoding for LHCSR3 protein, was characterized by a reduced NPQ phenotype, with the residual NPQ dependent on LHCSR1 subunit 18 , 27 . Indeed, the C. reinhardtii double mutant npq4 lhcsr1 , disrupted in all genes that encode for both LHCSR3 and LHCSR1 proteins, showed a null NPQ phenotype with high amounts of ROS formation and reduced growth rate 10 , 27 , 29 . Transcriptional analysis of C. reinhardtii cultures demonstrated that CO 2 concentration influences the expression of lhcSR1 and lhcSR3 ( lhcSR3.1 and lhcSR3.2 ) genes, which are respectively transcriptionally activated and downregulated at high CO 2 conditions 21 , 48 .…”

“…Among photosynthetic organisms, microalgae have high potential with a wide range of applications of their biomass and its derivatives, from food/feed to biofuels and high-value products 1 – 9 . These unicellular organisms are characterized generally by a higher photosynthetic efficiency compared to higher plants due to the absence of non-photosynthetic tissues 10 , 11 . However, some bottlenecks need to be faced in order to improve their domestication process: in fact, their photosynthetic efficiency is far lower than the theoretical value, ranging between 1 and 3% on industrial scale 11 , 12 .…”

“…However, some bottlenecks need to be faced in order to improve their domestication process: in fact, their photosynthetic efficiency is far lower than the theoretical value, ranging between 1 and 3% on industrial scale 11 , 12 . In a natural habitat, photosynthetic organisms are exposed to variable light conditions, which can induce photodamage, affecting their photosynthetic efficiency, so they have developed various acclimation mechanisms, such as thermal dissipation of energy absorbed in excess, a process called non-photochemical quenching (NPQ) 10 , 13 – 19 . However, this mechanism is activated in excess, especially in controlled photobioreactor conditions, leading to a massive energy dissipation, up to 80% of light absorbed, through heat 10 .…”

“…In a natural habitat, photosynthetic organisms are exposed to variable light conditions, which can induce photodamage, affecting their photosynthetic efficiency, so they have developed various acclimation mechanisms, such as thermal dissipation of energy absorbed in excess, a process called non-photochemical quenching (NPQ) 10 , 13 – 19 . However, this mechanism is activated in excess, especially in controlled photobioreactor conditions, leading to a massive energy dissipation, up to 80% of light absorbed, through heat 10 . In algae, NPQ mechanism is triggered by antenna-like pigment-binding proteins located in thylakoid membranes, called Light Harvesting Complex Stress Related proteins (LHCSRs), which are over-accumulated upon high light exposure 18 , 20 – 23 .…”