Potential and Challenges of Improving Photosynthesis in Algae

Abstract

Sunlight energy largely exceeds the energy required by anthropic activities, and therefore its exploitation represents a major target in the field of renewable energies. The interest in the mass cultivation of green microalgae has grown in the last decades, as algal biomass could be employed to cover a significant portion of global energy demand. Advantages of microalgal vs. plant biomass production include higher light-use efficiency, efficient carbon capture and the valorization of marginal lands and wastewaters. Realization of this potential requires a decrease of the current production costs, which can be obtained by increasing the productivity of the most common industrial strains, by the identification of factors limiting biomass yield, and by removing bottlenecks, namely through domestication strategies aimed to fill the gap between the theoretical and real productivity of algal cultures. In particular, the light-to-biomass conversion efficiency represents one of the major constraints for achieving a significant improvement of algal cell lines. This review outlines the molecular events of photosynthesis, which regulate the conversion of light into biomass, and discusses how these can be targeted to enhance productivity through mutagenesis, strain selection or genetic engineering. This review highlights the most recent results in the manipulation of the fundamental mechanisms of algal photosynthesis, which revealed that a significant yield enhancement is feasible. Moreover, metabolic engineering of microalgae, focused upon the development of renewable fuel biorefineries, has also drawn attention and resulted in efforts for enhancing productivity of oil or isoprenoids.

Nature‐Inspired, Computer‐Assisted Optimization of Hierarchically Structured Zeolites

“…Nanoscale image: Reproduced under the terms of the Creative Commons CC‐BY license. [ 48 ] Copyright 2020, The Authors published by MDPI.…”

The desert green algaeChlorella ohadiithrives at excessively high light intensities by exceptionally enhancing the mechanisms that protect photosynthesis from photoinhibition

“…Once PSII proteins are damaged, it becomes inactive and a repair cycle is initiated, during which replacement of damaged proteins occurs and PSII activity is restored (Nishiyama and Murata, 2014;Li et al, 2009;Lu, 2016;Keren and Krieger-Liszkay, 2011;Li et al, 2018;Cruz et al, 2005;Erickson et al, 2015;. PI occurs when the rate of damage caused by oxidative stress exceeds the rate of PSII repair (Vecchi et al, 2020;Liu et al, 2019;Rochaix, 2020). In many photosynthetic organisms there are special stress related LHCII subunits, termed LHCSR, that accumulate in high light and direct the absorbed light to a non-photochemical processes that reduce the production of oxygen radicals (Erickson et al, 2015;Larkum et al, 2020;Rochaix and Bassi, 2019;Girolomoni et al, 2019;Pinnola and Griffiths, 2019).…”

“…The second mode of defense against PI in photosynthetic organisms involves a decrease in PSII antenna size. This is achieved by phosphorylation activated disconnection of light-harvesting II (LHCII) from PSII, followed by a decline in chlorophyll b (chl b) content and a reduction the light harvesting complex II (LHCII) expression, thereby curbing the uptake of excess light energy (Rochaix and Bassi, 2019;Ananyev et al, 2017;Vecchi et al, 2020;Nicol et al, 2019;Croce, 2020). Overall, the change leads to more efficient photosynthetic activity due to reduced generation of ROS and maintains a high steady state level of photosynthesis.…”

“…In the underlying "xanthophylls cycle", violaxanthin is converted in high light (HL) to antheraxanthin, which is further converted to zeaxanthin. Zeaxanthin accumulation in the PSII quenches the photo-exited chlorophyll radicals to produce heat (Niyogi, 1999;Nicol et al, 2019;Leonelli et al, 2017;Vecchi et al, 2020;Nishiyama and Murata, 2014).…”

“…These include a short generation time (Treves et al, 2017), a significant PSII-cyclic electrons flow during HL intensities (Ananyev et al, 2017), maintained maximum PSII efficiency in high light intensities of 3500 µE m -2 s -1 (Treves et al, 2016;Treves et al, 2020), as well as multiphasic growth controlled by metabolic shifts (Treves et al, 2017). Considering its remarkable PI resistance and record growth rate, C. ohadii can serve as an ideal model organism for the study of PI resistance (Vecchi et al, 2020;Leister, 2019).…”