Biomass from microalgae: the potential of domestication towards sustainable biofactories

Abstract

Interest in bulk biomass from microalgae, for the extraction of high-value nutraceuticals, bio-products, animal feed and as a source of renewable fuels, is high. Advantages of microalgal vs. plant biomass production include higher yield, use of non-arable land, recovery of nutrients from wastewater, efficient carbon capture and faster development of new domesticated strains. Moreover, adaptation to a wide range of environmental conditions evolved a great genetic diversity within this polyphyletic group, making microalgae a rich source of interesting and useful metabolites. Microalgae have the potential to satisfy many global demands; however, realization of this potential requires a decrease of the current production costs. Average productivity of the most common industrial strains is far lower than maximal theoretical estimations, suggesting that identification of factors limiting biomass yield and removing bottlenecks are pivotal in domestication strategies aimed to make algal-derived bio-products profitable on the industrial scale. In particular, the light-to-biomass conversion efficiency represents a major constraint to finally fill the gap between theoretical and industrial productivity. In this respect, recent results suggest that significant yield enhancement is feasible. Full realization of this potential requires further advances in cultivation techniques, together with genetic manipulation of both algal physiology and metabolic networks, to maximize the efficiency with which solar energy is converted into biomass and bio-products. In this review, we draft the molecular events of photosynthesis which regulate the conversion of light into biomass, and discuss how these can be targeted to enhance productivity through mutagenesis, strain selection or genetic engineering. We outline major successes reached, and promising strategies to achieving significant contributions to future microalgae-based biotechnology.

Keywords: Bio-based products; Biomass; Light-use efficiency; Microalgae; Molecular genetic; Photobioreactor; Photosynthesis; Strain domestication.

Fig. 1

General scheme of the algal production chain. A number of factors, including the high cost of the infrastructure and the energy required for growth, harvesting and processing the algal biomass, significantly contribute to the cost of the whole production pipeline

Fig. 2

Light response curves for photosynthesis. The light compensation point is the minimum light intensity at which the organism shows a gain of carbon fixation. The net photosynthetic rate shows a linear rise in response to increased light, in the range of light limitation. At higher light levels, saturation occurs as the efficiency of the photosynthetic mechanism is reduced due to the activation of energy quenching processes. Under excess light conditions, net photosynthesis can decline as a result of photoxidative stress

Fig. 3

Schematic depiction of the major desirable traits to be either implemented or improved, toward higher productivity of microalgae in mass culture

Fig. 4

Potential traits to be implemented in GM—C. reinhardtii cell. The diagram displays a number of genetic strategies, aimed to enhance productivity in mass culture of microalgae. Gene over-expression (OE) using hydrid promoters or viral cis-acting elements and gene disruption/down-regulation (KO/KD) by Crispr–Cas9 and RNAi approaches are indicated. Some traits that may result in higher productivity include an increased photosynthetic efficiency, improved phototaxis, the use of non-canonical substrates, and optimized carotenoid, lipid and isoprene metabolism. Up- and down-ward pointing arrow mean up- and down-regulation, respectively, and are referred to the expression level of the corresponding endogenous enzyme. Bulb and red cross mean enzymatic in vitro improvement and loss of function, respectively. Abbreviations: Chl-f S chlorophyll f synthase, CWDE cell-wall degrading enzyme, FTSY chloroplast signal recognition particle, GL gametolysin signal peptide, HS hydrocarbon-synthase, HUP1 hexose-proton symporter, LHC light harvesting complexes, ME malate dehydrogenase, ML multifunctional lipase, NAB1 RNA-binding protein, PHY D phytoene desaturase, PHY S phytoene synthase, PS patchoulol synthase, PTXD phosphite dehydrogenase, β-PS β-phellandrene synthase, TF transcription factor, TLA1 truncated light-harvesting antenna 1, ZEP zeaxanthin epoxidase