Boosting Biomass Quantity and Quality by Improved Mixotrophic Culture of the Diatom Phaeodactylum tricornutum.

Valeria Villanova,Mark Poolman,Giovanni Finazzi,Julien Pagliardini,Dipali Singh,David Fell,Adeline Le Monnier

doi:10.3389/fpls.2021.642199

Abstract

Diatoms are photoautotrophic unicellular algae and are among the most abundant, adaptable, and diverse marine phytoplankton. They are extremely interesting not only for their ecological role but also as potential feedstocks for sustainable biofuels and high-value commodities such as omega fatty acids, because of their capacity to accumulate lipids. However, the cultivation of microalgae on an industrial scale requires higher cell densities and lipid accumulation than those found in nature to make the process economically viable. One of the known ways to induce lipid accumulation in Phaeodactylum tricornutum is nitrogen deprivation, which comes at the expense of growth inhibition and lower cell density. Thus, alternative ways need to be explored to enhance the lipid production as well as biomass density to make them sustainable at industrial scale. In this study, we have used experimental and metabolic modeling approaches to optimize the media composition, in terms of elemental composition, organic and inorganic carbon sources, and light intensity, that boost both biomass quality and quantity of P. tricornutum. Eventually, the optimized conditions were scaled-up to 2 L photobioreactors, where a better system control (temperature, pH, light, aeration/mixing) allowed a further improvement of the biomass capacity of P. tricornutum to 12 g/L.

Highlights

Diatoms are photosynthetic unicellular microalgae that dominate the oceans
The linear programming (LP) in Equation (1) was solved for 1 unit of TAG demand under four conditions: (i) +BIC+GLY: mixotrophic condition in the presence of HCO3, (ii) – BIC+GLY: mixotrophic condition in the absence of HCO3, (iii) +BIC-GLY: phototrophic condition in the presence of HCO3 and, (iv) –BIC-GLY: phototrophic condition in the absence of HCO3
Under phototrophic condition, carbon demand is met through inorganic carbon fixation through the Calvin cycle for which the enzymatic cost and the requirement for light energy are higher compared to mixotrophic conditions, as shown in Tables 1, 2, respectively

Summary

Introduction

Diatoms are photosynthetic unicellular microalgae that dominate the oceans Their ability to synthesize lipid as a storage compound makes them a potential source of biofuel and high-value commodities such as omega fatty acids (Hildebrand et al, 2012; d’Ippolito et al, 2015; Wang and Seibert, 2017; Yi et al, 2017; Pudney et al, 2019). They have notably different evolutionary history from that of other photosynthetic eukaryotes such as plants and green algae, and are thought to have arisen from a complex endosymbiotic event, which is ascertained, though donor is not clearly identified (Wilhelm et al, 2006; Armbrust, 2009; Moustafa et al, 2009). The Entner–Doudoroff (ED) pathway and phosphoketolase pathways, commonly found in prokaryotes, have been reported to be present in P. tricornutum (Fabris et al, 2012), though the contribution of these pathways in P. tricornutum is not well-known

Methods

Results

Conclusion