Investigating mixotrophic metabolism in the model diatom Phaeodactylum tricornutum.

Diatoms are prominent marine microalgae, interesting not only from an ecological point of view, but also for their possible use in biotechnology applications. They can be cultivated in phototrophic conditions, using sunlight as the sole energy source. Some diatoms, however, can also grow in a mixotrophic mode, wherein both light and external reduced carbon contribute to biomass accumulation. In this study, we investigated the consequences of mixotrophy on the growth and metabolism of the pennate diatom Phaeodactylum tricornutum, using glycerol as the source of reduced carbon. Transcriptomics, metabolomics, metabolic modelling and physiological data combine to indicate that glycerol affects the central-carbon, carbon-storage and lipid metabolism of the diatom. In particular, provision of glycerol mimics typical responses of nitrogen limitation on lipid metabolism at the level of triacylglycerol accumulation and fatty acid composition. The presence of glycerol, despite provoking features reminiscent of nutrient limitation, neither diminishes photosynthetic activity nor cell growth, revealing essential aspects of the metabolic flexibility of these microalgae and suggesting possible biotechnological applications of mixotrophy.This article is part of the themed issue 'The peculiar carbon metabolism in diatoms'.

Villanova V ，Fortunato AE ，Singh D ，Bo DD ，Conte M ，Obata T ，Jouhet J ，Fernie AR ，Marechal E ，Falciatore A ，Pagliardini J ，Le Monnier A ，Poolman M ，Curien G ，Petroutsos D ，Finazzi G ... -  《-》

Phaeodactylum tricornutum cultivation under mixotrophic conditions with glycerol supplied with ultrafiltered digestate: A simple biorefinery approach recovering C and N.

Phaeodactylum tricornutum was cultivated mixotrophically in batch mode providing glycerol as the C source, i.e., 0.02, 0.03 and 0.04 Mol L-1 glycerol, and ultrafiltered digestate (UF) as an N source. Biomass productivity, biomass composition, N efficiency use and total energy balance were recorded and compared to those under autotrophic conditions. Under mixotrophic conditions (0.03 Mol L-1 and 0.04 Mol L-1 glycerol), biomass productivity of P. tricornutum increased by 1.29 and 1.60 times in comparison with autotrophic conditions. Algal protein content declined as glycerol concentration increased, contrary to the case of the carbohydrate content. Lipid content did not change but unexpectedly, a lower unsaturated fatty acid in mixotrophic culture was observed than that from autotrophic culture. Mixotrophic conditions offered a higher energy recovery efficiency (EFt) than autotrophic conditions (5.7 % in 0.04 Mol L-1 glycerol and 4.2 % in autotrophic trial, respectively). Additionally, the efficiency of glycerol conversion into biomass (EFgly) increased with the glycerol dose, achieving 22.8 % for 0.04 Mol L-1 glycerol.

Su M ，D'Imporzano G ，Veronesi D ，Afric S ，Adani F ... -  《-》

Mixotrophy in diatoms: Molecular mechanism and industrial potential.

Diatoms are microalgae well known for their high variability and high primary productivity, being responsible for about 20% of the annual global carbon fixation. Moreover, they are interesting as potential feedstocks for the production of biofuels and high-value lipids and carotenoids. Diatoms exhibit trophic flexibility and, under certain conditions, they can grow mixotrophically by combing photosynthesis and respiration. So far, only a few species of diatoms have been tested for their mixotrophic metabolism; in some cases, they produced more biomass and with higher lipid content when grown under this condition. Phaeodactylum tricornutum is the most studied diatom species for its mixotrophic metabolism due to available genome sequence and molecular tools. However, studies in additional species are needed to better understand the conservation of this process in diatoms and its potential in industrial applications. Here, we describe the photosynthetic and respiratory pathways involved in mixotrophy and provide an overview of the trophic variability in diatoms. This review also highlights promising areas of industrial applications for diatoms when cultivated under mixotrophy.

Villanova V ，Spetea C 《-》

Physiological and molecular analysis of carbon source supplementation and pH stress-induced lipid accumulation in the marine diatom Phaeodactylum tricornutum.

Mus F ，Toussaint JP ，Cooksey KE ，Fields MW ，Gerlach R ，Peyton BM ，Carlson RP ... -  《-》

Mixotrophic growth of Phaeodactylum tricornutum on fructose and glycerol in fed-batch and semi-continuous modes.

Mixotrophic cultures of Phaeodactylum tricornutum were carried out in bubble columns using fructose and glycerol in indoor fed-batch and semi-continuous modes. In the fed-batch cultures, different nutrient-addition strategies, combined with stepwise increments in the light intensity, were assayed. It was found that glycerol promoted significantly higher biomass productivity than fructose. A glycerol-induced photoinhibition that arrested the growth of P. tricornutun was also observed. As this was considered a limitation as regards transferring the fed-batch mode to outdoor conditions, this information was used to culture P. tricornutum in semi-continuous mode. Similar glycerol-induced photoinhibition was not observed in these cultures, even at highest dilution rates. Although the highest biomass (1.5 g L(-1) d(-1)) and EPA (40 mg L(-1) d(-1)) productivities found in the semi-continuous cultures were lower than those obtained photoautotrophically in outdoor photobioreactors, the findings showed that semi-continuous mode was an excellent candidate for transferring mixotrophic culture to an outdoor setting.

Cerón-García MC ，Fernández-Sevilla JM ，Sánchez-Mirón A ，García-Camacho F ，Contreras-Gómez A ，Molina-Grima E ... -  《-》