Microalgal biotechnology has gained considerable importance in recent decades. Applications range from simple biomass production for food and animal feed to valuable products for fuel, pharmaceuticals, health, biomolecules and materials relevant to nanotechnology. There are few reports of the exploration of wider microalgae biodiversity in the literature on high value microalgal compounds, however, because it is believed that there is little to be gained in terms of biomass productivity by examining new strains. Still, without diversity, innovation in biotechnology applications is currently limited. Using microalgal diversity is a very promising way to match species and processes for a specific biotechnological application. In this context, three benthic marine diatom strains (Entomoneis paludosa NCC18.2, Nitzschia alexandrina NCC33, and Staurosira sp NCC182) were selected for their lipid production and growth capacities. Using PAM fluorometry and FTIR spectroscopy, this study investigated the impact of nitrogen repletion and depletion as well as light intensity (30, 100, and 400 μmol.photons.m-2.s-1) on their growth, photosynthetic performance and macromolecular content, with the aim of improving the quality of their lipid composition. Results suggest that under high light and nitrogen limitation, the photosynthetic machinery is negatively impacted, leading cells to reduce their growth and accumulate lipids and/or carbohydrates. However, increasing lipid content under stressful conditions does not increase the production of lipids of interest: PUFA, ARA and EPA production decreases. Culture conditions to optimize production of such fatty acids in these three original strains led to a balance between economic and ecophysiological constraints: low light and no nitrogen limitation led to better photosynthetic capacities associated with energy savings, and hence a more profitable approach.

The selection of suitable and indigenous microalgae species is a fundamental requirement in developing added-value bioactive compounds recoverable in the food, health, and cosmetics markets. In this work, an integrated screening approach was developed to characterize the lipid rate of 33 diatom species (including 15 species studied for the first time) belonging to 16 genera from the Nantes Culture Collection, with the main objective of discovering bioactive lipid producers. For that purpose, a simple reliable method for establishing growth kinetics of strains and semi-quantitative analysis of lipid rates was developed. Growth kinetics measurements were achieved by daily minimal measurement fluorescence (F0) whereas lipid rate analyses were performed by high-throughput Fourier Transform Infrared spectroscopy on entire cells and lipid extracts. Results indicated that the method could be used directly on entire cells in spite of the presence of silica for the FTIR approach (due to frustule). The total lipid rate was species-dependant and ranged from 3.7% to 30.5% DW. Six strains out of 33 were found to present a higher total lipid rate superior to 15% DW, and 11 showed medium lipid rates ranging from 10% to 15% DW. The results revealed that five diatom species i.e. Amphora sp. NCC169, Nitzschia sp. NCC109, Nitzschia alexandrina NCC33, Opephora sp NCC366 and Staurosira sp. NCC182 presented interesting growth capabilities and should be further investigated as potential sources for their original lipid rate.

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