Aqueous phase recycling: impact on microalgal lipid accumulation and biomass quality.

The potential success of microalgal biofuels greatly depends on the sustainability of the chosen pathway to produce them. Hydrothermal liquefaction (HTL) is a promising route to convert wet algal biomass into biocrude. Recycling the resulting HTL aqueous phase (AP) aims not only to recover nutrients from this effluent but also to use it as a substrate to close the photosynthetic loop and produce algal biomass again and process this biomass again into new biocrude. With that purpose, the response to AP recycling of five Chlorellaceae strains was monitored over five cultivation cycles. After four successive cycles of dynamic growth under nutrient-replete conditions, the microalgae were cultivated for a prolonged fifth cycle of 18 days in order to assess the impact of the AP on lipid and biomass accumulation under nutrient-limited conditions. Using AP as a substrate reduced the demand for external sources of N, S, and P while producing a significant amount of biomass (2.95-4.27 g/L) among the strains, with a lipid content ranging from 16 to 36%. However, the presence of the AP resulted in biomass with suboptimal properties, as it slowed down the accumulation of lipids and thus reduced the overall energy content of the biomass in all strains. Although Chlorella vulgaris NIES 227 did not have the best growth on AP, it did maintain the best lipid productivity of all the tested strains. Understanding the impact of AP on microalgal cultivation is essential for further optimizing biofuel production via the HTL process.

Resolving phytosterols in microalgae using offline two-dimensional reversed phase liquid chromatography-supercritical fluid chromatography coupled with quadrupole time-of-flight mass spectrometry.

Sterols are unsaponifiable lipids resulting from plant metabolism that exhibit interesting bioactive properties. Microalgae are a major source of specific phytosterols, most of which are still not fully characterized. The similarity in sterol structures and the existence of positional isomers make the separation of phytosterols challenging. A method was developed based on an offline two-dimensional (2D) system, reversed-phase liquid chromatography (RPLC)-supercritical fluid chromatography (SFC)/quadrupole time-of-flight (Q-ToF) mass spectrometry, for the identification of sterols in microalgae. Subsequent positive-mode MS/MS was used to confirm the identified phytosterols. The 2D chromatogram exhibited a pattern related to the positions of the double bonds, which were confirmed by standard injection, enabling structural elucidation. The analysis of the unsaponifiable fraction of two algae, namely Scenedesmus obliquus, a freshwater microalgae, and Padina pavonica, a marine macroalgae, highlighted the ability of the method to distinguish a large number of sterol isomers.

Microalgae adaptation as a strategy to recycle the aqueous phase from hydrothermal liquefaction.

Hydrothermal liquefaction (HTL) produces bio-crude oil from wet algae along with an aqueous phase (AP). This effluent contains minerals that can be reused for cultivating new microalgae but whose utility remains limited due to the presence of inhibitors. Reduced photosynthetic performance, growth, and null lipid accumulation were observed in wild-type Chlorella vulgaris NIES 227 cultivated in AP (1/200). Adaptive laboratory evolution was studied by batch transfers and turbidostat mode. Both methods effectively counterbalanced AP toxicity and restored the fitness of the microalgae. After adaptation, a higher AP addition was achieved, from 1/600 to 1/200, without inhibition. As compared with the wild typein control medium (0.261 g/L/d), both adapted-strains maintained competitive productivity (0.310 and 0.258 g/L/d) of lipid-rich biomass (37 %-56 %). The improved tolerance of the adapted strains persisted after the removal of AP and under axenic conditions. Adaptive laboratory evolution is suggested for AP reutilization in the algae production process.