## 3 Results
### Yield of Hydrothermal Liquefaction Products of Microalgae  
Figure 6a shows previous results regarding HTL product yields with this strain of _Arthrospira platensis_ cultivated under atmospheric conditions. The HTL conditions are similar to those that are used in the current work with one exception: 100 g of dry biomass and 330 mL of distilled water were placed in the reactor [50]. In Figure 6b are the results regarding the yield of the HTL products of wet concentrated biomass obtained during the current work. At the end of HTL, the residual pressure in the reactor was 16 atm, which is similar to our previous results with _Arthrospira platensis_ cultivated at atmospheric CO2 concentration. In our previous works regarding the HTL of this strain of _Arthrospira platensis_ cultivated at atmospheric gas concentrations with similar HTL parameters, the yields of bio-oil were 34.7 wt.% [28] and 37.2 wt.% [29].  
The bio-oil yield in the current work was 13.8 wt.%, which was caused by an increase in the amount of carbohydrates and a decrease in the amount of lipids when _Arthrospira platensis_ was cultivated in a medium with high carbon dioxide content [27]. This leads to the bio-oil yield decrease, as the main contribution to the bio-oil yield is due to lipids [50]. This can also be seen when comparing Figure 6a,b: the bio-oil yield decreased from 32.6 to 18.5 wt.%, whereas the aqueous phase yield increased from 28.3 to 58.2 wt.%.  
It is considered that the changes in the concentration of carbon dioxide during the cultivation of _Arthrospira platensis_ affects the changes in the proportions of the aqueous phase and bio-oil and that the yield of biochar and of gaseous products is almost constant. In the currently work, we show a decrease of the yield of biochar and of gaseous products  
Figure 6: The product yield of _Arthrospira platensis_ hydrothermal liquefaction (wt.%) when cultivating _Arthrospira platensis_ in an atmospheric medium (**a**) [50] or in a medium with an initial CO2 concentration of 8 vol.% (**b**).  
when compared with [50]. An increase in the yield of the aqueous phase shows the promise of this direction for studying its application. The promising directions for the use of aqueous phase are as one of the components of the nutrient medium in the process of microalgae cultivation [51; 52] or as an additive to fertilizers [52]. The promising directions for the use of biochar are the application biochar as a biofertilizer for the soil [51], its reuse as a nutrient medium for the cultivation of microalgae [53; 54] or its use in wastewater treatment [53; 54]. The main component of the gas phase is CO\({}_{2}\)[51]. A promising direction for its application is the use of carbon dioxide as a nutrient medium in the cultivation of microalgae [51]. The economic potential of microalgae processing by hydrothermal liquefaction has not yet been fully studied [55; 56; 57]. In our opinion, when assessing the economic potential the main research should focus on the following points: determination of the products' production costs changing depending on production volumes and the physical and chemical characteristics of the products.