Growth curves of G. sulphuraria cultivated in recycled aqueous
products of HTL as a function of HTL temperature and dilution are
summarized in Fig. 3 a)–f) relative to that in the standard growth
medium without any carbon source. Two generalizations can be made
from these growth curves. First, at any given dilution, biomass growth
decreased with increase of HTL temperature. At the dilution of 1%, for
example, the 3-day biomass density decreased 16.5% as the HTL temperature
increased from 180 to 300 °C; while at the dilution of 4% the
corresponding decrease was 48.7%. Second, at any given HTL temperature,
growth increased with increasing AP concentration. For instance,
at the HTL temperature of 180 °C, 3-day biomass density increased
81.8% as the AP concentration increased from 1% to 4%; while at
300 °C, the corresponding increase was 11.7%. Given the negligible
amount of carbohydrate detected in the AP produced at 275 and
300 °C (Fig. 2 a), we interpret the growth observed with AP from
these processing temperatures as likely due to catabolism of lipid
molecules. Free fatty acids have been identified as major products
in both the bio-crude and aqueous product fractions from algal HTL
processing [27].