Results from the different batches done under different temperatures (16.5, 20, 25, and 30 °C) and a constant light intensity of 300 μmol m−2 s−1 show that Tisochrysis lutea grew faster (0.78 d−1) at 30 °C than at lower temperatures (Table 2). The biomass productivity increased from 0.06 to 0.42 g L−1 d−1 with increasing temperature from 16.5 to 30 °C (Table 2). The final biomass at 30 °C was 5.2-fold higher than at 16.5 °C. In literature, the cultivation temperature of Isochrysis ranges from 5 to 35 °C (Araie et al., 2018, Ippoliti et al., 2016, Li et al., 2016), with the most commonly used temperatures between 25 and 30 °C (Abu-Rezq et al., 1999, All et al., 2012, Renaud et al., 2002). The optimum temperature varies for different strains of Isochrysis species; for example, Kaplan et al. (1986) reported a reduced biomass yield in Isochrysis galbana at temperatures higher than 32 °C or lower than 19 °C. Likewise, the cell density of Isochrysis galbana Parke was reported to be 5 times higher at 20 °C than at 35 °C (Su et al., 2017). These differences are strain-specific and are usually associated with the origin of the strains. Tisochrysis lutea strain was originally isolated from a tropical region of the Pacific (Bendif et al., 2013), which explains its adaptation to higher temperatures. In addition, the growth rate of Isochrysis sp. decreased from 0.89 to 0.04 d−1 with increasing temperature from 30 to 35 °C (Renaud et al., 2002). Hence, temperatures higher than 30 °C were not studied in the present study.
Table 2. Cell diameters and biomass productions at different temperatures and an incident light intensity of 300 μmol m−2 s−1.
Temperature (°C) 16.5 20 25 30
Growth rate (d−1) 0.38 0.74 0.76 0.78
Diameter (μm) 6.05 ± 0.31a 4.89 ± 0.16b 4.30 ± 0.22c 4.57 ± 0.22bc
Final Biomass Concentration DW (g/L) 0.35 ± 0.02d 1.36 ± 0.07c 1.54 ± 0.08b 1.81 ± 0.09a
Biomass Productivity (g L−1 d−1) 0.06 ± 0.01 d 0.32 ± 0.02c 0.36 ± 0.02b 0.42 ± 0.02 a
Note: values are the means ± SD. Means with different letters are significantly different from each other (comparisons were made between groups for each variable) (p < 0.05).
Fx contents were similar at 30 °C and 25 °C (p > 0.05) and significantly higher than at 20 °C and 16.5 °C (p < 0.05; Fig. 1b). Pfx was higher (p < 0.05) with increasing temperatures due to higher biomass productivities. The Pfx at 30 °C was 7.9-, 1.6-, and 1.2-fold higher than at 16.5, 20, 25 °C, respectively. The optimal temperature was 30 °C for both biomass and Pfx. The same optimum temperature was obtained from a prediction model in a previous study (Marchetti et al., 2013).
In our results, Pfx improved at higher temperatures due to higher biomass productivity. Moreover, the cellular diameter was larger at 16.5 °C (p < 0.05) (Table 2), which is a known response to low temperatures due to intracellular lipid accumulation (Skau et al., 2017). Lipid bodies can be seen under microscope at 16.5 °C. This alteration in cell diameter, associated with lower growth, indicates that lower temperatures are not lethal but lead to cellular stress in Tisochrysis lutea. A similar phenomenon was reported before where negative growth rates (~0.1 d−1) and increased lipid contents in Isochrysis galbana were observed at 10 °C (Roleda et al., 2013).
Overall, the batch experiments provided the optimum parameters, 300 μmol m−2 s−1 and 30 °C, for growth and biomass production, and supplied evidence for the effect of light on Fx production, which were used when designing the continuous experiments, described in the following sections
- Tisochrysis lutea
- Tisochrysis galbana
- Temperature
- Research