Transition: Phase II
In Phase II the cultures were shifted to opposing temperatures and grown from day 15 to 33. The cultures grown previously at 20 °C were shifted to 35 °C (AB) and 30 °C (CD), and cultures at 35 °C were shifted to 20 °C (EF) and 30 °C (GH). As a recovery phase and to acclimate the cultures after transition from Phase I to Phase II, the cultures in the newer temperature conditions were grown in batch mode without dilution for four days from day 15-19. The responses to temperature change were assessed at the end of the fourth day. As expected, the culture shifted from lower temperature (LT) (20 °C) to higher temperatures (HT) (30 °C and 35 °C) showed higher growth to 1.90 gDW L-1 (from 0.93 to 1.90 gDW L-1 over 4 days). The cultures shifted from higher temperature (HT) (35 °C) to low temperature (LT) (20 °C and 30 °C) grew more slowly, with biomass concentration reaching around 1.71 gDW L-1 (from 1.17 to 1.71 gDW L-1) on day 19 after four days of batch cultivation (Figure 2 a and b). The time scale of temperature response forArthrospira , as judged from these OD750nm measurements, is essentially instantaneous within the noise of these measurements.
During semi-continuous operation in Phase II, different algal growth patterns are found under the chosen temperature conditions. The average pre-dilution DW biomass concentrations for temperature treatments are shown in Table 1. The 20 °C average is the only one that is clearly distinguishable. The most favorable temperature appears to be 30 °C, which is close to the optimum temperature for Arthrospira for achieving maximum productivity under our growth conditions. The results are consistent with that of Colla et al., (2007), where high temperatures had a negative effect on Arthrospira platensisbiomass production. An optimization study carried out by Sánchez‐Luna et al., (2007) in fed-batch cultivation reported 29 °C as best growth temperature. For the two cultures in our Phase II study maintained at 30 °C (one originating from the 20 °C Phase I experiment and the other from the 35 °C Phase I experiment), the results are essentially the same. Thus, the extreme of temperatures and prolonged exposure to high temperatures at 35 °C in Phase I is thought to have caused some stress to the cells, and that usually results in decline in biomass production and protein content, with simultaneous accumulation of carbohydrate and EPS (Panyakampol et al., 2015; Trabelsi et al., 2009). There was little, if any decline in growth rate at 35 °C (though a stress response is clear in the pigment content, as discussed below). Noticeable decline in growth and a lower cell density were observed in the cultures that were shifted from the extreme HT to LT (35 °C→20 °C). This decline in growth is normal temperature dependence (Kumar et al., 2011) and, not surprisingly, the lowest growth and dilution rates were observed at 20 °C. The relative dilution rates were 0.12 day-1 at 20 °C, 0.16 day-1 at 30 °C, and 0.13 day-1 at 35 °C. The highest dilution rate, and therefore productivity, was seen at 30 °C. This agrees with the results of Trabelsi et al., (2009) where maximum growth rate forArthrospira platensis was found at 30 °C.