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.