Transition: Phase II
As noted earlier, in Phase II (day 15 to 33) the four cultures (ABCD)
grown previously at 20 °C were shifted to 35 °C (AB) and 30 °C (CD), and
the four cultures at 35 °C (EFGH) 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 (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 (35 °C) to low temperature (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 for Arthrospira , 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 the different 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 Arthrospirafor achieving maximum productivity under our growth conditions. The
results are consistent with that of Colla et al. (2007), where
higher temperatures had a clear negative effect on Arthrospirabiomass production. An optimization study carried out by Sánchez‐Luna et
al. (2007) in batch cultivations 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 stress to the
cells, and that has been observed by others to result 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 some decline in growth rate at 35 °C, though a stress
response is clearer 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 35 °C→20 °C. This is normal
temperature dependence (Kumar et al., 2011). The relative dilution rates
were 0.12 day-1 at 20 °C, 0.16 day-1at 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 cited above and also with Trabelsi et al. (2009)
where maximum growth rate for Arthrospira platensis was found at
30 °C.