Photosynthetic parameters
Photosynthesis-irradiance (PI) curves have been extensively used to
evaluate the photosynthetic response to various abiotic stresses
experienced by algae (Falkowski and Raven, 2007). Photosynthetic
parameters, including Pmax (µmol O2L-1 hr-1), α’ (mol
O2 mol photon-1),
R0’ (µmol O2L-1 min-1) and Ek(µE m-2 s-1), were evaluated at
different temperature treatments during Phases I and II using PE curves
(Figure 6 and Table 2) to test consistency with the above observations
for changing conditions and provide parameters for productivity
modeling. The culture samples from different temperature treatments were
first incubated at 30 °C under dark conditions for 1 h. Testing for the
different treatments was conducted at a single temperature (30 °C) to
avoid the normal temperature dependence wherein Pmax and
Ek display a Q10 = 2 dependence (about
60 kJ mol-1). The PE curves were measured (in
duplicate) for all treatments, with average values reported. The
photosynthetic response patterns from cultures grown at 20 °C, 30 °C and
35 °C Phase I are shown in Figure 6, with results summarized in Table 2.
It is clear that with this experimental protocol none of the samples in
Figure 6 shows a significant difference from the others, the only
possible exceptions being the AB-ExSP sample exposed to the most severe
summer profile conditions and the A sample from Phase II (constant 35
°C). PE curves measured at 20 °C for culture samples from Phase I (20 °C
treatment) yield a Pmax of 240 µmol O2L-1 hr-1 and Ek as
85 µE m-2 s-1, which is roughly
Q10 of 2 when compared to results from PE curves
measured at 30 °C. In fact a more extensive testing (not presented here)
of PE curves measured over the temperature range 15-35 °C yields an
activation energy for Pmax of 60 kJ
mol-1. This activated process can be attributed
entirely to Ek, as the limiting quantum yield (α) has
been shown to be independent of temperature over the range studied. As
noted earlier, these observations are typical of temperature response in
photosynthetic organisms (Falkowski and Raven, 2007). The constant
exposure to 35 °C, also measured at 35 °C, (Table 2) yields
photosynthetic parameters similar to the other tests at 30 °C. In Phase
I there is some indication of a stress response at sustained high
temperatures in these results, though this is not as clear as the
pigment variation. There is no indication in Phase III of dynamic high
temperature exposure having an adverse effect. These observations are
consistent those made in conjunction with biomass and pigment
production.
There was no significant difference in values of α’ (limited quantum
yield for O2 production) which were close to
~ 0.070 mol O2/mol photon for all the
treatments. The lowest R0’(respiration rate) of 0.12 µmol O2/L-min was observed at
20°C while the maximum of 0.55 µmol O2/L-min was found
at 35°C. R0’ determinations have
effects due to the light exposure history (Falkowski and Raven, 2007).
Little temperature dependence is expected for α’, consistent with the
results from this study. The ratio α’/α is the photosynthetic quotient
(O2 per fixed carbon) which is expected to be in the
range 1.1-1.3 (Falkowski and Raven, 2007). We use 1.2 for the modeling
analysis to follow. The same value applies to R0, the
respiration rate on a carbon basis required for application in the
Algenol Productivity Model, Equation 3.
It is noteworthy that large changes in pigment content and light
absorption level are seen with very little change in biomass
productivity, whether measured directly or inferred from the PE curves.
This is consistent with the relatively minor impacts of low pigment
mutants on productivity in other organisms (Kirst et al., 2014;
Lea-Smith et al., 2014).