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).