Abstract
The cell transformation from green motile cells to non-motile cells and astaxanthin accumulation can be induced in the green alga Haematococcus pluvialis cultured outdoors. In the initial 3 d of incubation (cell transformation phase), light absorption and photosynthetic electron transport became more efficient. After five days of incubation (astaxanthin accumulation phase), the light absorption per active reaction center (ABS/RC) increased, but the efficiency of electron transport (ψo) and the quantum yield of electron transport (φEo) decreased with increased time, indicating that the capacity of photosynthetic energy utilization decreased significantly during astaxanthin accumulation, leading to an imbalance between photosynthetic light absorption and energy utilization. It would inevitably aggravate photoinhibition under high light, e.g., at midday. However, the level of photoinhibition in H. pluvialis decreased as the incubation time increased, which is reflected by the fact that Fv/Fm determined at midday decreased significantly in the initial 3 d of incubation, but was affected very little after seven days of incubation, compared with that determined at predawn. This might be because the non-photochemical quenching, plastid terminal oxidase, photosystem I cyclic electron transport, defensive enzymes and the accumulated astaxanthin can protect cells against photoinhibition.
Highlights
Haematococcus pluvialis, a unicellular green alga, is widely known for its ability to accumulate large amounts of astaxanthin (3,3 -dihydroxy-β,β-carotene-4,4 -dione) [1,2,3]
The astaxanthin content increased from 0.47 μg·mL−1 (0.02 μg·104 cells−1) to 12.89 μg·mL−1 (0.56 μg·104 cells−1) during a nine-day incubation period (Figure 1D,E)
In detail, during the initial three days of incubation, there was relative variable fluorescence kinetics at the J-step, the L-band and the electron transport per active reaction center (ETo/RC) did not change, the K-band, the light absorption per active reaction center (ABS/RC) and the trapping of excitation energy per active reaction center (TRo/RC) decreased, whereas the maximal amplitudes of fluorescence in the I-P phase of the OJIP transient, the efficiency of electron transport and the quantum yield of electron transport increased over time (Figures 3 and 4). These results indicate that the light absorption and photosynthetic electron transport became much more efficient [17,22] during cell transformation, which was indicated by the fact that the performance index PIABS as well as its individual partial components RC/ABS, φPo/(1 − φPo) and ψo/(1 − ψo) increased significantly during the initial three days of incubation (Figure 4)
Summary
Haematococcus pluvialis, a unicellular green alga, is widely known for its ability to accumulate large amounts of astaxanthin (3,3 -dihydroxy-β,β-carotene-4,4 -dione) [1,2,3]. The cell transformation and astaxanthin accumulation are strongly light-induced processes for H. pluvialis, and can be regulated by photosynthetic redox status [9,10,11]. Understanding the changes of photosynthetic behaviors during the cell transformation and astaxanthin accumulation would provide some new insights into mechanisms of astaxanthin accumulation in H. pluvialis. Most of the work published so far on H. pluvialis has been focused on the optimization of cell growth and the induction of astaxanthin accumulation of this alga [3,12]. Far less attention has been paid to the changes of photosynthetic behaviors, especially the energy fluxes of absorption, trapping and electron transport, in H. pluvialis during cell transformation and astaxanthin accumulation. Many works about the response of photosynthesis during cell transformation and astaxanthin accumulation in H. pluvialis grown indoors have been reported. There is no work concerning outdoor photosynthetic response of this species reported
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