Abstract
Natural light intensities can rise several orders of magnitude over subsecond time spans, posing a major challenge for photosynthesis. Fluctuating light tolerance in the green alga Chlamydomonas reinhardtii requires alternative electron pathways, but the role of nonphotochemical quenching (NPQ) is not known. Here, fluctuating light (10 min actinic light followed by 10 min darkness) led to significant increase in NPQ/qE-related proteins, LHCSR1 and LHCSR3, relative to constant light of the same subsaturating or saturating intensity. Elevated levels of LHCSR1/3 increased the ability of cells to safely dissipate excess light energy to heat (i.e., qE-type NPQ) during dark to light transition, as measured with chlorophyll fluorescence. The low qE phenotype of the npq4 mutant, which is unable to produce LHCSR3, was abolished under fluctuating light, showing that LHCSR1 alone enables very high levels of qE. Photosystem (PS) levels were also affected by light treatments; constant light led to lower PsbA levels and Fv/Fm values, while fluctuating light led to lower PsaA and maximum P700+ levels, indicating that constant and fluctuating light induced PSII and PSI photoinhibition, respectively. Under fluctuating light, npq4 suffered more PSI photoinhibition and significantly slower growth rates than parental wild type, whereas npq1 and npq2 mutants affected in xanthophyll carotenoid compositions had identical growth under fluctuating and constant light. Overall, LHCSR3 rather than total qE capacity or zeaxanthin is shown to be important in C. reinhardtii in tolerating fluctuating light, potentially via preventing PSI photoinhibition.
Highlights
Photosynthetic bacteria, algae, and plants are able to cope with rapid fluctuations in light intensity, this requires significantly more regulation of photosynthesis than light of constant intensity [1,2]
Colony growth rates of the two genotypes were very similar under constant saturating (500 μmoL quanta m−2 s−1 ) and subsaturating (100 μmoL quanta m−2 s−1 ) light intensities (Figure 1 and Figure S1) and when darkness was replaced with low light intensity (50 μmoL quanta m−2 s−1 ; Figure S1)
C. reinhardtii responded to fluctuating light by increasing levels of LHCSR1 and LHCSR3 (Figure 3), inferring that these qE proteins are important in the dynamic regulation of photosynthesis under fluctuating light
Summary
Photosynthetic bacteria, algae, and plants are able to cope with rapid fluctuations in light intensity, this requires significantly more regulation of photosynthesis than light of constant intensity [1,2]. Rates of CO2 assimilation are limited and cannot always keep pace with rapid changes in light intensity that happen, e.g., as clouds pass across the sun or in sun spots under canopies. Nonphotochemical quenching (NPQ) is a general term for mechanisms that regulate how much light energy is available for photosynthesis [3]. The so-called qE component of NPQ, which reduces quantum yields of chlorophyll fluorescence in the light-harvesting antennae, is the dominant thermal dissipation pathway driven by pH changes in the thylakoid lumen [6,7,8].
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