Abstract
The LHCSR protein belongs to the light harvesting complex family of pigment-binding proteins found in oxygenic photoautotrophs. Previous studies have shown that this complex is required for the rapid induction and relaxation of excess light energy dissipation in a wide range of eukaryotic algae and moss. The ability of cells to rapidly regulate light harvesting between this dissipation state and one favoring photochemistry is believed to be important for reducing oxidative stress and maintaining high photosynthetic efficiency in a rapidly changing light environment. We found that a mutant of Chlamydomonas reinhardtii lacking LHCSR, npq4lhcsr1, displays minimal photoinhibition of photosystem II and minimal inhibition of short term oxygen evolution when grown in constant excess light compared to a wild type strain. We also investigated the impact of no LHCSR during growth in a sinusoidal light regime, which mimics daily changes in photosynthetically active radiation. The absence of LHCSR correlated with a slight reduction in the quantum efficiency of photosystem II and a stimulation of the maximal rates of photosynthesis compared to wild type. However, there was no reduction in carbon accumulation during the day. Another novel finding was that npq4lhcsr1 cultures underwent fewer divisions at night, reducing the overall growth rate compared to the wild type. Our results show that the rapid regulation of light harvesting mediated by LHCSR is required for high growth rates, but it is not required for efficient carbon accumulation during the day in a sinusoidal light environment. This finding has direct implications for engineering strategies directed at increasing photosynthetic productivity in mass cultures.
Highlights
The natural aquatic light environment fluctuates across space and time
The measured growth rates were reduced by 11% in npq4lhcsr1 relative to wild type at the highest irradiance (p
Previous work on the qE deficient mutant, npq4, showed that absence of LHCSR3 leads to a 50% reduction in non-photochemical quenching of light energy (NPQ) capacity and this capacity could be recovered by complementation [21]
Summary
Light intensity follows a sinusoidal oscillation across the day with superimposed rapid fluctuations due to changes in cloud cover, wave focusing, turbidity and vertical mixing[1, 2]. These short term changes can cause light absorption to exceed the capacity of utilization leading to the generation of reactive. Plants and cyanobacteria dynamically regulate a process termed non-photochemical quenching of light energy (NPQ) to avoid excess damage while maintaining efficient photosynthesis in lower light fluxes This balance between energy dissipation and light harvesting capacity allows photosynthetic organisms to maintain optimal fitness in diverse environmental niches
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