Kinetics of the Xanthophyll Cycle and its Role in the Photoprotective Memory and Response

Efficiently balancing photochemistry and photoprotection is crucial for survival and productivity of photosynthetic organisms in the rapidly fluctuating light levels found in natural environments. The ability to respond quickly to sudden changes in light level is clearly advantageous. In the alga Nannochloropsis oceanica we observed an ability to respond rapidly to sudden increases in light level which occur soon after a previous high-light exposure. This ability implies a kind of memory. In this work, we explore the xanthophyll cycle in N. oceanica as a photoprotective memory system. By combining snapshot fluorescence lifetime measurements with a biochemistry-based quantitative model we show that both short-term and medium-term "memory" arises from the xanthophyll cycle. In addition, the model enables us to characterize the relative quenching abilities of the three xanthophyll cycle components. Given the ubiquity of the xanthophyll cycle in photosynthetic organisms the model described here will be of utility in improving our understanding of vascular plant photoprotection with important implications for crop productivity

Photoprotective and Photosynthetic Mechanisms of Eustigmatophyte Microalga, Nannochloropsis oceanica

Photosynthetic stramenopiles (stramenochromes) are a very diverse group of algae. They range from microscopic phytoplankton that serve as primary producers in oceanic environments to macroscopic seaweeds that provide food worldwide. Stramenochromes originated from secondary endosymbiosis involving a red-algal ancestor and exhibit unique features that make them different from well-studied green algae and plants. Because they occupy such a wide range of ecological niches, they have evolved different strategies in photosynthesis and photoprotection to regulate light harvesting under varying environmental conditions. Chapter 1 highlights these unique features and strategies among stramenochromes while encompassing classes beyond well-known groups like Bacillariophyceae and Eustigmatophyceae. Subsequent chapters focus on a particular Eustigmatophyte microalga that has emerged as a model organism, Nannochloropsis oceanica CCMP1779.Nannochloropsis oceanica is a stramenochrome that can produce high amounts of lipids including the valuable omega-3 fatty acid, eicosapentaenoic acid. This dissertation focuses on the mechanisms of photoprotection and photosynthesis in N. oceanica, holding potential applications in enhancing photosynthetic efficiency in mass cultures and algae farms. This begins with Chapter 2 which characterizes and investigates the mechanisms of a process known as non-photochemical quenching (NPQ), which dissipates excess absorbed light energy as heat. In this chapter we use chemical inhibitor treatments and mutant analysis to reveal that LHCX1 is crucial for qE induction, whereas LHCX2 and LHCX3 do not seem to play a role in NPQ in low-light grown cells. We also show that the vde mutant, deficient in the synthesis of the photoprotective pigment zeaxanthin, lacks both qE and qZ. These results show that photoprotection in N. oceanica is highly dependent on the formation of zeaxanthin and its collaboration with LHCX1. Chapter 3 focuses on resolving the molecular function of LHCX1 by testing the hypothesis that LHCX1 is involved in sensing a decrease in thylakoid lumen pH in excess light. Knock-in mutations targeting putative pH-sensing sites were generated in the native LHCX1 gene, although they revealed no discernable impact on qE capacity. This aligns with previous results in diatoms but is different from similar proteins in the green algae and plants, suggesting functional divergence in stramenochromes. Moreover, generation and analysis of the W143M mutant revealed that a mutation in this amino acid doesn’t impact a putative pigment binding site involved in qE. This contrasts with previous results in diatoms, suggesting species-specific divergence in N. oceanica. This work prompts a reconsideration of ΔpH-sensing mechanisms in photosynthetic organisms. While the overall function of LHCX1 in qE remains elusive, strides are made towards resolving its structure in Chapter 4. Chapter 4 focuses on developing methods towards unraveling the architecture and protein makeup of PSI-VCPI and PSII-VCPII supercomplexes that contain LHCX1. To do this, high-resolution single-particle cryogenic electron microscopy (EM) will be used. Developing the methods towards cryo-EM involves many steps, which were optimized in this chapter. Overall findings indicated that using maltose density gradients with gradient fixation (GraFix) improves sample quality and supercomplex isolation. These methods will be used for structural analysis to reveal the overall architecture, protein-protein interactions, and pigment locations in the photosynthetic supercomplexes, leading to inferences on energy-transfer and energy-dissipation pathways in N. oceanica and other diverse algae groups

Kinetics of the xanthophyll cycle and its role in photoprotective memory and response.

Efficiently balancing photochemistry and photoprotection is crucial for survival and productivity of photosynthetic organisms in the rapidly fluctuating light levels found in natural environments. The ability to respond quickly to sudden changes in light level is clearly advantageous. In the alga Nannochloropsis oceanica we observed an ability to respond rapidly to sudden increases in light level which occur soon after a previous high-light exposure. This ability implies a kind of memory. In this work, we explore the xanthophyll cycle in N. oceanica as a short-term photoprotective memory system. By combining snapshot fluorescence lifetime measurements with a biochemistry-based quantitative model, we show that short-term memory arises from the xanthophyll cycle. In addition, the model enables us to characterize the relative quenching abilities of the three xanthophyll cycle components. Given the ubiquity of the xanthophyll cycle in photosynthetic organisms the model described here will be of utility in improving our understanding of vascular plant and algal photoprotection with important implications for crop productivity

Xanthophyll-cycle based model of the rapid photoprotection of Nannochloropsis in response to regular and irregular light/dark sequences.

We explore the photoprotection dynamics of Nannochloropsis oceanica using time-correlated single photon counting under regular and irregular actinic light sequences. The varying light sequences mimic natural conditions, allowing us to probe the real-time response of non-photochemical quenching (NPQ) pathways. Durations of fluctuating light exposure during a fixed total experimental time and prior light exposure of the algae are both found to have a profound effect on NPQ. These observations are rationalized with a quantitative model based on the xanthophyll cycle and the protonation of LHCX1. The model is able to accurately describe the dynamics of non-photochemical quenching across a variety of light sequences. The combined model and observations suggest that the accumulation of a quenching complex, likely zeaxanthin bound to a protonated LHCX1, is responsible for the gradual rise in NPQ. Additionally, the model makes specific predictions for the light sequence dependence of xanthophyll concentrations that are in reasonable agreement with independent chromatography measurements taken during a specific light/dark sequence

Regulation of photoprotection gene expression in Chlamydomonas by a putative E3 ubiquitin ligase complex and a homolog of CONSTANS.

Photosynthetic organisms use nonphotochemical quenching (NPQ) mechanisms to dissipate excess absorbed light energy and protect themselves from photooxidation. In the model green alga Chlamydomonas reinhardtii, the capacity for rapidly reversible NPQ (qE) is induced by high light, blue light, and UV light via increased expression of LHCSR and PSBS genes that are necessary for qE. Here, we used a forward genetics approach to identify SPA1 and CUL4, components of a putative green algal E3 ubiquitin ligase complex, as critical factors in a signaling pathway that controls light-regulated expression of the LHCSR and PSBS genes in C. reinhardtii The spa1 and cul4 mutants accumulate increased levels of LHCSR1 and PSBS proteins in high light, and unlike the wild type, they express LHCSR1 and exhibit qE capacity even when grown in low light. The spa1-1 mutation resulted in constitutively high expression of LHCSR and PSBS RNAs in both low light and high light. The qE and gene expression phenotypes of spa1-1 are blocked by mutation of CrCO, a B-box Zn-finger transcription factor that is a homolog of CONSTANS, which controls flowering time in plants. CONSTANS-like cis-regulatory sequences were identified proximal to the qE genes, consistent with CrCO acting as a direct activator of qE gene expression. We conclude that SPA1 and CUL4 are components of a conserved E3 ubiquitin ligase that acts upstream of CrCO, whose regulatory function is wired differently in C. reinhardtii to control qE capacity via cis-regulatory CrCO-binding sites at key photoprotection genes

Role of an ancient light-harvesting protein of PSI in light absorption and photoprotection

Diverse algae of the red lineage possess chlorophyll a-binding proteins termed LHCR, comprising the PSI light-harvesting system, which represent an ancient antenna form that evolved in red algae and was acquired through secondary endosymbiosis. However, the function and regulation of LHCR complexes remain obscure. Here we describe isolation of a Nannochloropsis oceanica LHCR mutant, named hlr1, which exhibits a greater tolerance to high-light (HL) stress compared to the wild type. We show that increased tolerance to HL of the mutant can be attributed to alterations in PSI, making it less prone to ROS production, thereby limiting oxidative damage and favoring growth in HL. HLR1 deficiency attenuates PSI light-harvesting capacity and growth of the mutant under light-limiting conditions. We conclude that HLR1, a member of a conserved and broadly distributed clade of LHCR proteins, plays a pivotal role in a dynamic balancing act between photoprotection and efficient light harvesting for photosynthesis. LHCR proteins are ancient chlorophyll a-binding antennas that evolved in diverse algae of the red lineage. Here Lu et al. characterize a red lineage LHCR mutant and show reduced oxidative damage in high light but attenuated growth under low light, thus demonstrating how LHCR proteins impact the balance between photoprotection and light harvesting