Photosystem II core quenching in desiccated Leptolyngbya ohadii

Light sensitive orange carotenoid proteins (OCPs) in cyanobacterial photoprotection: evolutionary insights, structural-functional dynamics and biotechnological prospects

Among all photosynthetic life forms, cyanobacteria exclusively possess a water-soluble, light-sensitive carotenoprotein complex known as orange carotenoid proteins (OCPs), crucial for their photoprotective mechanisms. These protein complexes exhibit both structural and functional modularity, with distinct C-terminal (CTD) and N-terminal domains (NTD) serving as light-responsive sensor and effector regions, respectively. The majority of cyanobacterial genomes contain genes for OCP homologs and related proteins, highlighting their essential role in survival of the organism over time. Cyanobacterial photoprotection primarily involves the translocation of carotenoid entity into the NTD, leading to remarkable conformational changes in both domains and formation of metastable OCPR. Subsequently, OCPR interacts with phycobiliprotein, inducing the quenching of excitation energy and a significant reduction in PS II fluorescence yield. In dark conditions, OCPR detaches from phycobilisomes and reverts to OCPO in the presence of fluorescent recovery proteins (FRP), sustaining a continuous cycle. Research suggests that the modular structure of the OCPs, coupled with its unique light-driven dissociation and re-association capability, opens avenues for exploiting its potential as light-controlled switches, offering various biotechnological applications.

Mass spectrometry and spectroscopic characterization of a tetrameric photosystem I supercomplex from Leptolyngbya ohadii, a desiccation-tolerant cyanobacterium

Cyanobacteria inhabiting desert biological soil crusts face the harsh conditions of the desert. They evolved a suite of strategies toward desiccation-hydration cycles mixed with high light irradiations, etc. In this study we purified and characterized the structure and function of Photosystem I (PSI) from Leptolyngbya ohadii, a desiccation-tolerant desert cyanobacterium. We discovered that PSI forms tetrameric (PSI-Tet) aggregate. We investigated it by using sucrose density gradient centrifugation, clear native PAGE, high performance liquid chromatography, mass spectrometry (MS), time-resolved fluorescence (TRF) and time-resolved transient absorption (TA) spectroscopy. MS analysis identified the presence of two PsaB and two PsaL proteins in PSI-Tet and uniquely revealed that PsaLs are N-terminally acetylated in contrast to non-modified PsaL in the trimeric PSI from Synechocystis sp. PCC 6803. Chlorophyll (Chl) a fluorescence decay profiles of the PSI-Tet performed at 77 K revealed two emission bands at ∼690 nm and 725 nm with the former appearing only at early delay time. The main fluorescence emission peak, associated with emission from the low energy Chls a, decays within a few nanoseconds. TA studies demonstrated that the 725 nm emission band is associated with low energy Chls a with absorption band clearly resolved at ∼710 nm at 77 K. In summary, our work suggests that the heterogenous composition of PsaBs and PsaL in PSI-Tet is related with the adaptation mechanisms needed to cope with stressful conditions under which this bacterium naturally grows.

A kaleidoscope of photosynthetic antenna proteins and their emerging roles

Photosynthetic light-harvesting antennae are pigment-binding proteins that perform one of the most fundamental tasks on Earth, capturing light and transferring energy that enables life in our biosphere. Adaptation to different light environments led to the evolution of an astonishing diversity of light-harvesting systems. At the same time, several strategies have been developed to optimize the light energy input into photosynthetic membranes in response to fluctuating conditions. The basic feature of these prompt responses is the dynamic nature of antenna complexes, whose function readily adapts to the light available. High-resolution microscopy and spectroscopic studies on membrane dynamics demonstrate the crosstalk between antennae and other thylakoid membrane components. With the increased understanding of light-harvesting mechanisms and their regulation, efforts are focusing on the development of sustainable processes for effective conversion of sunlight into functional bio-products. The major challenge in this approach lies in the application of fundamental discoveries in light-harvesting systems for the improvement of plant or algal photosynthesis. Here, we underline some of the latest fundamental discoveries on the molecular mechanisms and regulation of light harvesting that can potentially be exploited for the optimization of photosynthesis.

The use of R in photosynthesis research

R is one of the most commonly used analytical tools in the plant sciences. To identify key trends in general reported R use and patterns in photosynthesis research, we explored the frequency of R use in 2966 articles published in the 377 journals with 'photosynthesis' in the title from 2010 to 2019 using the Web of Science search. Solutions provided by each R package cited in the articles or online sources was recorded and classified. The percentage of research articles reporting R use increased linearly from 3.6% in 2010 to 12.5% in 2019. The three main categories of R package solutions were 'general statistical calculations and graph packages' (G); 'photosynthesis special-purpose packages' (S); and 'genetic and evolutionary packages' (E). The top five R packages cited were nlme (G), lme4 (G), multcomp (G), plantecophys (S), and ape (E). The increasing popularity of R use in photosynthesis research is due to its user-friendly and abundant open-source codes online for handling specific issues, particularly in fitting photosynthesis models. These findings are limited by the number of articles and online sources, but they reveal a significant increase in usage in photosynthesis research over the past decade and have a bright prospect in the future.

Reading and surviving the harsh conditions in desert biological soil crust: The cyanobacterial viewpoint

Biological soil crusts (BSCs) are found in drylands, cover ∼12% of the Earth's surface in arid and semi-arid lands and their destruction is considered an important promoter of desertification. These crusts are formed by the adhesion of soil particles to polysaccharides excreted mostly by filamentous cyanobacteria, which are the pioneers and main primary producers in BSCs. Desert BSCs survive in one of the harshest environments on Earth, and are exposed to daily fluctuations of extreme conditions. The cyanobacteria inhabiting these habitats must precisely read the changing conditions and predict, for example, the forthcoming desiccation. Moreover, they evolved a comprehensive regulation of multiple adaptation strategies to enhance their stress tolerance. Here we focus on what distinguishes cyanobacteria able to revive after dehydration from those that cannot. While important progress has been made in our understanding of physiological, biochemical and omics aspects, clarification of the sensing, signal transduction and responses enabling desiccation tolerance are just emerging. We plot the trajectory of current research and open questions ranging from general strategies and regulatory adaptations in the hydration/desiccation cycle, to recent advances in our understanding of photosynthetic adaptation. The acquired knowledge provides new insights to mitigate desertification and improve plant productivity under drought conditions.

Juggling Lightning: How Chlorella ohadii handles extreme energy inputs without damage

The green alga Chlorella ohadii was isolated from a desert biological soil crust, one of the harshest environments on Earth. When grown under optimal laboratory settings it shows the fastest growth rate ever reported for a photosynthetic eukaryote and a complete resistance to photodamage even under unnaturally high light intensities. Here we examined the energy distribution along the photosynthetic pathway under four light and carbon regimes. This was performed using various methodologies such as membrane inlet mass spectrometer with stable O₂ isotopes, variable fluorescence, electrochromic shift and fluorescence assessment of NADPH level, as well as the use of specific inhibitors. We show that the preceding illumination and CO₂ level during growth strongly affect the energy dissipation strategies employed by the cell. For example, plastid terminal oxidase (PTOX) plays an important role in energy dissipation, particularly in high light- and low-CO₂-grown cells. Of particular note is the reliance on PSII cyclic electron flow as an effective and flexible dissipation mechanism in all conditions tested. The energy management observed here may be unique to C. ohadii, as it is the only known organism to cope with such conditions. However, the strategies demonstrated may provide an insight into the processes necessary for photosynthesis under high-light conditions.

State transitions in cyanobacteria studied with picosecond fluorescence at room temperature

Cyanobacteria can rapidly regulate the relative activity of their photosynthetic complexes photosystem I and II (PSI and PSII) in response to changes in the illumination conditions. This process is known as state transitions. If PSI is preferentially excited, they go to state I whereas state II is induced either after preferential excitation of PSII or after dark adaptation. Different underlying mechanisms have been proposed in literature, in particular i) reversible shuttling of the external antenna complexes, the phycobilisomes, between PSI and PSII, ii) reversible spillover of excitation energy from PSII to PSI, iii) a combination of both and, iv) increased excited-state quenching of the PSII core in state II. Here we investigated wild-type and mutant strains of Synechococcus sp. PCC 7942 and Synechocystis sp. PCC 6803 using time-resolved fluorescence spectroscopy at room temperature. Our observations support model iv, meaning that increased excited-state quenching of the PSII core occurs in state II thereby balancing the photochemistry of photosystems I and II.