Chlororespiration protects the photosynthetic apparatus against photoinhibition by alleviating inhibition of photodamaged-PSII repair in Haematococcus pluvialis at the green motile stage

Integrated multilevel investigation of photosynthesis revealed the algal response distinction to differentially charged nanoplastics

Nanoplastics (NPs), especially those with different charges, as one of emerging contaminants pose a threat to aquatic ecosystems. Although differentially charged NPs could induce distinct biological effects, mechanistic understanding of the critical physiological processes of aquatic organisms from an integrated multilevel perspective on aquatic organisms is still uncertain. Herein, multi-effects of differentially charged nanosized polystyrene (nPS) including neutral nPS, nPS-COOH, and nPS-NH2 on the photosynthesis-related physiological processes of algae were explored at the population, individual, subcellular, protein, and transcriptional levels. Results demonstrated that both nPS and nPS-COOH exhibited hormesis to algal photosynthesis but nPS-NH2 triggered severe inhibition. As for nPS-NH2, the integrity of algal subcellular structure, chlorophyll biosynthesis, and expression of photosynthesis-related proteins and genes were interfered. Intracellular NPs' content in nPS treatment was 25.64 % higher than in nPS-COOH treatment, and the content of chloroplasts in PS and nPS-COOH treatment were 3.09 % and 4.56 % higher than control, respectively. Furthermore, at the molecular levels, more photosynthesis-related proteins and genes were regulated under nPS-COOH exposure than those exposed to nPS. Light-harvesting complex II could be recognized as an underlying explanation for different effects between nPS and nPS-COOH. This study first provides a novel approach to assess the ecological risks of NPs at an integrated multilevel.

Investigation of the toxicity of Cr (VI) against cyanobacteria and the mechanism of tolerance of the cyanobacterial consortia: a quantum mechanical approach

Hexavalent chromium (Cr (VI)) is a heavy metal that is distributed globally and poses a significant threat to the environment through various mechanisms. It can react with soil and water, leading to severe environmental damage. In this study, the toxicity of Cr (VI) was investigated by analyzing two major cyanobacteria species, Nostoc commune and Anabaena variabilis, commonly found in soil along with their consortia. The findings revealed that the toxicity mechanisms of Cr (VI) differed in individual monocultures, with Cr (VI) competing with various components. However, when the cyanobacteria species were combined, i.e., in consortia, they demonstrated an impressive retention of their functioning even in Cr (VI) concentration at 10 ppm. The study also concluded that non-photochemical quenching played a critical role in minimizing Cr (VI) toxicity. Furthermore, the research examined the role of the S-cycle in the process. The quantum yield of electron flux revealed that the Cr (VI) was competing with Qa in A. variabilis and with Qb in N. commune, albeit the photosystem dysfunction is only visible in the latter. The mechanism seemed to be quantum tunneling alteration because of the Cr (VI) having different energized quantum wells. The consortia proved to be behaving in a better manner as compared to the control. Overall, this study reveals the mode of toxicity of Cr (VI) in these two important cyanobacterial strains as well as it also discusses the mechanism of tolerance of consortia against Cr (VI) toxicity.

Loss of plastid ndh genes in an autotrophic desert plant

Plant plastid genomes are highly conserved with most flowering plants having the same complement of essential plastid genes. Here, we report the loss of five of the eleven NADH dehydrogenase subunit genes (ndh) in the plastid of a desert plant jojoba (Simmondsia chinensis). The plastid genome of jojoba was 156,496 bp with one large single copy region (LSC), a very small single copy region (SSC) and two expanded inverted repeats (IRA + IRB). The NADH dehydrogenase (NDH) complex is comprised of several protein subunits, encoded by the ndh genes of the plastome and the nucleus. The ndh genes are critical to the proper functioning of the photosynthetic electron transport chain and protection of plants from oxidative stress. Most plants are known to contain all eleven ndh genes. Plants with missing or defective ndh genes are often heterotrophs either due to their complete or holo- or myco- parasitic nature. Plants with a defective NDH complex, caused by the deletion/pseudogenisation of some or all the ndh genes, survive in milder climates suggesting the likely extinction of plant lineages lacking these genes under harsh climates. Interestingly, some autotrophic plants do exist without ndh gene/s and can cope with high or low light. This implies that these plants are protected from oxidative stress by mechanisms excluding ndh genes. Jojoba has evolved mechanisms to cope with a non-functioning NDH complex and survives in extreme desert conditions with abundant sunlight and limited water.

The use of silicon and mycorrhizal fungi to mitigate changes in licorice leaf micromorphology, chlorophyll fluorescence, and rutin content under water-deficit conditions

In this study, the effects of water-deficit conditions, silicon (Si) fertilizer (300 ppm), and arbuscular mycorrhizal (AM) inoculation by Claroiedoglomus etunicatum were evaluated on several features of licorice (Glycyrrhiza glabra L.). The measurable features were photosynthetic parameters, rutin content in aerial parts, and leaf micromorphology. Drought was administered at five levels determined by the percentage of field capacity (FC), i.e. 100, 80, 60, 40, and 20% of FC. Leaf extracts were utilized for measuring rutin content (via HPLC), and photosynthetic pigments; measurement of stomatal density, and trichome analysis were performed by scanning electron microscopy (SEM). Under severe drought stress, leaf area decreased by 50.84%, compared to well-irrigated plants. A significant decrease in leaf numbers (32.52%) was observed because of deficit irrigation. AM and Si improved chlorophyll fluorescence, which corresponded to the maximum efficiency of photosystem II. Rutin content decreased significantly under deficit irrigation. Also, the integration of AM and Si treatments positively affected rutin quantity under various irrigation regimes. Under moderate stress (60% FC), using AM and/or Si treatments reduced the stomatal length by 61.22 and 52.98%, respectively. Interestingly, a significant reduction in stomatal density towards control was observed as a result of the integrated treatments of Si and AM (58.28% at W₂₀ and 59.82% at W₁₀₀), which helped plants reduce water loss when facing drought stress. Principal component analysis (PCA) showed that photosynthetic pigments, chlorophyll fluorescence, and rutin changed quantitatively under moderate drought stress, while more variations were observed in leaf epidermal micromorphology under severe drought stress. These findings revealed that Si and AM, by exogenous application, synergistically mitigated the effects of drought stress on licorice.

Nanodiamond Particles Reduce Oxidative Stress Induced by Methyl Viologen and High Light in the Green Alga Chlamydomonas reinhardtii

Widely used in biomedical and bioanalytical applications, the detonation nanodiamonds (NDs) are generally considered to be biocompatible and non-toxic to a wide range of eukaryotic cells. Due to their high susceptibility to chemical modifications, surface functionalisation is often used to tune the biocompatibility and antioxidant activity of the NDs. The response of photosynthetic microorganisms to redox-active NDs is still poorly understood and is the focus of the present study. The green microalga Chlamydomonas reinhardtii was used to assess the potential phytotoxicity and antioxidant activity of NDs hosting hydroxyl functional groups at concentrations of 5–80 μg NDs/mL. The photosynthetic capacity of microalgae was assessed by measuring the maximum quantum yield of PSII photochemistry and the light-saturated oxygen evolution rate, while oxidative stress was assessed by lipid peroxidation and ferric-reducing antioxidant capacity. We demonstrated that hydroxylated NDs might reduce cellular levels of oxidative stress, protect PSII photochemistry and facilitate the PSII repair under methyl viologen and high light associated stress conditions. Factors involved in this protection may include the low phytotoxicity of hydroxylated NDs in microalgae and their ability to accumulate in cells and scavenge reactive oxygen species. Our findings could pave the way for using hydroxylated NDs as antioxidants to improve cellular stability in algae-based biotechnological applications or semi-artificial photosynthetic systems.

Single-walled carbon nanotubes protect photosynthetic reactions in Chlamydomonas reinhardtii against photoinhibition

Single-walled carbon nanotubes (SWCNTs) are among the most exploited carbon allotropes in nanosensing, bioengineering, and photobiological applications, however, the interactions of nanotubes with the photosynthetic process and structures are still poorly understood. We found that SWCNTs are not toxic to the photosynthetic apparatus of the model unicellular alga Chlamydomonas reinhardtii and demonstrate that this carbon nanomaterial can protect algal photosynthesis against photoinhibition. The results show that the inherent phytotoxicity of the nanotubes may be overcome by an intentional selection of nanomaterial characteristics. A low concentration (2 μg mL-1) of well-dispersed, purified and small SWCNTs did not alter the growth and pigment accumulation of the cultures. Indeed, under the photoinhibitory conditions of our experiments, SWCNT-enriched samples were characterized by a lower rate of PSII inactivation, reduced excitation pressure in PSII, a higher rate of photosynthetic electron transport, and an increased non-photochemical quenching in comparison with the controls. In addition, SWCNTs change the distribution of energy between the photosystems in favour of PSII (state 1). The underlying mechanism of this action is not yet understood but possibly, electrons or energy can be exchanged between the redox active nanotubes and photosynthetic components, and probably other redox active intra-chloroplast constituents. Alternatively, nanotubes may promote the formation of an NPQ conformation of PSII. Our results provided evidence for such electron/energy transfer from photosynthetic structures toward the nanotubes. The discovered photoprotective effects can potentially be used in photobiotechnology to maintain the photosynthetic activity of microorganisms under unfavourable conditions.

Photoprotective carbon redistribution in mixotrophic Haematococcus pluvialis under high light stress

Mixotrophy of Haematococcus pluvialis is a potential strategy for producing astaxanthin. However, this strategy has not been extensively commercialized because the mixotrophic mechanisms by which H. pluvialis overcomes high light stress are unclear. This study analyzed the biochemical compositions and differential proteomics of mixotrophic H. pluvialis under different light conditions. High light exposure substantially increased astaxanthin, carbohydrate, and fatty acid contents. A total of 119 and 81 proteins were significantly up- and down-regulated after two days of high light exposure. These proteins mainly enriched pathways for photosynthetic metabolism, glyoxylate cycle, and biosynthesis of secondary metabolites. This study proposed a regulatory model through which mixotrophic H. pluvialis copes with high light stress. The model includes pathways for modulating photosynthetic apparatus, increasing astaxanthin accumulation by enhancing photorespiration, pentose phosphate and Embden-Meyerhof-Parna pathways, while thickening the cell wall by malate-oxaloacetate shuttle.

Characterization of the Wave Phenomenon in Flash-Induced Fluorescence Relaxation and Its Application to Study Cyclic Electron Pathways in Microalgae

Photosynthesis is a series of redox reactions, in which several electron transport processes operate to provide the energetic balance of light harvesting. In addition to linear electron flow, which ensures the basic functions of photosynthetic productivity and carbon fixation, alternative electron transport pathways operate, such as the cyclic electron flow (CEF), which play a role in the fine tuning of photosynthesis and balancing the ATP/NADPH ratio under stress conditions. In this work, we characterized the electron transport processes in microalgae species that have high relevance in applied research and industry (e.g., Chlorella sorokiniana, Haematococcus pluvialis, Dunaliella salina, Nannochloropsis sp.) by using flash-induced fluorescence relaxation kinetics. We found that a wave phenomenon appeared in the fluorescence relaxation profiles of microalgae to different extents; it was remarkable in the red cells of H. pluvialis, D. salina and C. sorokiniana, but it was absent in green cells of H. pluvialis and N. limnetica. Furthermore, in microalgae, unlike in cyanobacteria, the appearance of the wave required the partial decrease in the activity of Photosystem II, because the relatively high Photosystem II/Photosystem I ratio in microalgae prevented the enhanced oxidation of the plastoquinone pool. The wave phenomenon was shown to be related to the antimycin A-sensitive pathway of CEF in C. sorokiniana but not in other species. Therefore, the fluorescence wave phenomenon appears to be a species-specific indicator of the redox reactions of the plastoquinone pool and certain pathways of cyclic electron flow.

On the Edge of Dispensability, the Chloroplast ndh Genes

The polypeptides encoded by the chloroplast ndh genes and some nuclear genes form the thylakoid NADH dehydrogenase (Ndh) complex, homologous to the mitochondrial complex I. Except for Charophyceae (algae related to higher plants) and a few Prasinophyceae, all eukaryotic algae lack ndh genes. Among vascular plants, the ndh genes are absent in epiphytic and in some species scattered among different genera, families, and orders. The recent identification of many plants lacking plastid ndh genes allows comparison on phylogenetic trees and functional investigations of the ndh genes. The ndh genes protect Angiosperms under various terrestrial stresses, maintaining efficient photosynthesis. On the edge of dispensability, ndh genes provide a test for the natural selection of photosynthesis-related genes in evolution. Variable evolutionary environments place Angiosperms without ndh genes at risk of extinction and, probably, most extant ones may have lost ndh genes recently. Therefore, they are evolutionary endpoints in phylogenetic trees. The low number of sequenced plastid DNA and the long lifespan of some Gymnosperms lacking ndh genes challenge models about the role of ndh genes protecting against stress and promoting leaf senescence. Additional DNA sequencing in Gymnosperms and investigations into the molecular mechanisms of their response to stress will provide a unified model of the evolutionary and functional consequences of the lack of ndh genes.