Ecotoxicological risk assessment on coagulation-flocculation in water/wastewater treatment: a systematic review

It is undeniable that removal efficiency is the main factor in coagulation-flocculation (C-F) process for wastewater treatment. However, as far as environmental safety is concerned, the ecotoxicological aspect of the C-F process needs to be examined further. In this study, a systematic review was performed based on publications related to the toxicity research in C-F technology for wastewater treatment. Through a series of screening steps, available toxicity studies were categorized into four themes, namely acute toxicity, phytotoxicity, cytotoxicity, and genotoxicity, which comprised 48 articles. A compilation of the methodologies executed for each theme was also outlined. The findings show that conventional metallic coagulants (e.g., alum, iron chloride, and iron sulfate) were less toxic when tested on test species such as Daphnia magna (water flea), Lattuca sativa (lettuce), and animal cells compared to synthetic polymers. Natural coagulants such as chitosan or Moringa oleifera were less toxic compared to metallic coagulants; however, inconsistent results were observed. Moreover, an advanced C-F (electrocoagulation) as well as integration between C-F and Fenton, adsorption, and photocatalytic does not significantly change the toxicological profile of the system. It was found that diverse coagulants and flocculants, species sensitivity, complexity in toxicity testing, and dynamic environmental conditions were some key challenges faced in this field. Finally, it was expected that advances in technology, interdisciplinary collaboration, and a growing awareness of environmental sustainability will drive efforts to develop more effective and eco-friendly coagulants and flocculants, improve toxicity testing methodologies, and enhance the overall efficiency and safety of water and wastewater treatment processes.

Responding to light signals: a comprehensive update on photomorphogenesis in cyanobacteria

Cyanobacteria are ancestors of chloroplast and perform oxygen-evolving photosynthesis similar to higher plants and algae. However, an obligatory requirement of photons for their growth results in the exposure of cyanobacteria to varying light conditions. Therefore, the light environment could act as a signal to drive the developmental processes, in addition to photosynthesis, in cyanobacteria. These Gram-negative prokaryotes exhibit characteristic light-dependent developmental processes that maximize their fitness and resource utilization. The development occurring in response to radiance (photomorphogenesis) involves fine-tuning cellular physiology, morphology and metabolism. The best-studied example of cyanobacterial photomorphogenesis is chromatic acclimation (CA), which allows a selected number of cyanobacteria to tailor their light-harvesting antenna called phycobilisome (PBS). The tailoring of PBS under existing wavelengths and abundance of light gives an advantage to cyanobacteria over another photoautotroph. In this work, we will provide a comprehensive update on light-sensing, molecular signaling and signal cascades found in cyanobacteria. We also include recent developments made in other aspects of CA, such as mechanistic insights into changes in the size and shape of cells, filaments and carboxysomes.

Biocrude Production Using a Novel Cyanobacterium: Pilot-Scale Cultivation and Lipid Extraction via Hydrothermal Liquefaction

The use of renewable energy to reduce fossil fuel consumption is a key strategy to mitigate pollution and climate change, resulting in the growing demand for new sources. Fast-growing proprietary cyanobacterial strains of Fremyella diplosiphon with an average life cycle of 7-10 days, and a proven capacity to generate lipids for biofuel production are currently being studied. In this study, we investigated the growth and photosynthetic pigmentation of a cyanobacterial strain (SF33) in both greenhouse and outdoor bioreactors, and produced biocrude via hydrothermal liquefaction. The cultivation of F. diplosiphon did not significantly differ under suboptimal conditions (p < 0.05), including in outdoor bioreactors with growth differences of less than 0.04 (p = 0.035) among various batches. An analysis of the biocrude's components revealed the presence of fatty acid biodiesel precursors such as palmitic acid and behenic acid, and alkanes such as hexadecane and heptadecane, used as biofuel additives. In addition, the quantification of value-added photosynthetic pigments revealed chlorophyll a and phycocyanin concentrations of 0.0011 ± 5.83 × 10^-5 μg/μL and 7.051 ± 0.067 μg/μg chlorophyll a. Our results suggest the potential of F. diplosiphon as a robust species that can grow at varying temperatures ranging from 13 °C to 32 °C, while producing compounds for applications ranging from biofuel to nutritional supplements. The outcomes of this study pave the way for production-level scale-up and processing of F. diplosiphon-derived biofuels and marketable bioproducts. Fuel produced using this technology will be eco-friendly and cost-effective, and will make full use of the geographical location of regions with access to brackish waters.

Reflections on Cyanobacterial Chromatic Acclimation: Exploring the Molecular Bases of Organismal Acclimation and Motivation for Rethinking the Promotion of Equity in STEM

Cyanobacteria are photosynthetic organisms that exhibit characteristic acclimation and developmental responses to dynamic changes in the external light environment. Photomorphogenesis is the tuning of cellular physiology, development, morphology, and metabolism in response to external light cues. The tuning of photosynthetic pigmentation, carbon fixation capacity, and cellular and filament morphologies to changes in the prevalent wavelengths and abundance of light have been investigated to understand the regulation and fitness implications of different aspects of cyanobacterial photomorphogenesis. Chromatic acclimation (CA) is the most common form of photomorphogenesis that has been explored in cyanobacteria. Multiple types of CA in cyanobacteria have been reported, and insights gained into the regulatory pathways and networks controlling some of these CA types. I examine the recent expansion of CA types that occur in nature and provide an overview of known regulatory factors involved in distinct aspects of cyanobacterial photomorphogenesis. Additionally, I explore lessons for cultivating success in scientific communities that can be drawn from a reflection on existing knowledge of and approaches to studying CA.

Effects of zinc and iron on the abundance of Microcystis in Lake Taihu under green light and turbulence conditions

Trace element is one of the important factors affecting the growth of Microcystis. The effects of zinc (0.4 mg/L) and iron (2 mg/L) on the abundance of Microcystis in Lake Taihu were investigated under continuous turbulence and green light conditions in a microcosm experiment. The study results showed that the abundance of Microcystis in the zinc treatment and the iron treatment group was 8.30% and 214% of that in the control group at the end of the experiment, respectively. The proportion of Cyanobacteria in the total phytoplankton biomass in the control, iron treatment, and zinc treatment group decreased from 99.99% at the beginning of the experiment to 13%, 18%, and 1% at the end of the experiment, respectively. At the end of the microcosm experiment, the phytoplankton community was dominated by Bacillariophyta in the control group, accounting for 63%, but it was dominated by Chlorophyta in the zinc treatment and the iron treatment group, accounting for 89% and 42%, respectively. The study results showed that under green light and turbulence, 0.4 mg/L zinc remarkably decreased the abundance of Microcystis, but 2 mg/L iron effectively increased the number of Microcystis and other algae. This research results provided a new idea for controlling Microcystis blooms.

Zero-Valent Iron Nanoparticles Induce Reactive Oxygen Species in the Cyanobacterium, Fremyella diplosiphon

Nanoscale zero-valent iron nanoparticles (nZVIs) are known to boost biomass production and lipid yield in Fremyella diplosiphon, a model biodiesel-producing cyanobacterium. However, the impact of nZVI-induced reactive oxygen species (ROS) in F. diplosiphon has not been evaluated. In the present study, ROS in F. diplosiphon strains (B481-WT and B481-SD) generated in response to nZVI-induced oxidative stress were quantified and the enzymatic response determined. Lipid peroxidation as a measure of oxidative stress revealed significantly higher malondialdehyde content (p < 0.01) in both strains treated with 3.2, 12.8, and 51.2 mg L-1 nZVIs compared to untreated control. In addition, ROS in all nZVI-treated cultures treated with 1.6-25.6 mg L-1 nZVIs was significantly higher than the untreated control as determined by the 2',7'-dichlorodihydrofluorescein diacetate fluorometric probe. Immunodetection using densitometric analysis of iron superoxide dismutase (SOD) revealed significantly higher SOD levels in both strains treated with nZVIs at 51.2 mg L-1. In addition, we observed significantly higher (p < 0.001) SOD levels in the B481-SD strain treated with 6.4 mg L-1 nZVIs compared to 3.2 mg L-1 nZVIs. Validation using transmission electron microscopy equipped with energy-dispersive X-ray spectroscopy (EDS) revealed adsorption of nZVIs with a strong iron peak in both B481-WT and B481-SD strains. While the EDS spectra showed strong signals for iron at 4 and 12 days after treatment, a significant decrease in peak intensity was observed at 20 days. Future efforts will be aimed at studying transduction mechanisms that cause metabolic and epigenetic alterations in response to nZVIs in F. diplosiphon.

Oxidative stress measurement in different morphological forms of wild-type and mutant cyanobacterial strains: Overcoming the limitation of fluorescence microscope-based method

Monitoring of oxidative stress caused by a wide range of reactive oxygen species (ROS) is essential to have an idea about the fitness and growth of photosynthetic organisms. The imaging-based oxidative stress measurement in cyanobacteria using 2',7'-dichlorodihydrofluorescein diacetate (DCFH-DA) dye has the limitation of small sample size as the only selected number of cells are analyzed to measure the ROS levels. Here, we developed a method for oxidative stress measurement by DCFH-DA and flow cytometer (FCM) using unicellular Synechococcus elongatus PCC 7942 and filamentous Fremyella diplosiphon BK14 cyanobacteria. F. diplosiphon BK14 inherently possess high levels of ROS and showed higher sensitivity to hydrogen peroxide treatment in comparison to S. elongatus PCC 7942. We successfully measured oxidative stress in glutaredoxin lacking strain (Δgrx3) of S. elongatus PCC 7942, and wild-type Synechocystis sp. PCC 6803 using FCM based method. Importantly, ROS were not detected in these two strains of cyanobacteria by fluorescence microscope-based method due to their small spherical morphology. Δgrx3 strain showed high ROS levels in comparison to its wild-type strain. Treatment of abiotic factors such as high PAR in wild-type and Δgrx3 strains of S. elongatus PCC 7942, low PAR or low PAR + UVR in wild-type S. elongatus PCC 7942, and high PAR or high PAR + NaCl in Synechocystis sp. PCC 6803 increased oxidative stress. In summary, the FCM based method can measure ROS levels produced due to physiological conditions associated with genetic changes or abiotic stress in a large population of cells regardless of their morphology. Therefore, the present study shows the usefulness of the method in monitoring the health of organisms in a large scale cultivation system.

Impact of Zero-Valent Iron Nanoparticles on Fremyella diplosiphon Transesterified Lipids and Fatty Acid Methyl Esters

Efforts to enhance the transformative potential of biofuels is an important step to achieving an environment-friendly and sustainable energy source. Fremyella diplosiphon is an ideal third-generation biofuel agent due to its ability to produce lipids and desirable essential fatty acids. In this study, the impact of Nanofer 25s nanoscale zero-valent iron nanoparticles (nZVIs) on total lipid content and fatty acid composition of F. diplosiphon strains SF33 and B481 was investigated. We observed significant increases (P < 0.05) in the growth of F. diplosiphon treated with 0.2-1.6 mg L^-1 Nanofer 25s, indicating that trace concentrations of nZVIs were not toxic to the organism. Chlorophyll a, carotenoids, and phycobiliprotein levels were not altered in F. diplosiphon treated with nZVIs ranging from 0.4 to 1.6 mg L^-1, confirming that these concentrations did not negatively impact photosynthetic efficacy. In addition, Nanofer 25s ranging from 0.2 to 1.6 mg L^-1 had an optimal impact on SF33 and B481 total lipid content. We identified significant increases in unsaturated fatty acid methyl esters (FAMEs) from F. diplosiphon Nanofer 25s-treated transesterified lipids. Theoretical chemical and physical biofuel properties revealed a product with elevated cetane number and oxidative stability for both strains. Scanning electron microscopy and energy-dispersive X-ray spectroscopy validated the localization of nZVIs. Our findings indicate that Nanofer 25s nZVIs significantly enhance F. diplosiphon total lipid content and essential FAMEs, thus offering a promising approach to augment the potential of the cyanobacterium as a large-scale biofuel agent.

Chromatic Acclimation in Cyanobacteria: A Diverse and Widespread Process for Optimizing Photosynthesis

Chromatic acclimation (CA) encompasses a diverse set of molecular processes that involve the ability of cyanobacterial cells to sense ambient light colors and use this information to optimize photosynthetic light harvesting. The six known types of CA, which we propose naming CA1 through CA6, use a range of molecular mechanisms that likely evolved independently in distantly related lineages of the Cyanobacteria phylum. Together, these processes sense and respond to the majority of the photosynthetically relevant solar spectrum, suggesting that CA provides fitness advantages across a broad range of light color niches. The recent discoveries of several new CA types suggest that additional CA systems involving additional light colors and molecular mechanisms will be revealed in coming years. Here we provide a comprehensive overview of the currently known types of CA and summarize the molecular details that underpin CA regulation.

Diverse light responses of cyanobacteria mediated by phytochrome superfamily photoreceptors

Cyanobacteria are an evolutionarily and ecologically important group of prokaryotes. They exist in diverse habitats, ranging from hot springs and deserts to glaciers and the open ocean. The range of environments that they inhabit can be attributed in part to their ability to sense and respond to changing environmental conditions. As photosynthetic organisms, one of the most crucial parameters for cyanobacteria to monitor is light. Cyanobacteria can sense various wavelengths of light and many possess a range of bilin-binding photoreceptors belonging to the phytochrome superfamily. Vital cellular processes including growth, phototaxis, cell aggregation and photosynthesis are tuned to environmental light conditions by these photoreceptors. In this Review, we examine the physiological responses that are controlled by members of this diverse family of photoreceptors and discuss the signal transduction pathways through which these photoreceptors operate. We highlight specific examples where the activities of multiple photoreceptors function together to fine-tune light responses. We also discuss the potential application of these photosensing systems in optogenetics and synthetic biology.

Structural and spectroscopic characterization of HCP2

The Helical Carotenoid Proteins (HCPs) are a large group of newly identified carotenoid-binding proteins found in ecophysiologically diverse cyanobacteria. They likely evolved before becoming the effector (quenching) domain of the modular Orange Carotenoid Protein (OCP). The number of discrete HCP families-at least nine-suggests they are involved in multiple distinct functions. Here we report the 1.7 Å crystal structure of HCP2, one of the most widespread HCPs found in nature, from the chromatically acclimating cyanobacterium Tolypothrix sp. PCC 7601. By purifying HCP2 from the native source we are able to identify its natively-bound carotenoid, which is exclusively canthaxanthin. In solution, HCP2 is a monomer with an absorbance maximum of 530 nm. However, the HCP2 crystals have a maximum absorbance at 548 nm, which is accounted by the stacking of the β1 rings of the carotenoid in the two molecules in the asymmetric unit. Our results demonstrate how HCPs provide a valuable system to study carotenoid-protein interactions and their spectroscopic implications, and contribute to efforts to understand the functional roles of this large, newly discovered family of pigment proteins, which to-date remain enigmatic.

Morphophysiological and transcriptome analysis reveals a multiline defense system enabling cyanobacterium Leptolyngbya strain JSC-1 to withstand iron induced oxidative stress

Iron intoxications induce severe oxidative stress by producing reactive oxygen species (ROS) in cyanobacteria, leading to membrane lipid peroxidation, altered morphology, impaired photosynthesis and other oxidative stress injuries. Given these stresses, mitigation of ROS is a prerequisite for all aerobic organisms. Study of siderophilic cyanobacterium Leptolyngbya strain JSC-1 inhabiting iron-rich hot springs may provide insight into the mechanism of iron homeostasis and alleviation of oxidative stress. In this study, we investigated the morphophysiological and molecular mechanisms enabling this cyanobacterium to cope with iron-induced oxidative stress. Strain JSC-1 biomineralized extracellular iron via an exopolymeric sheath (acting as a first line of defense) and intracellular iron via polyphosphate inclusions (second line of defense), thus minimizing the burden of free ferric ions. Physiological parameters, SOD, CAT and POD activities, bacterioferritin and total protein contents fluctuated in response to iron elevation, displaying a third line of defense to mitigate ROS. Differential gene expression analysis of JSC-1 indicated up-regulation of 94 and 125 genes and down-regulation of 89 and 183 genes at low (4 μM) and high (400 μM) iron concentration, respectively. The differentially expressed genes (DEGs) were enriched in 100 KEGG pathways and were found to be involved in lipopolysaccharide and fatty acid biosynthesis, starch, sucrose, chlorophyll and other metabolic pathways. Together with metabolic reprogramming (fourth line of defense), JSC-1 established a unique multiline defense system that allows JSC-1 to withstand severe oxidative stress. These findings also provide insight into potential survival strategies of ancient microorganisms inhabiting similar environment present in early earth history.

RcaE-Dependent Regulation of Carboxysome Structural Proteins Has a Central Role in Environmental Determination of Carboxysome Morphology and Abundance in Fremyella diplosiphon

Carboxysomes are central to the carbon dioxide-concentrating mechanism (CCM) and carbon fixation in cyanobacteria. Although the structure is well understood, roles of environmental cues in the synthesis, positioning, and functional tuning of carboxysomes have not been systematically studied. Fremyella diplosiphon is a model cyanobacterium for assessing impacts of environmental light cues on photosynthetic pigmentation and tuning of photosynthetic efficiency during complementary chromatic acclimation (CCA), which is controlled by the photoreceptor RcaE. Given the central role of carboxysomes in photosynthesis, we investigated roles of light-dependent RcaE signaling in carboxysome structure and function. A ΔrcaE mutant exhibits altered carboxysome size and number, ccm gene expression, and carboxysome protein accumulation relative to the wild-type (WT) strain. Several Ccm proteins, including carboxysome shell proteins and core-nucleating factors, overaccumulate in ΔrcaE cells relative to WT cells. Additionally, levels of carboxysome cargo RuBisCO in the ΔrcaE mutant are lower than or unchanged from those in the WT strain. This shift in the ratios of carboxysome shell and nucleating components to the carboxysome cargo appears to drive carboxysome morphology and abundance dynamics. Carboxysomes are also occasionally mislocalized spatially to the periphery of spherical mutants within thylakoid membranes, suggesting that carboxysome positioning is impacted by cell shape. The RcaE photoreceptor links perception of external light cues to regulating carboxysome structure and function and, thus, to the cellular capacity for carbon fixation. IMPORTANCE Carboxysomes are proteinaceous subcellular compartments, or bacterial organelles, found in cyanobacteria that consist of a protein shell surrounding a core primarily composed of the enzyme ribulose-1,5-biphosphate carboxylase/oxygenase (RuBisCO) that is central to the carbon dioxide-concentrating mechanism (CCM) and carbon fixation. Whereas significant insights have been gained regarding the structure and synthesis of carboxysomes, limited attention has been given to how their size, abundance, and protein composition are regulated to ensure optimal carbon fixation in dynamic environments. Given the centrality of carboxysomes in photosynthesis, we provide an analysis of the role of a photoreceptor, RcaE, which functions in matching photosynthetic pigmentation to the external environment during complementary chromatic acclimation and thereby optimizing photosynthetic efficiency, in regulating carboxysome dynamics. Our data highlight a role for RcaE in perceiving external light cues and regulating carboxysome structure and function and, thus, in the cellular capacity for carbon fixation and organismal fitness.

Additional families of orange carotenoid proteins in the photoprotective system of cyanobacteria

The orange carotenoid protein (OCP) is a structurally and functionally modular photoactive protein involved in cyanobacterial photoprotection. Using phylogenomic analysis, we have revealed two new paralogous OCP families, each distributed among taxonomically diverse cyanobacterial genomes. Based on bioinformatic properties and phylogenetic relationships, we named the new families OCP2 and OCPx to distinguish them from the canonical OCP that has been well characterized in Synechocystis, denoted hereafter as OCP1. We report the first characterization of a carotenoprotein photoprotective system in the chromatically acclimating cyanobacterium Tolypothrix sp. PCC 7601, which encodes both OCP1 and OCP2 as well as the regulatory fluorescence recovery protein (FRP). OCP2 expression could only be detected in cultures grown under high irradiance, surpassing expression levels of OCP1, which appears to be constitutive; under low irradiance, OCP2 expression was only detectable in a Tolypothrix mutant lacking the RcaE photoreceptor required for complementary chromatic acclimation. In vitro studies show that Tolypothrix OCP1 is functionally equivalent to Synechocystis OCP1, including its regulation by Tolypothrix FRP, which we show is structurally similar to the dimeric form of Synechocystis FRP. In contrast, Tolypothrix OCP2 shows both faster photoconversion and faster back-conversion, lack of regulation by the FRP, a different oligomeric state (monomer compared to dimer for OCP1) and lower fluorescence quenching of the phycobilisome. Collectively, these findings support our hypothesis that the OCP2 is relatively primitive. The OCP2 is transcriptionally regulated and may have evolved to respond to distinct photoprotective needs under particular environmental conditions such as high irradiance of a particular light quality, whereas the OCP1 is constitutively expressed and is regulated at the post-translational level by FRP and/or oligomerization.

Mechanisms and fitness implications of photomorphogenesis during chromatic acclimation in cyanobacteria

Photosynthetic organisms absorb photons and convert light energy to chemical energy through the process of photosynthesis. Photosynthetic efficiency is tuned in response to the availability of light, carbon dioxide and nutrients to promote maximal levels of carbon fixation, while simultaneously limiting the potential for light-associated damage or phototoxicity. Given the central dependence on light for energy production, photosynthetic organisms possess abilities to tune their growth, development and metabolism to external light cues in the process of photomorphogenesis. Photosynthetic organisms perceive light intensity and distinct wavelengths or colors of light to promote organismal acclimation. Cyanobacteria are oxygenic photosynthetic prokaryotes that exhibit abilities to alter specific aspects of growth, including photosynthetic pigment composition and morphology, in responses to changes in available wavelengths and intensity of light. This form of photomorphogenesis is known as chromatic acclimation and has been widely studied. Recent insights into the photosensory photoreceptors found in cyanobacteria and developments in our understanding of the molecular mechanisms initiated by light sensing to affect the changes characteristic of chromatic acclimation are discussed. I consider cyanobacterial responses to light, the broad diversity of photoreceptors encoded by these organisms, specific mechanisms of photomorphogenesis, and associated fitness implications in chromatically acclimating cyanobacteria.

Interrelated modules in cyanobacterial photosynthesis: the carbon-concentrating mechanism, photorespiration, and light perception

Here we consider the cyanobacterial carbon-concentrating mechanism (CCM) and photorespiration in the context of the regulation of light harvesting, using a conceptual framework borrowed from engineering: modularity. Broadly speaking, biological 'modules' are semi-autonomous functional units such as protein domains, operons, metabolic pathways, and (sub)cellular compartments. They are increasingly recognized as units of both evolution and engineering. Modules may be connected by metabolites, such as NADPH, ATP, and 2PG. While the Calvin-Benson-Bassham Cycle and photorespiratory salvage pathways can be considered as metabolic modules, the carboxysome, the core of the cyanobacterial CCM, is both a structural and a metabolic module. In photosynthetic organisms, which use light cues to adapt to the external environment and which tune the photosystems to provide the ATP and reducing power for carbon fixation, light-regulated modules are critical. The primary enzyme of carbon fixation, RuBisCO, uses CO2 as a substrate, which is accumulated via the CCM. However RuBisCO also has a secondary reaction in which it utilizes O2, a by-product of the photochemical modules, which leads to photorespiration. A complete understanding of the interplay among CCM and photorespiration is predicated on uncovering their connections to the light reactions and the regulatory factors and pathways that tune these modules to external cues. We probe this connection by investigating light inputs into the CCM and photorespiratory pathways in the chromatically acclimating cyanobacterium Fremyella diplosiphon.

Pivotal Role of Iron in the Regulation of Cyanobacterial Electron Transport

Iron-containing metalloproteins are the main cornerstones for efficient electron transport in biological systems. The abundance and diversity of iron-dependent proteins in cyanobacteria makes those organisms highly dependent of this micronutrient. To cope with iron imbalance, cyanobacteria have developed a survey of adaptation strategies that are strongly related to the regulation of photosynthesis, nitrogen metabolism and other central electron transfer pathways. Furthermore, either in its ferrous form or as a component of the haem group, iron plays a crucial role as regulatory signalling molecule that directly or indirectly modulates the composition and efficiency of cyanobacterial redox reactions. We present here the major mechanism used by cyanobacteria to couple iron homeostasis to the regulation of electron transport, making special emphasis in processes specific in those organisms.

The Tryptophan-Rich Sensory Protein (TSPO) is Involved in Stress-Related and Light-Dependent Processes in the Cyanobacterium Fremyella diplosiphon

The tryptophan-rich sensory protein (TSPO) is a membrane protein, which is a member of the 18 kDa translocator protein/peripheral-type benzodiazepine receptor (MBR) family of proteins that is present in most organisms and is also referred to as Translocator protein 18 kDa. Although TSPO is associated with stress- and disease-related processes in organisms from bacteria to mammals, full elucidation of the functional role of the TSPO protein is lacking for most organisms in which it is found. In this study, we describe the regulation and function of a TSPO homolog in the cyanobacterium Fremyella diplosiphon, designated FdTSPO. Accumulation of the FdTSPO transcript is upregulated by green light and in response to nutrient deficiency and stress. A F. diplosiphon TSPO deletion mutant (i.e., ΔFdTSPO) showed altered responses compared to the wild type (WT) strain under stress conditions, including salt treatment, osmotic stress, and induced oxidative stress. Under salt stress, the FdTSPO transcript is upregulated and a ΔFdTSPO mutant accumulates lower levels of reactive oxygen species (ROS) and displays increased growth compared to WT. In response to osmotic stress, FdTSPO transcript levels are upregulated and ΔFdTSPO mutant cells exhibit impaired growth compared to the WT. By comparison, methyl viologen-induced oxidative stress results in higher ROS levels in the ΔFdTSPO mutant compared to the WT strain. Taken together, our results provide support for the involvement of membrane-localized FdTSPO in mediating cellular responses to stress in F. diplosiphon and represent detailed functional analysis of a cyanobacterial TSPO. This study advances our understanding of the functional roles of TSPO homologs in vivo.

Interdependence of tetrapyrrole metabolism, the generation of oxidative stress and the mitigative oxidative stress response

Tetrapyrroles are involved in light harvesting and light perception, electron-transfer reactions, and as co-factors for key enzymes and sensory proteins. Under conditions in which cells exhibit stress-induced imbalances of photosynthetic reactions, or light absorption exceeds the ability of the cell to use photoexcitation energy in synthesis reactions, redox imbalance can occur in photosynthetic cells. Such conditions can lead to the generation of reactive oxygen species (ROS) associated with alterations in tetrapyrrole homeostasis. ROS accumulation can result in cellular damage and detrimental effects on organismal fitness, or ROS molecules can serve as signals to induce a protective or damage-mitigating oxidative stress signaling response in cells. Induced oxidative stress responses include tetrapyrrole-dependent and -independent mechanisms for mitigating ROS generation and/or accumulation. Thus, tetrapyrroles can be contributors to oxidative stress, but are also essential in the oxidative stress response to protect cells by contributing to detoxification of ROS. In this review, we highlight the interconnection and interdependence of tetrapyrrole metabolism with the occurrence of oxidative stress and protective oxidative stress signaling responses in photosynthetic organisms.

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Tetrapyrroles are involved in light sensing and oxidative stress mitigation.

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Reactive oxygen species (ROS) can form upon light exposure of free tetrapyrroles.

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Tetrapyrrole homeostasis must be tightly regulated to avoid oxidative stress.

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ROS can result in cellular damage or oxidative stress signaling in cells.