Redox Homeostasis Disclosed in the Saltmarsh Plant Halimione portulacoides upon Short Waterborne Exposure to Inorganic Mercury.

The saltmarsh plant Halimione portulacoides was shortly exposed to realistic levels of inorganic mercury (iHg) with the aim of investigating the adaptative processes of the roots and leaves regarding redox homeostasis, physiology, and Hg accumulation. Plants were collected at a contaminated (CONT) and a reference (REF) site to address the interference of contamination backgrounds. The influence of major abiotic variables (i.e., temperature and light) was also examined. Total Hg levels, antioxidant enzymes, lipid peroxidation (LPO), and photosynthetic activity were analyzed after 2 and 4 h of exposure. A poor accumulation of Hg in the roots was noticed, and no translocation to the stems and leaves was found, but plants from the CONT site seemed more prone to iHg uptake (in winter). Despite this, antioxidant modulation in the roots and leaves was found, disclosing, in winter, higher thresholds for the induction of enzymatic antioxidants in CONT leaves compared to REF plants, denoting that the former are better prepared to cope with iHg redox pressure. Consistently, CONT leaves exposed to iHg had remarkably lower LPO levels. Exposure did not impair photosynthetic activity, pinpointing H. portulacoides' ability to cope with iHg toxicity under very-short-term exposure. Biochemical changes were noticed before enhancements in accumulation, reinforcing the relevance of these responses in precociously signaling iHg toxicity.

The Photoprotective Behavior of a Motile Benthic Diatom as Elucidated from the Interplay Between Cell Motility and Physiological Responses to a Light Microgradient Using a Novel Experimental Setup.

It has long been hypothesized that benthic motile pennate diatoms use phototaxis to optimize photosynthesis and minimize photoinhibitory damage by adjusting their position within vertical light gradients in coastal benthic sediments. However, experimental evidence to test this hypothesis remains inconclusive, mainly due to methodological difficulties in studying cell behavior and photosynthesis over realistic spatial microscale gradients of irradiance and cell position. In this study, a novel experimental approach was developed and used to test the hypothesis of photosynthesis optimization through motility, based on the combination of single-cell in vivo chlorophyll fluorometry and microfluidic chips. The approach allows the concurrent study of behavior and photosynthetic activity of individual cells of the epipelic diatom species Craspedostauros britannicus exposed to a light microgradient of realistic dimensions, simulating the irradiance and distance scales of light microgradients in benthic sediments. Following exposure to light, (i) cells explored their light environment before initiating light-directed motility; (ii) cells used motility to lower their light dose, when exposed to the highest light intensities; and (iii) motility was combined with reversible non-photochemical quenching, to allow cells to avoid photoinhibition. The results of this proof-of-concept study not only strongly support the photoprotective nature of photobehavior in the studied species but also revealed considerable variability in how individual cells reacted to a light microgradient. The experimental setup can be readily applied to study motility and photosynthetic light responses of other diatom species or natural assemblages, as well as other photoautotrophic motile microorganisms, broadening the toolset for experimental microbial ecology research.

Photoinactivation vs repair of photosystem II as target of thermal stress in epipelic and epipsammic microphytobenthos communities.

Microphytobenthos (MPB) inhabiting intertidal flats are exposed to large and sudden changes in temperature, often simultaneously with exposure to direct sunlight. These conditions are expected to negatively impact photosynthesis by exacerbating the photoinhibition under high light. This study addressed the photoinhibitory effects of short-term exposure to cold (5°C) and moderate heat (35°C) on MPB dominated by motile epipelic (EPL) and immotile epipsammic (EPM) diatom species, by evaluating the seasonal variation of photoinactivation and repair of photosystem II (PSII). The susceptibility to PSII photoinactivation and the counteracting repair capacity were measured by the constant rates kPI and kREC, respectively. The photoacclimation state was characterized by hysteresis light-response curves (HLC) of the relative electron transport rate, rETR, and of the nonphotochemical quenching index Y(NPQ). Under non-stress conditions (20°C), kREC was on average almost 10x higher than the corresponding kPI (20.4 vs 2.70 × 10-4 s-1, respectively), indicating the operation of efficient repair mechanisms. Overall, the exposure to low and high temperatures affected both PSII photoinactivation and repair but causing smaller impacts in the former than in the latter. Also, cold stress caused larger effects on repair (decrease of kREC) than on photoinactivation (increase of kPI), but heat stress affected similarly the two processes. These effects varied seasonally, suggesting a role of thermal acclimation, as heat stress had stronger effects in cold-acclimated samples and cold stress resulted in stronger effects in heat-acclimated samples. The changes in kPI and kREC occurred despite the high light-acclimated phenotype found all year round, indicating that these processes vary independently from the photoacclimation state. The results also showed that photoprotection processes, as measured by energy-dependent non-photochemical index qE, appear to have an important role, both by preventing PSII photoinactivation and by alleviating the impacts on PSII repair under acute thermal stress.

Hysteresis light curves: a protocol for characterizing the time dependence of the light response of photosynthesis.

Photosynthesis vs. light curves (LCs) have played a central role in photosynthesis research for decades. They are the commonest form of describing how photosynthesis responds to changes in light, being frequently used for characterizing photoacclimation. However, LCs are often interpreted exclusively regarding the response to light intensity, the effects of time of exposure not being explicitly considered. This study proposes the use of 'hysteresis light curves' (HLC), an experimental protocol focused on the cumulative effects of light exposure to obtain information on the time dependence of photosynthetic light responses. HLC are generated by exposing samples to a symmetrical sequence of increasing and decreasing light levels. The comparison of the light-increasing and the light-decreasing phases allows the quantification of the hysteresis caused by high-light exposure, the magnitude and direction of which inform on the activation, and subsequent relaxation of high-light-induced photosynthetic processes. HLCs of the chlorophyll fluorescence indices rETR (relative electron transport rate of photosystem II) and Y(NPQ) (index of non-photochemical quenching) were measured on cyanobacteria, algae, and plants, with the aim of identifying main patterns of hysteresis and their diversity. A non-parametric index is proposed to quantify the magnitude and direction of hysteresis in HLCs of rETR and Y(NPQ). The results of this study show that HLCs can provide additional relevant information on the time dependence of the light response of photosynthetic samples, not obtainable from conventional LCs, useful for phenotyping photosynthetic traits, including photoacclimation state and kinetics of light activation and relaxation of electron flow and energy dissipation processes.

Cyanobacteria as Candidates to Support Mars Colonization: Growth and Biofertilization Potential Using Mars Regolith as a Resource.

Cyanobacteria are indicated as organisms that can possibly support Mars colonization, contributing to the production of oxygen and other commodities therein. In this general context, the aim of this work was to evaluate the ability of three species of cyanobacteria (Anabaena cylindrica, Nostoc muscorum, and Arthrospira platensis) and a green microalga (Chlorella vulgaris) to grow using only the resources existing in Mars, i.e., water and Martian regolith stimulant (MGS-1), under an Earth-like atmosphere. A Martian regolith extract was produced and used as a culture medium to grow these species. Their growth was assessed during a period of 25 days, using optical density and fluorometric parameters. After this period, the possible contribution of end-of-life cyanobacteria/microalga as biofertilizing agents was also assessed, using the macrophyte Lemna minor as a vegetable model. Among the three species, N. muscorum showed the best growth performance when compared to the other species, while A. platensis and C. vulgaris were not able to thrive on Mars regolith extract. Therefore, N. muscorum should be the target of future studies not only due to their role in oxygen production but also due to their possible use as a food source, as many members of the Nostoc genus. Cyanobacteria and microalgae (A. platensis and C. vulgaris) showed good abilities as biofertilizing agents, i.e., they stimulated biomass (i.e., dry weight) production at levels comparable to the plants that grew on standard synthetic medium. The highest yield was reached with A. platensis, while the lowest was achieved using the media with N. muscorum. FTIR-ATR (Fourier transform infrared with attenuated total reflectance) spectroscopy showed that the differences between the plants grown on media with or without Martian regolith seem to be related mainly to polysaccharides.

The complete mitochondrial genome of the photosymbiotic sea slug Berghia stephanieae (Valdés, 2005) (Gastropoda, Nudibranchia).

Berghia stephanieae (Nudibranchia, Cladobranchia) is a photosymbiotic sea slug that feeds exclusively on sea anemones from the genus Exaiptasia. It then specifically incorporates dinoflagellates belonging to the Symbiodiniaceae obtained from their prey. Here, we present the complete mitochondrial genome sequence of B. stephanieae combining Oxford Nanopore long read and Illumina short-read sequencing data. The mitochondrial genome has a total length of 14,786 bp, it contains the 13 protein-encoding genes, 23 tRNAs, and two rRNAs and is similar to other nudibranchs except for the presence of a duplicated tRNA-Ser 1.

Application of a standard risk assessment scheme to a North Africa contaminated site (Sfax, Tunisia) -Tier 1.

Phosphorus is a critical element to agriculture, consequently global phosphate rock demand will remain rising to feed a growing world population. The beneficiation of phosphorous ore gives rise to several tons of a waste by-product [phosphogypsum (PG)] which valorisation is limited, within other reasons, by the risks posed to environment and human health. Although threatening, the accumulation in stacks is the only procedure so far practiced by several countries as a means to get rid of this industrial externality. As part of a NATO Science for Peace Project (SfP 983311) this study describes the application of an environmental risk assessment (ERA) framework, to assess the risks posed by a PG stack to the surrounding soils, in Sfax, Republic of Tunisia. The ERA followed a weight of evidence approach, supported by two lines of evidence (LoE): the chemical (ChemLoE) and the ecotoxicological (EcotoxLoE). Integrated risks point for risk values greater than 0.5 in soils collected in PG stack surrounding area. Soil salinization, has likely contributed to the exacerbation of risks, as well as to the lack of consistency between both LoEs. This study highlights the need of rethinking the weight given to each LoE in ERA, in areas where soil salinization is a reality.

Identification of scavenger receptors and thrombospondin-type-1 repeat proteins potentially relevant for plastid recognition in Sacoglossa.

Functional kleptoplasty is a photosymbiotic relationship, in which photosynthetically active chloroplasts serve as an intracellular symbiont for a heterotrophic host. Among Metazoa, functional kleptoplasty is only found in marine sea slugs belonging to the Sacoglossa and recently described in Rhabdocoela worms. Although functional kleptoplasty has been intensively studied in Sacoglossa, the fundamentals of the specific recognition of the chloroplasts and their subsequent incorporation are unknown. The key to ensure the initiation of any symbiosis is the ability to specifically recognize the symbiont and to differentiate a symbiont from a pathogen. For instance, in photosymbiotic cnidarians, several studies have shown that the host innate immune system, in particular scavenger receptors (SRs) and thrombospondin-type-1 repeat (TSR) protein superfamily, is playing a major role in the process of recognizing and differentiating symbionts from pathogens. In the present study, SRs and TSRs of three Sacoglossa sea slugs, Elysia cornigera, Elysia timida, and Elysia chlorotica, were identified by translating available transcriptomes into potential proteins and searching for receptor specific protein and/or transmembrane domains. Both receptors classes are highly diverse in the slugs, and many new domain arrangements for each receptor class were found. The analyses of the gene expression of these three species provided a set of species-specific candidate genes, that is, SR-Bs, SR-Es, C-type lectins, and TSRs, that are potentially relevant for the recognition of kleptoplasts. The results set the base for future experimental studies to understand if and how these candidate receptors are indeed involved in chloroplast recognition.

Evaluation of physiological changes induced by the fluoroquinolone antibiotic ciprofloxacin in the freshwater macrophyte species Lemna minor and Lemna gibba.

The worldwide increase in the consumption of antibiotics is becoming a concern for the scientific community, since the presence of their residues in the wild poses specific challenges, especially in ecotoxicological terms. Currently, antibiotics are used for a wide range of purposes, being used against bacterial diseases but also as growth promoters. As a result, their environmental presence can affect wild organisms, especially those from the aquatic environment. This scenario leads to the need of characterizing the toxicity of antibiotics, especially towards non-target organisms. In this study we selected two species of aquatic macrophytes, Lemna minor and Lemna gibba, which are standard plant species inscribed in ecotoxicological testing guidelines. In this work we characterized the toxic effects of the quinolone antibiotic ciprofloxacin (in levels of 0.005, 0.013, 0.031, 0.078, and 0.195 mg/L), focusing on its potential toxicity towards photosynthetic mechanisms, and pro-oxidant effects. These objectives were attained by measuring the concentrations of chlorophyll a and b, and carotenoids levels. The determination of the quantum yield allowed assessing the effects of ciprofloxacin on the photochemical efficiency of the Photosystem II (PSII). The pro-oxidant effects induced by ciprofloxacin were evaluated by measuring oxidative stress biomarkers, such as catalase activity, and also by determining lipoperoxidation levels. The obtained results showed no differences in terms of the content of both chlorophylls a and b, or any change in the photochemical efficiency of the PSII; however, the global carotenoids content of L. gibba were significantly decreased. The activity of the anti-oxidant enzyme catalase was also significantly increased in L. minor. L. gibba showed a decrease in lipid peroxidation levels, but only for the two lowest concentrations of ciprofloxacin. The global set of data shows the activation of the anti-oxidant defensive system of both plant species, a response that was likely activated by the pro-oxidant character of ciprofloxacin. Our data demonstrate the interference of this therapeutic compound at different levels of plant metabolism, at ecologically relevant concentrations. In fact, the obtained results are of ecological relevance since they illustrate deleterious effects that may compromise the physiology of aquatic non-target plant species.

A chlorophyll fluorescence-based method for the integrated characterization of the photophysiological response to light stress.

This work introduces a new experimental method for the comprehensive description of the physiological responses to light of photosynthetic organisms. It allows the integration in a single experiment of the main established manipulative chlorophyll fluorescence-based protocols. It enables the integrated characterization of the photophysiology of samples regarding photoacclimation state (generating non-sequential light-response curves of effective PSII quantum yield, electron transport rate or non-photochemical quenching), photoprotection capacity (running light stress-recovery experiments, quantifying non-photochemical quenching components) and the operation of photoinactivation and photorepair processes (measuring rate constants of photoinactivation and repair for different light levels and the relative quantum yield of photoinactivation). The new method is based on a previously introduced technique, combining the illumination of a set of replicated samples with spatially separated actinic light beams of different intensity, and the simultaneous measurement of the fluorescence emitted by all samples using an imaging fluorometer. The main novelty described here is the independent manipulation of light intensity and duration of exposure for each sample, and the control of the cumulative light dose applied. The results demonstrate the proof of concept for the method, by comparing the responses of cultures of Chlorella vulgaris acclimated to low and high light regimes, highlighting the mapping of light stress responses over a wide range of light intensity and exposure conditions, and the rapid generation of paired light-response curves of photoinactivation and repair rate constants. This approach represents a chlorophyll fluorescence 'protocol of everything', contributing towards the high throughput characterization of the photophysiology of photosynthetic organisms.

Response of intertidal benthic microalgal biofilms to a coupled light-temperature stress: evidence for latitudinal adaptation along the Atlantic coast of Southern Europe.

Although estuarine microphytobenthos (MPB) is frequently exposed to excessive light and temperature conditions, little is known on their interactive effects on MPB primary productivity. Laboratory and in situ experiments were combined to investigate the short-term joint effects of high light (HL) and high temperature (37 °C versus 27 °C) on the operating efficiency of photoprotective processes [vertical migration versus non-photochemical quenching (NPQ)] exhibited by natural benthic diatom communities from two intertidal flats in France (FR) and Portugal (PT). A clear latitudinal pattern was observed, with PT biofilms being more resistant to HL stress, regardless the effect of temperature, and displaying a lower relative contribution of vertical migration to photoprotection and a stronger NPQ in situ. However, higher temperature leads to comparable effects, with photoinhibition increasing to about three times (i.e. from 3% to 10% and from 8% to 22% in PT and FR sites respectively). By using a number of methodological novelties in MPB research (lipid peroxidation quantification, Lhcx proteins immunodetection), this study brings a physiological basis to the previously reported depression of MPB photosynthetic productivity in summer. They emphasize the joint role of temperature and light in limiting, at least transiently (i.e. during emersion), MPB photosynthetic activity in situ.

High-to-low CO2 acclimation reveals plasticity of the photorespiratory pathway and indicates regulatory links to cellular metabolism of Arabidopsis.

BACKGROUND: Photorespiratory carbon metabolism was long considered as an essentially closed and nonregulated pathway with little interaction to other metabolic routes except nitrogen metabolism and respiration. Most mutants of this pathway cannot survive in ambient air and require CO(2)-enriched air for normal growth. Several studies indicate that this CO(2) requirement is very different for individual mutants, suggesting a higher plasticity and more interaction of photorespiratory metabolism as generally thought. To understand this better, we examined a variety of high- and low-level parameters at 1% CO(2) and their alteration during acclimation of wild-type plants and selected photorespiratory mutants to ambient air. METHODOLOGY AND PRINCIPAL FINDINGS: The wild type and four photorespiratory mutants of Arabidopsis thaliana (Arabidopsis) were grown to a defined stadium at 1% CO(2) and then transferred to normal air (0.038% CO(2)). All other conditions remained unchanged. This approach allowed unbiased side-by-side monitoring of acclimation processes on several levels. For all lines, diel (24 h) leaf growth, photosynthetic gas exchange, and PSII fluorescence were monitored. Metabolite profiling was performed for the wild type and two mutants. During acclimation, considerable variation between the individual genotypes was detected in many of the examined parameters, which correlated with the position of the impaired reaction in the photorespiratory pathway. CONCLUSIONS: Photorespiratory carbon metabolism does not operate as a fully closed pathway. Acclimation from high to low CO(2) was typically steady and consistent for a number of features over several days, but we also found unexpected short-term events, such as an intermittent very massive rise of glycine levels after transition of one particular mutant to ambient air. We conclude that photorespiration is possibly exposed to redox regulation beyond known substrate-level effects. Additionally, our data support the view that 2-phosphoglycolate could be a key regulator of photosynthetic-photorespiratory metabolism as a whole.