INTRACELLULAR METAL COMPARTMENTALIZATION IN THE GREEN ALGAL MODEL SYSTEM MICRASTERIAS DENTICULATA (STREPTOPHYTA) MEASURED BY TRANSMISSION ELECTRON MICROSCOPY-COUPLED ELECTRON ENERGY LOSS SPECTROSCOPY1

Metal toxicity in Bryum coronatum Schwaegrichen: impact on chlorophyll content, lamina cell structure, and metal accumulation

This research examined the impact of heavy metals, including Cd, Pb, and Zn, on chlorophyll content and lamina cell structure in Bryum coronatum. After exposure to varying metal concentrations (0.015, 0.065, 0.250, 1, and 4 mg/L), chlorophyll content, chloroplast numbers, lamina cell change, and metal accumulation were investigated. Chlorophyll content was assessed using spectrophotometry, whereas chloroplast numbers and lamina cell changes were examined under a light microscope. Metal accumulation was quantified through ICP-MS. The findings revealed that Cd notably reduced chlorophyll a content, while Pb and Zn showed minimal influence. Cd and Pb exposure decreased the number of chloroplasts in lamina cells, with no impact from Zn. The moss's capacity to absorb metals increased with higher exposure levels, indicating its potential as a biomonitor for heavy metal pollution. Cell mortality occurred in response to Cd and Pb, primarily in the median and apical lamina regions, while Zn had no effect. This study sheds light on heavy metal toxicity in B. coronatum, underscoring its significance for environmental monitoring. Further research on the mechanisms and consequences of heavy metal toxicity in bryophytes is essential for a comprehensive understanding of this critical issue.

Copper and zinc interact significantly in their joint toxicity to Chlamydomonas reinhardtii: Insights from physiological and transcriptomic investigations

Copper (Cu) and zinc (Zn) often discharge simultaneously from industrial and agricultural sectors and cause stress to aquatic biota. Although microalgae have been extensively investigated for their responses to Cu or Zn exposure, how they cope with the mixtures of two metals, especially at transcriptomic level, remains largely unknown. In this study, Chlamydomonas reinhardtii was exposed to environmentally relevant concentrations of two metals. It was found that Zn promoted the entry of Cu into the algal cells. With the increase of combined toxicity, extracellular polymeric substances (EPS) and cell wall functional groups immobilized significant amounts of Cu and Zn. Furthermore, C. reinhardtii adjusted resistance strategies internally, including starch consumption and synthesis of chlorophyll and lipids. Upon high level of Cu and Zn coexistence, synergistic effects were observed in lipid peroxidation and catalase (CAT) activity. Under 1.05 mg/L Cu + 0.87 mg/L Zn, 256 differentially expressed genes (DEGs) were mainly involved in oxidative phosphorylation, ribosome, nitrogen metabolism; while 4294 DEGs induced by 4.21 mg/L Cu + 3.48 mg/L Zn were mainly related to photosynthesis, citric acid cycle, etc. Together, this study revealed a more comprehensive understanding of mechanisms of Cu/Zn detoxification in C. reinhardtii, emphasizing critical roles of photosynthetic carbon sequestration and energy metabolism in the metal resistance.

Excess manganese increases photosynthetic activity via enhanced reducing center and antenna plasticity in Chlorella vulgaris

Photosynthesis relies on many easily oxidizable/reducible transition metals found in the metalloenzymes that make up much of the photosynthetic electron transport chain (ETC). One of these is manganese, an essential cofactor of photosystem II (PSII) and a component of the oxygen-evolving complex, the only biological entity capable of oxidizing water. Additionally, manganese is a cofactor in enzymatic antioxidants, notably the superoxide dismutases-which are localized to the chloroplastic membrane. However, unlike other metals found in the photosynthetic ETC, previous research has shown exposure to excess manganese enhances photosynthetic activity rather than diminishing it. In this study, the impact of PSII heterogeneity on overall performance was investigated using chlorophyll fluorescence, a rapid, non-invasive technique that probed for overall photosynthetic efficiency, reducing site activity, and antenna size and distribution. These measurements unveiled an enhanced plasticity of PSII following excess manganese exposure, in which overall performance and reducing center activity increased while antenna size and proportion of PSIIβ centers decreased. This enhanced activity suggests manganese may hold the key to improving photosynthetic efficiency beyond that which is observed in nature.

Changes in the morphology and cell ultrastructure of a microalgal community exposed to a commercial glyphosate formulation and a toxigenic cyanobacterium

Human activities significantly influence the health of aquatic ecosystems because many noxious chemical wastes are discharged into freshwater bodies. Intensive agriculture contributes to the deterioration by providing indirectly fertilizers, pesticides, and other agrochemicals that affect the aquatic biota. Glyphosate is one of the most used herbicides worldwide, and microalgae are particularly sensitive to its formulation, inducing displacement of some green microalgae from the phytoplankton that leads to alterations in the floristic composition, which fosters the abundance of cyanobacteria, some of which can be toxigenic. The combination of chemical stressors such as glyphosate and biological ones, like cyanotoxins and other secondary metabolites of cyanobacteria, could induce a combined effect potentially more noxious to microalgae, affecting not only their growth but also their physiology and morphology. In this study, we evaluated the combined effect of glyphosate (Faena^®) and a toxigenic cyanobacterium on the morphology and ultrastructure of microalgae in an experimental phytoplankton community. For this purpose, Microcystis aeruginosa (a cosmopolitan cyanobacterium that forms harmful blooms) and the microalgae Ankistrodesmus falcatus, Chlorella vulgaris, Pseudokirchneriella subcapitata, and Scenedesmus incrassatulus were cultivated, individually and jointly, exposing them to sub-inhibitory concentrations of glyphosate (IC₁₀, IC_20, and IC₄₀). Effects were evaluated through scanning electron (SEM) and transmission electron (TEM) microscopy. Exposure to Faena^® produced alterations in the external morphology and ultrastructure of microalgae both individually and in combined cultures. SEM evidenced the loss of the typical shape and integrity of the cell wall and an increase in the biovolume. TEM revealed reduction and disorganization of the chloroplast, variation in starch and polyphosphate granules, formation of vesicles and vacuoles, cytoplasm degradation, and cell wall continuity loss. The presence of M. aeruginosa was, for microalgae, an additional stress factor adding to the chemical stress produced by Faena^®, increasing the damage in their morphology and ultrastructure. These results alert to the effects that can be caused by glyphosate and the presence of toxigenic bacteria on the algal phytoplankton in contaminated and anthropic and eutrophic freshwater ecosystems.

Toxic effects of chlorpyrifos, cypermethrin and glyphosate on the non-target organism Selenastrum capricornutum (Chlorophyta)

The toxic effects of the insecticides chlorpyrifos and cypermethrin, and the herbicide glyphosate on the growth, biovolume and ultrastructure of the green microalgae Selenastrum capricornutum were evaluated. Concentrations between 9.37-150 mg L-1 of chlorpyrifos, 3.12-100 mg L-1 of cypermethrin and 4.7-60 mg L-1 of glyphosate were assayed along with a control culture. The assayed concentrations were prepared using commercial formulations. After 48 h all tested concentrations of the three pesticides reduced significantly the population growth. The 96 h effective concentration 50 (EC50) was 14.45 mg L-1 for chlorpyrifos, 12.37 mg L-1 for cypermethrin and 15.60 mg L-1 for glyphosate. Cells exposed to the three pesticides showed an increase in the cellular size related to the increase in pesticide concentration and exposure time. The most significant damages observed on the ultrastructure of cells exposed to the three pesticides included thylakoids and mitochondria disruption, formation of electrodense bodies, accumulation of lipids and increase in the size and number of starch granules. The present study demonstrates that the effects of pesticides also extend to non-target organisms having significant ecological implications.

Responses to Cadmium in Early-Diverging Streptophytes (Charophytes and Bryophytes): Current Views and Potential Applications

Several transition metals are essential for plant growth and development, as they are involved in various fundamental metabolic functions. By contrast, cadmium (Cd) is a metal that can prove extremely toxic for plants and other organisms in a dose-dependent manner. Charophytes and bryophytes are early-diverging streptophytes widely employed for biomonitoring purposes, as they are able to cope with high concentrations of toxic metal(loid)s without showing any apparent heavy damage. In this review, we will deal with different mechanisms that charophytes and bryophytes have evolved to respond to Cd at a cellular level. Particular attention will be addressed to strategies involving Cd vacuolar sequestration and cell wall immobilization, focusing on specific mechanisms that help achieve detoxification. Understanding the effects of metal(loid) pollution and accumulation on the morpho-physiological traits of charophytes and bryophytes can be in fact fundamental for optimizing their use as phytomonitors and/or phytoremediators.

Aluminium triggers oxidative stress and antioxidant response in the microalgae Scenedesmus sp

Aluminium (Al) water pollution is an increasing environmental problem and comprehensive analysis of toxic responses of aquatic primary producer organisms is imperative. We characterized the antioxidant response of Scenedesmus sp. microalga to Al-induced oxidative stress. After 72 h of exposure to Al (0, 10, and 100 μM) in a modified Bold Basal Medium (pH 5.0), we observed cell aggregation and alterations in the subcellular structure, strong lipid peroxidation and oxidative stress induction (detected with the fluorescent probe 2',7'-dichlorodihydrofluorescein diacetate) in parallel with Al accumulation in cells. At the same time, Al toxicity caused depletion of important macronutrients like Ca, which is important for cell-wall structure. Analysis of antioxidant enzymatic activities in Al-treated Scenedesmus cells revealed that catalase, ascorbate peroxidase, as well as different isoforms of superoxide dismutase were inhibited especially at the highest Al dose (100 μM), cells that accumulated the highest concentration of Al. On the other hand, glutathione reductase activity increased at that Al concentration. Immunodetection after Western-blotting confirmed that only ascorbate peroxidase inhibition was apparently due to a decrease in enzyme levels. However, the inhibition of catalase and activation of glutathione reductase activities seemed related with post-translational modifications in protein function as protein expression decreased or increased, respectively under Al stress. Our results may help to understand toxic mechanisms triggered by Al in freshwater microalgae, which in turn could aid to select suitable biomarkers of Al contamination in aquatic ecosystems.

Symmetry breaking of the cellular lobes closely relates to phylogenetic structure within green microalgae of the Micrasterias lineage (Zygnematophyceae)

Green microalgae of the Micrasterias lineage are unicellular microorganisms with modular morphology consisting of successively differentiated lobes. Due to their morphological diversity and peculiar morphogenesis, these species are important model systems for studies of cytomorphogenesis and cellular plasticity. Interestingly, the phylogenetic structure of the Micrasterias lineage and most other Desmidiales is poorly related to the traditional morphological characters used for delimitation of taxa. In this study, we focused on symmetry breaking between adjacent cellular lobes in relation to phylogeny of the studied species. While pronounced morphological asymmetry between the adjacent lobes is typical for some species, others have been characterized by the almost identical morphologies of these structures. We asked whether there is any detectable average shape asymmetry between the pairs of lobes and terminal lobules in 19 Micrasterias species representing all major clades of this desmidiacean lineage. Then, we evaluated whether the asymmetric patterns among species are phylogenetically structured. The analyses showed that the phylogeny was in fact strongly related to the patterns of morphological asymmetry between the adjacent cellular lobes. Thus, evolution of the asymmetric development between the adjacent lobes proved to be the key event differentiating cellular shape patterns of Micrasterias. Conversely, the phylogeny was only weakly related to asymmetry between the pairs of terminal lobules. The subsequent analyses of the phylogenetic morphological integration showed that individual hierarchical levels of cellular morphology were only weakly coordinated with regard to asymmetric variation among species. This finding indicates that evolutionary differentiation of morphogenetic processes leading to symmetry breaking may be relatively independent at different branching levels. Such modularity is probably the key to the evolvability of cellular shapes, leading to the extraordinary morphological diversity of these intriguing microalgae.

Biomineralization of strontium and barium contributes to detoxification in the freshwater alga Micrasterias

The unicellular model alga Micrasterias denticulata inhabits acid peat bogs that are highly endangered by pollutants due to their high humidity. As it was known from earlier studies that algae like Micrasterias are capable of storing barium naturally in form of BaSO₄ crystals, it was interesting to experimentally investigate distribution and sequestration of barium and the chemically similar alkaline earth metal strontium. Additionally, we intended to analyze whether biomineralization by crystal formation contributes to diminution of the generally toxic effects of these minerals to physiology and structure of this alga which is closely related to higher plants. The results show that depending on the treatment differently shaped crystals are formed in BaCl₂ and Cl₂Sr exposed Micrasterias cells. Modern microscopic techniques such as analytical TEM by electron energy loss spectroscopy and Raman microscopy provide evidence for the chemical composition of these crystals. It is shown that barium treatment results in the formation of insoluble BaSO₄ crystals that develop within distinct compartments. During strontium exposure long rod-like crystals are formed and are surrounded by membranes. Based on the Raman signature of these crystals their composition is attributed to strontium citrate. These crystals are instable and are dissolved during cell death. During strontium as well as barium treatment cell division rates and photosynthetic oxygen production decreased in dependence of the concentration, whereas cell vitality was reduced only slightly. Together with the fact that TEM analyses revealed only minor ultrastructural alterations as consequence of relatively high concentrated BaCl₂ and Cl₂Sr exposure, this indicates that biomineralization of Sr and Ba protects the cells from severe damage or cell death at least within a particular concentration range and time period. In the case of Sr treatment where ROS levels were found to be elevated, hallmarks for autophagy of single organelles were observed by TEM, indicating beginning degradation processes.

Ionic stress induces fusion of mitochondria to 3-D networks: An electron tomography study

Mitochondria are central organelles for energy supply of cells and play an important role in maintenance of ionic balance. Consequently mitochondria are highly sensitive to any kind of stress to which they mainly response by disturbance of respiration, ROS production and release of cytochrome c into the cytoplasm. Many of the physiological and molecular stress reactions of mitochondria are well known, yet there is a lack of information on corresponding stress induced structural changes. 3-D visualization of high-pressure frozen cells by FIB-SEM tomography and TEM tomography as used for the present investigation provide an excellent tool for studying structure related mitochondrial stress reactions. In the present study it is shown that mitochondria in the unicellular fresh-water algal model system Micrasterias as well as in the closely related aquatic higher plant Lemna fuse to local networks as a consequence of exposure to ionic stress induced by addition of KCl, NaCl and CoCl₂. In dependence on concentration and duration of the treatment, fusion of mitochondria occurs either by formation of protuberances arising from the outer mitochondrial membrane, or by direct contact of the surface of elongated mitochondria. As our results show that respiration is maintained in both model systems during ionic stress and mitochondrial fusion, as well as formation of protuberances are reversible, we assume that mitochondrial fusion is a ubiquitous process that may help the cells to cope with stress. This may occur by interconnecting the respiratory chains of the individual mitochondria and by enhancing the buffer capacity against stress induced ionic imbalance.

Asymmetry and integration of cellular morphology in Micrasterias compereana

Background

Unicellular green algae of the genus Micrasterias (Desmidiales) have complex cells with multiple lobes and indentations, and therefore, they are considered model organisms for research on plant cell morphogenesis and variation. Micrasterias cells have a typical biradial symmetric arrangement and multiple terminal lobules. They are composed of two semicells that can be further differentiated into three structural components: the polar lobe and two lateral lobes. Experimental studies suggested that these cellular parts have specific evolutionary patterns and develop independently. In this study, different geometric morphometric methods were used to address whether the semicells of Micrasterias compereana are truly not integrated with regard to the covariation of their shape data. In addition, morphological integration within the semicells was studied to ascertain whether individual lobes constitute distinct units that may be considered as separate modules. In parallel, I sought to determine whether the main components of morphological asymmetry could highlight underlying cytomorphogenetic processes that could indicate preferred directions of variation, canalizing evolutionary changes in cellular morphology.

Results

Differentiation between opposite semicells constituted the most prominent subset of cellular asymmetry. The second important asymmetric pattern, recovered by the Procrustes ANOVA models, described differentiation between the adjacent lobules within the quadrants. Other asymmetric components proved to be relatively unimportant. Opposite semicells were shown to be completely independent of each other on the basis of the partial least squares analysis analyses. In addition, polar lobes were weakly integrated with adjacent lateral lobes. Conversely, higher covariance levels between the two lateral lobes of the same semicell indicated mutual interconnection and significant integration between these parts.

Conclusions

Micrasterias cells are composed of several successively disintegrated parts. These integration patterns concurred with presumed scenarios of morphological evolution within the lineage. In addition, asymmetric differentiation in the shape of the lobules involves two major patterns: asymmetry across the isthmus axis and among the adjacent lobules. Notably, asymmetry among the adjacent lobules may be related to evolutionary differentiation among species, but it may also point out developmental instability related to environmental factors.

Electronic supplementary material

The online version of this article (doi:10.1186/s12862-016-0855-1) contains supplementary material, which is available to authorized users.

Comparative Transcriptomic Analysis of the Response of Dunaliella acidophila (Chlorophyta) to Short-Term Cadmium and Chronic Natural Metal-Rich Water Exposures

Heavy metals are toxic compounds known to cause multiple and severe cellular damage. However, acidophilic extremophiles are able to cope with very high concentrations of heavy metals. This study investigated the stress response under natural environmental heavy metal concentrations in an acidophilic Dunaliella acidophila. We employed Illumina sequencing for a de novo transcriptome assembly and to identify changes in response to high cadmium concentrations and natural metal-rich water. The photosynthetic performance was also estimated by pulse amplitude-modulated (PAM) fluorescence. Transcriptomic analysis highlights a number of processes mainly related to a high constitutive expression of genes involved in oxidative stress and response to reactive oxygen species (ROS), even in the absence of heavy metals. Photosynthetic activity seems to be unaltered under short-term exposition to Cd and chronic exposure to natural metal-rich water, probably due to an increase in the synthesis of structural photosynthetic components preserving their functional integrity. An overrepresentation of Gene Ontology (GO) terms related to metabolic activities, transcription, and proteosomal catabolic process was observed when D. acidophila grew under chronic exposure to natural metal-rich water. GO terms involved in carbohydrate metabolic process, reticulum endoplasmic and Golgi bodies, were also specifically overrepresented in natural metal-rich water library suggesting an endoplasmic reticulum stress response.

The green alga Zygogonium ericetorum (Zygnematophyceae, Charophyta) shows high iron and aluminium tolerance: protection mechanisms and photosynthetic performance

Streptophyte green algae, ancestors of Embryophytes, occur frequently in terrestrial habitats being exposed to high light intensities, water scarcity and potentially toxic metal cations under acidic conditions. The filamentous Zygogonium ericetorum synthesizes a purple vacuolar ferrous pigment, which is lost after aplanospore formation. However, it is unknown whether this cellular reorganization also removes excessive iron from the protoplast and how Z. ericetorum copes with high concentrations of aluminium. Here we show that aplanospore formation shifts iron into the extracellular space of the algal filament. Upon germination of aplanospores, aluminium is bound in the parental cell wall. Both processes reduce iron and aluminium in unpigmented filaments. Comparison of the photosynthetic oxygen production in response to light and temperature gradients in two different Z. ericetorum strains from an Austrian alpine and a Scottish highland habitat revealed lower values in the latter strain. In contrast, the Scottish strain showed a higher optimum quantum yield of PSII during desiccation stress followed by rehydration. Furthermore, pigmented filaments of both strains exhibited a higher light and temperature dependent oxygen production when compared to the unpigmented phenotype. Our results demonstrate a high metal tolerance of Z. ericetorum, which is crucial for surviving in acidic terrestrial habitats.

Micrasterias as a Model System in Plant Cell Biology

The unicellular freshwater alga Micrasterias denticulata is an exceptional organism due to its complex star-shaped, highly symmetric morphology and has thus attracted the interest of researchers for many decades. As a member of the Streptophyta, Micrasterias is not only genetically closely related to higher land plants but shares common features with them in many physiological and cell biological aspects. These facts, together with its considerable cell size of about 200 μm, its modest cultivation conditions and the uncomplicated accessibility particularly to any microscopic techniques, make Micrasterias a very well suited cell biological plant model system. The review focuses particularly on cell wall formation and composition, dictyosomal structure and function, cytoskeleton control of growth and morphogenesis as well as on ionic regulation and signal transduction. It has been also shown in the recent years that Micrasterias is a highly sensitive indicator for environmental stress impact such as heavy metals, high salinity, oxidative stress or starvation. Stress induced organelle degradation, autophagy, adaption and detoxification mechanisms have moved in the center of interest and have been investigated with modern microscopic techniques such as 3-D- and analytical electron microscopy as well as with biochemical, physiological and molecular approaches. This review is intended to summarize and discuss the most important results obtained in Micrasterias in the last 20 years and to compare the results to similar processes in higher plant cells.

Limited accumulation of copper in heavy metal adapted mosses

Copper is an essential micronutrient but has toxic effects at high concentrations. Bryophytes are remarkably tolerant to elevated levels of copper but we wondered if this tolerance might be species dependent. Therefore, in three moss species, Physcomitrella patens, Mielichhoferia elongata and Pohlia drummondii, the accumulation of copper was compared with semiquantitative SEM-EDX analyses after six weeks of cultivation on copper containing media. We investigated the role of the copper-linked anion and applied copper as CuCl2, CuSO4 and CuEDTA, respectively. Line scans along the growth axis of moss gametophores allowed for a detailed analysis of copper detection from the base towards the tip. Mosses originating from metal-containing habitats (i.e. M. elongata and P. drummondii) revealed a lower accumulation of copper when compared to the non-adapted P. patens. CuEDTA had a shielding effect in all three species and copper levels differed greatly from CuCl2 or CuSO4. The detection of reactive oxygen species (ROS), H2O2 and O2(-), was further used to indicate stress levels in the gametophore stems. ROS staining was increased along the whole stem and the tip in the non-adapted species P. patens whereas the tolerant species M. elongata and P. drummondii generally showed less staining located mainly at the base of the stem. We discuss the relation between metal accumulation and ROS production using indicator dyes in the three moss species. As moss gametophores are very delicate structures, ROS staining provide an excellent alternative to spectrophotometric analyses to estimate stress levels.

Structural stress responses and degradation of dictyosomes in algae analysed by TEM and FIB-SEM tomography

Stress-induced physiological deficiencies in cells are reflected in structural, morphological and functional reactions of organelles. Although numerous investigations have focused on chloroplasts and mitochondria as main targets of different stressors in plant cells, there is insufficient information on the plant Golgi apparatus as stress sensor. By using the advantages of field emission scanning electron microscopy tomography in combination with classical ultrathin sectioning and transmission electron microscopic analyses, we provide structural evidence for common stress responses of the large and highly stable dictyosomes in the algal model system Micrasterias. Stress is induced by different metals such as manganese and lead, by starvation in 9 weeks of darkness or by inhibiting photosynthesis or glycolysis and by disturbing ionic homeostasis via KCl. For the first time a stress-induced degradation pathway of dictyosomes is described that does not follow "classical" autophagy but occurs by disintegration of cisternae into single membrane balls that seem to be finally absorbed by the endoplasmic reticulum (ER). Comparison of the morphological features that accompany dictyosomal degradation in Micrasterias to similar reactions observed during the same stress application in Nitella indicates an ubiquitous degradation process at least in algae. As the algae investigated belong to the closest relatives of higher land plants these results may also be relevant for understanding dictyosomal stress and degradation responses in the latter phylogenetic group. In addition, this study shows that two-dimensional transmission electron microscopy is insufficient for elucidating complex processes such as organelle degradation, and that information from three-dimensional reconstructions as provided by field emission scanning electron microscopy tomography is absolutely required for a comprehensive understanding of the phenomenon.

Effects of chlorpyrifos on the growth and ultrastructure of green algae, Ankistrodesmus gracilis

The effect of the organophosphorus insecticide chlorpyrifos on the growth, biovolume, and ultrastructure of the green microalga Ankistrodesmus gracilis was evaluated. Concentrations of 9.37, 18.75, 37.5, 75 and 150mgL(-1) of chlorpyrifos were assayed along with a control culture. At the end of the bioassay the ultrastructure of algal cells from control culture and from cultures exposed to 37.5 and 150mgL(-1) was observed under transmission (TEM) and scanning electron microscopy (SEM). After 24 and 48h, treatments with 75 and 150mgL(-1) inhibited the growth of A. gracilis; whereas after 72 and 96h, all the treatments except at 9.37mgL(-1) significantly affected the algae growth. The effective concentration 50 (EC50) after 96h was 22.44mgL(-1) of chlorpyrifos. After the exposure to the insecticide, an increase in the biovolume was observed, with a larger increase in cells exposed to 75 and 150mgL(-1). Radical changes were observed in the ultrastructure of cells exposed to chlorpyrifos. The insecticide affected the cell shape and the distribution of the crests in the wall. At 37.5mgL(-1) electodense bodies were observed along with an increase in the size and number of starch granules. At 150mgL(-1) such bodies occupied almost the whole cytoplasm together with lipids and remains of thylakoids. Autospores formation occurred normally at 37.5mgL(-1) while at 150mgL(-1) karyokinesis occurred, but cell-separation-phase was inhibited. The present study demonstrates that the exposure of phytoplankton to the insecticide chlorpyrifos leads to effects observed at both cellular and population level.

Analysis of the Proteome of the Marine Diatom Phaeodactylum tricornutum Exposed to Aluminum Providing Insights into Aluminum Toxicity Mechanisms

Trace aluminum (Al) concentrations can be toxic to marine phytoplankton, the basis of the marine food web, but the fundamental Al toxicity and detoxification mechanisms at the molecular levels are poorly understood. Using an array of proteomic, transcriptomic, and biochemical techniques, we describe in detail the cellular response of the model marine diatom Phaeodactylum tricornutum to a short-term sublethal Al stress (4 h of exposure to 200 μM total initial Al). A total of 2204 proteins were identified and quantified by isobaric tags for relative and absolute quantification (iTRAQ) in response to the Al stress. Among them, 87 and 78 proteins performing various cell functions were up- and down-regulated after Al treatment, respectively. We found that photosynthesis was a key Al toxicity target. The Al-induced decrease in electron transport rates in thylakoid membranes lead to an increase in reactive oxygen species (ROS) production, which cause increased lipid peroxidation. Several ROS-detoxifying proteins were induced to help decrease Al-induced oxidative stress. In parallel, glycolysis and pentose phosphate pathway were up-regulated in order to produce cell energy (NADPH, ATP) and carbon skeleton for cell growth, partially circumventing the Al-induced toxicity effects on photosynthesis. These cellular responses to Al stress were coordinated by the activation of various signal transduction pathways. The identification of Al-responsive proteins in the model marine phytoplankton P. tricornutum provides new insights on Al stress responses as well as a good start for further exploring Al detoxification mechanisms.

Tracking the Temporal Dynamics of Intracellular Lead Speciation in a Green Alga

Organisms have developed metal regulatory mechanisms in response to changes in the bioavailability of trace metals. Just as metal bioavailability dictates cellular uptake, intracellular metal speciation determines the availability of metals to exert biological effects. However, the missing link in understanding the relationship between metal uptake and biological responses is the ability to accurately measure intracellular metal speciation. We conducted Pb exposure studies on the well-characterized model green alga Chlamydomonas reinhardtii and identified temporal changes in intracellular Pb speciation under conditions relevant for fresh water ecosystems using resonant X-ray emission spectroscopy (RXES), which possesses enhanced sensitivity to functional group chemistry relative to X-ray absorption spectroscopy (XAS). Analysis of RXES maps show that only a small fraction of total intracellular Pb was complexed by thiol groups. Initial sequestration of Pb in oxides and inorganic phosphate was followed by binding of Pb to organic phosphate, suggesting potential interference in vital cellular functions. These results contrast proposed detoxification responses involving complexation by thiol groups from peptides.

Subcellular Sequestration and Impact of Heavy Metals on the Ultrastructure and Physiology of the Multicellular Freshwater Alga Desmidium swartzii

Due to modern life with increasing traffic, industrial production and agricultural practices, high amounts of heavy metals enter ecosystems and pollute soil and water. As a result, metals can be accumulated in plants and particularly in algae inhabiting peat bogs of low pH and high air humidity. In the present study, we investigated the impact and intracellular targets of aluminum, copper, cadmium, chromium VI and zinc on the filamentous green alga Desmidium swartzii, which is an important biomass producer in acid peat bogs. By means of transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) it is shown that all metals examined are taken up into Desmidium readily, where they are sequestered in cell walls and/or intracellular compartments. They cause effects on cell ultrastructure to different degrees and additionally disturb photosynthetic activity and biomass production. Our study shows a clear correlation between toxicity of a metal and the ability of the algae to compartmentalize it intracellularly. Cadmium and chromium, which are not compartmentalized, exert the most toxic effects. In addition, this study shows that the filamentous alga Desmidium reacts more sensitively to aluminum and zinc when compared to its unicellular relative Micrasterias, indicating a severe threat to the ecosystem.

Bioremediation for coal-fired power stations using macroalgae

Macroalgae are a productive resource that can be cultured in metal-contaminated waste water for bioremediation but there have been no demonstrations of this biotechnology integrated with industry. Coal-fired power production is a water-limited industry that requires novel approaches to waste water treatment and recycling. In this study, a freshwater macroalga (genus Oedogonium) was cultivated in contaminated ash water amended with flue gas (containing 20% CO₂) at an Australian coal-fired power station. The continuous process of macroalgal growth and intracellular metal sequestration reduced the concentrations of all metals in the treated ash water. Predictive modelling shows that the power station could feasibly achieve zero discharge of most regulated metals (Al, As, Cd, Cr, Cu, Ni, and Zn) in waste water by using the ash water dam for bioremediation with algal cultivation ponds rather than storage of ash water. Slow pyrolysis of the cultivated algae immobilised the accumulated metals in a recalcitrant C-rich biochar. While the algal biochar had higher total metal concentrations than the algae feedstock, the biochar had very low concentrations of leachable metals and therefore has potential for use as an ameliorant for low-fertility soils. This study demonstrates a bioremediation technology at a large scale for a water-limited industry that could be implemented at new or existing power stations, or during the decommissioning of older power stations.

Determination of elemental distribution in green micro-algae using synchrotron radiation nano X-ray fluorescence (SR-nXRF) and electron microscopy techniques--subcellular localization and quantitative imaging of silver and cobalt uptake by Coccomyxa actinabiotis

The newly discovered unicellular micro-alga Coccomyxa actinabiotis proves to be highly radio-tolerant and strongly concentrates radionuclides, as well as large amounts of toxic metals. This study helps in the understanding of the mechanisms involved in the accumulation and detoxification of silver and cobalt. Elemental distribution inside Coccomyxa actinabiotis cells was determined using synchrotron nano X-ray fluorescence spectroscopy at the ID22 nano fluorescence imaging beamline of the European Synchrotron Radiation Facility. The high resolution and high sensitivity of this technique enabled the assessment of elemental associations and exclusions in subcellular micro-algae compartments. A quantitative treatment of the scans was implemented to yield absolute concentrations of each endogenous and exogenous element with a spatial resolution of 100 nm and compared to the macroscopic content in cobalt and silver determined using inductively coupled plasma-mass spectrometry. The nano X-ray fluorescence imaging was complemented by transmission electron microscopy coupled to X-ray microanalysis (TEM-EDS), yielding differential silver distribution in the cell wall, cytosol, nucleus, chloroplast and mitochondria with unique resolution. The analysis of endogenous elements in control cells revealed that iron had a unique distribution; zinc, potassium, manganese, molybdenum, and phosphate had their maxima co-localized in the same area; and sulfur, copper and chlorine were almost homogeneously distributed among the whole cell. The subcellular distribution and quantification of cobalt and silver in micro-alga, assessed after controlled exposure to various concentrations, revealed that exogenous metals were mainly sequestered inside the cell rather than on mucilage or the cell wall, with preferential compartmentalization. Cobalt was homogeneously distributed outside of the chloroplast. Silver was localized in the cytosol at low concentration and in the whole cell excluding the nucleus at high concentration. Exposure to low concentrations of cobalt or silver did not alter the localization nor the concentration of endogenous elements within the cells. To our knowledge, this is the first report on element co-localization and segregation at the sub-cellular level in micro-algae by means of synchrotron nano X-ray fluorescence spectroscopy.

Rescue of heavy metal effects on cell physiology of the algal model system Micrasterias by divalent ions

Recent studies have shown that metals such as copper, zinc, aluminum, cadmium, chromium, iron and lead cause severe dose-dependent disturbances in growth, morphogenesis, photosynthetic and respiratory activity as well as on ultrastructure and function of organelles in the algal model system Micrasterias denticulata (Volland et al., 2011, 2012; Andosch et al., 2012). In the present investigation we focus on amelioration of these adverse effects of cadmium, chromium and lead by supplying the cells with different antioxidants and essential micronutrients to obtain insight into metal uptake mechanisms and subcellular metal targets. This seems particularly interesting as Micrasterias is adapted to extremely low-concentrated, oligotrophic conditions in its natural bog environment. The divalent ions of iron, zinc and calcium were able to diminish the effects of the metals cadmium, chromium and lead on Micrasterias. Iron showed most ameliorating effects on cadmium and chromium in short- and long-term treatments and improved cell morphogenesis, ultrastructure, cell division rates and photosynthesis. Analytical transmission electron microscopic (TEM) methods (electron energy loss spectroscopy (EELS) and electron spectroscopic imaging (ESI)) revealed that chromium uptake was decreased when Micrasterias cells were pre-treated with iron, which resulted in no longer detectable intracellular chromium accumulations. Zinc rescued the detrimental effects of chromium on net-photosynthesis, respiration rates and electron transport in PS II. Calcium and gadolinium were able to almost completely compensate the inhibiting effects of lead and cadmium on cell morphogenesis after mitosis, respectively. These results indicate that cadmium is taken up by calcium and iron transporters, whereas chromium appears to enter the algae cells via iron and zinc carriers. It was shown that lead is not taken up into Micrasterias at all but exerts its adverse effects on cell growth by substituting cell wall bound calcium. The antioxidants salicylic acid, ascorbic acid and glutathione were not able to ameliorate any of the investigated metal effects on the green alga Micrasterias when added to the culture medium.

3-D analysis of dictyosomes and multivesicular bodies in the green alga Micrasterias denticulata by FIB/SEM tomography

In the present study we employ FIB/SEM tomography for analyzing 3-D architecture of dictyosomes and formation of multivesicular bodies (MVB) in high pressure frozen and cryo-substituted interphase cells of the green algal model system Micrasterias denticulata. The ability of FIB/SEM of milling very thin ‘slices’ (5–10 nm), viewing the block face and of capturing cytoplasmic volumes of several hundred μm³ provides new insight into the close spatial connection of the ER–Golgi machinery in an algal cell particularly in z-direction, complementary to informations obtained by TEM serial sectioning or electron tomography.

Our FIB/SEM series and 3-D reconstructions show that interphase dictyosomes of Micrasterias are not only closely associated to an ER system at their cis-side which is common in various plant cells, but are surrounded by a huge “trans-ER” sheath leading to an almost complete enwrapping of dictyosomes by the ER. This is particularly interesting as the presence of a trans-dictyosomal ER system is well known from mammalian secretory cells but not from cells of higher plants to which the alga Micrasterias is closely related. In contrast to findings in plant storage tissue indicating that MVBs originate from the trans-Golgi network or its derivatives our investigations show that MVBs in Micrasterias are in direct spatial contact with both, trans-Golgi cisternae and the trans-ER sheath which provides evidence that both endomembrane compartments are involved in their formation.

Identification of phytochelatins in the cadmium-stressed conjugating green alga Micrasterias denticulata

Aquatic environments like peat bogs are affected by anthropogenic metal input into the environment. These ecosystems are inhabited by unicellular green algae of the class Zygnematophyceae. In this study the desmid Micrasterias denticulata was stressed with 600 nM Cd, 10 μM Cr and 300 nM Cu for 3 weeks. GSH levels were measured with HPLC and did not differ between the different treatments or the control. According to the metallo-thiolomics concept, mass spectrometry was used as a method for unambiguous thiol peptide identification. PC2, PC3 and PC4 were clearly identified in the Cd stressed sample with UPLC-MS by their MS spectrum and molecular masses. PC2 and PC3 were determined to be the main thiol compounds, while PC4 was only abundant in traces in Micrasterias. In addition, the identity of PC2 and PC3 was confirmed by MS/MS. No PCs were detected in the Cu stressed algae sample. However, in the Cr stressed sample traces of PC2 were indicated by a peak in UPLC-MS at the retention time of the PC2 standard, but the intensity was too low to acquire reliable MS and MS/MS spectra. In this study PCs have been detected for the first time in a green alga of the division Streptophyta, a close relative to higher plants.

A freshwater green alga under cadmium stress: ameliorating calcium effects on ultrastructure and photosynthesis in the unicellular model Micrasterias

Cadmium is a highly toxic heavy metal pollutant arising mainly from increasing industrial disposal of electronic components. Due to its high solubility it easily enters soil and aquatic environments. Via its similarity to calcium it may interfere with different kinds of Ca dependent metabolic or developmental processes in biological systems. In the present study we investigate primary cell physiological, morphological and ultrastructural responses of Cd on the unicellular freshwater green alga Micrasterias which has served as a cell biological model system since many years and has proved to be highly sensitive to any kind of abiotic stress. Our results provide evidence that the severe Cd effects in Micrasterias such as unidirectional disintegration of dictyosomes, occurrence of autophagy, decline in photosystem II activity and oxygen production as well as marked structural damage of the chloroplast are based on a disturbance of Ca homeostasis probably by displacement of Ca by Cd. This is indicated by the fact that physiological and structural cadmium effects could be prevented in Micrasterias by pre-treatment with Ca. Additionally, thapsigargin an inhibitor of animal and plant Ca(2+)-ATPase mimicked the adverse Cd induced morphological and functional effects on dictyosomes. Recovery experiments indicated rapid repair mechanisms after Cd stress.

Sustainable sources of biomass for bioremediation of heavy metals in waste water derived from coal-fired power generation

Biosorption of heavy metals using dried algal biomass has been extensively described but rarely implemented. We contend this is because available algal biomass is a valuable product with a ready market. Therefore, we considered an alternative and practical approach to algal bioremediation in which algae were cultured directly in the waste water stream. We cultured three species of algae with and without nutrient addition in water that was contaminated with heavy metals from an Ash Dam associated with coal-fired power generation and tested metal uptake and bioremediation potential. All species achieved high concentrations of heavy metals (to 8% dry mass). Two key elements, V and As, reached concentrations in the biomass of 1543 mg.kg⁻¹ DW and 137 mg.kg⁻¹ DW. Growth rates were reduced by more than half in neat Ash Dam water than when nutrients were supplied in excess. Growth rate and bioconcentration were positively correlated for most elements, but some elements (e.g. Cd, Zn) were concentrated more when growth rates were lower, indicating the potential to tailor bioremediation depending on the pollutant. The cosmopolitan nature of the macroalgae studied, and their ability to grow and concentrate a suite of heavy metals from industrial wastes, highlights a clear benefit in the practical application of waste water bioremediation.

Intracellular chromium localization and cell physiological response in the unicellular alga Micrasterias

Various contaminants like metals and heavy metals are constantly released into the environment by anthropogenic activities. The heavy metal chromium has a wide industrial use and exists in two stable oxidation states: trivalent and hexavalent. Chromium can cause harm to cell metabolism and development, when it is taken up by plants instead of necessary micronutrients such as for example iron. The uptake of Cr VI into plant cells has been reported to be an active process via carriers of essential anions, while the cation Cr III seems to be taken up inactively. Micrasterias denticulata, an unicellular green alga of the family Desmidiaceae is a well-studied cell biological model organism. Cr III and VI had inhibiting effects on its cell development, while cell division rates were only impaired by Cr VI. Transmission electron microscopy (TEM) revealed ultrastructural changes such as increased vacuolization, condensed cytoplasm and dark precipitations in the cell wall after 3 weeks of Cr VI treatment. Electron energy loss spectroscopy (EELS) and electron spectroscopic imaging (ESI) were applied to measure intracellular chromium distribution. Chromium was only detected after 3 weeks of 10 μM Cr VI treatment in electron dense precipitations found in bag-like structures along the inner side of the cell walls together with iron and elevated levels of oxygen, pointing toward an accumulation respectively extrusion of chromium in form of an iron-oxygen compound. Atomic emission spectroscopy (EMS) revealed that Micrasterias cells are able to accumulate considerable amounts of chromium and iron. During chromium treatment the Cr:Fe ratio shifted in favor of chromium, which implied that chromium may be taken up instead of iron. Significant and rapid increase of ROS production within the first 5 min of treatment confirms an active Cr VI uptake. SOD and CAT activity after Cr VI treatment did not show a response, while the glutathione pool determined by immuno-TEM decreased significantly in chromium treated cells, showing that glutathione is playing a major role in intracellular ROS and chromium detoxification.