Adaptive modifications of the photosynthetic apparatus in Euglena gracilis Klebs exposed to manganese excess

Responses of Vallisneria natans (Lour.) Hara to the combined effects of Mn and pH

Aquatic plants play a vital role in maintaining the health and stability of ecosystems and in ecological restoration of contaminated water bodies. Herein, a 21-day-long laboratory-scale experiment was designed to explore the growth and physiological responses of Vallisneria natans (Lour.) Hara (V. natans) to the combined effects of manganese (Mn, 5, 20, and 80 mg L^-1) and pH (pH 4.0, 5.5, and 7.0). Our results showed the combined toxicity intensity was closely related to Mn concentration and the toxicity exhibited by Mn gradually strengthened with the decrease of pH level. High concentration of Mn stress significantly reduced plants leaf area, final leaf number, photosynthetic pigment content, RGR (relative growth rate) and biomass accumulation, but significantly increased the contents of malondialdehyde (MDA) and hydrogen peroxide (H₂O₂). At the same time, V. natans plants can resist the adverse stress by activating the antioxidant defense system, including superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), and polyphenol oxidase (PPO) activities. Besides, V. natans tended to adjust the biomass allocation strategy and transferred more energy to the subsurface and the ramets and stolons parts under the combined stress. This experiment also showed that the increasing pH within a certain range could largely improve the removal rate of Mn (at highest by 84.28%). This may indicate the V. natans plant species can act as a promising tool for the Mn phytoremediation in aquatic environments which needs to be further explored by longer cycle field studies.

Manganese Toxicity in Sugarcane Plantlets Grown on Acidic Soils of Southern China

Ratoon sugarcane plantlets in southern China have suffered a serious chlorosis problem in recent years. To reveal the causes of chlorosis, plant nutrition in chlorotic sugarcane plantlets and the role of manganese (Mn) in this condition were investigated. The study results showed that the pH of soils growing chlorotic plantlets ranged from 3.74 to 4.84. The symptoms of chlorosis were similar to those of iron (Fe) deficiency while the chlorotic and non-chlorotic plantlets contained similar amount of Fe. Chlorotic plantlets had 6.4-times more Mn in their leaf tissues compared to the control plants. There was a significantly positive correlation between Mn concentration in the leaves and the exchangeable Mn concentration in the soils. Moreover, leaf Mn concentration was related to both seasonal changes in leaf chlorophyll concentration and to the occurrence of chlorosis. Basal stalks of mature sugarcanes contained up to 564.36 mg·kg(-1) DW Mn. Excess Mn in the parent stalks resulted in a depress of chlorophyll concentration in the leaves of sugarcanes as indicated by lower chlorophyll concentration in the leaves of plantlets emerged from basal stalks. Ratoon sugarcane plantlets were susceptible to chlorosis due to high Mn accumulation in their leaves (456.90-1626.95 mg·kg(-1) DW), while in planted canes chlorosis did not occur because of low Mn accumulation (94.64-313.41mg·kg(-1) DW). On the other hand, active Fe content in chlorotic plantlets (3.39 mg kg(-1) FW) was only equivalent to 28.2% of the concentration found in the control. These results indicate that chlorosis in ratoon sugarcane plantlets results from excessive Mn accumulated in parent stalks of planted cane sugarcanes grown on excessive Mn acidic soils, while active Fe deficiency in plantlets may play a secondary role in the chlorosis.

Cadmium removal by Euglena gracilis is enhanced under anaerobic growth conditions

The facultative protist Euglena gracilis, a heavy metal hyper-accumulator, was grown under photo-heterotrophic and extreme conditions (acidic pH, anaerobiosis and with Cd(2+)) and biochemically characterized. High biomass (8.5×10(6)cellsmL(-1)) was reached after 10 days of culture. Under anaerobiosis, photosynthetic activity built up a microaerophilic environment of 0.7% O₂, which was sufficient to allow mitochondrial respiratory activity: glutamate and malate were fully consumed, whereas 25-33% of the added glucose was consumed. In anaerobic cells, photosynthesis but not respiration was activated by Cd(2+) which induced higher oxidative stress. Malondialdehyde (MDA) levels were 20 times lower in control cells under anaerobiosis than in aerobiosis, although Cd(2+) induced a higher MDA production. Cd(2+) stress induced increased contents of chelating thiols (cysteine, glutathione and phytochelatins) and polyphosphate. Biosorption (90%) and intracellular accumulation (30%) were the mechanisms by which anaerobic cells removed Cd(2+) from medium, which was 36% higher versus aerobic cells. The present study indicated that E. gracilis has the ability to remove Cd(2+) under anaerobic conditions, which might be advantageous for metal removal in sediments from polluted water bodies or bioreactors, where the O₂ concentration is particularly low.

Zn-bis-glutathionate is the best co-substrate of the monomeric phytochelatin synthase from the photosynthetic heavy metal-hyperaccumulator Euglena gracilis

The phytochelatin synthase from photosynthetic Euglena gracilis (EgPCS) was analyzed at the transcriptional, kinetic, functional, and phylogenetic levels. Recombinant EgPCS was a monomeric enzyme able to synthesize, in the presence of Zn(2+) or Cd(2+), phytochelatin2-phytochelatin4 (PC2-PC4) using GSH or S-methyl-GS (S-methyl-glutathione), but not γ-glutamylcysteine or PC2 as a substrate. Kinetic analysis of EgPCS firmly established a two-substrate reaction mechanism for PC2 synthesis with Km values of 14-22 mM for GSH and 1.6-2.5 μM for metal-bis-glutathionate (Me-GS2). EgPCS showed the highest Vmax and catalytic efficiency with Zn-(GS)2, and was inactivated by peroxides. The EgPCS N-terminal domain showed high similarity to that of other PCSases, in which the typical catalytic core (Cys-70, His-179 and Asp-197) was identified. In contrast, the C-terminal domain showed no similarity to other PCSases. An EgPCS mutant comprising only the N-terminal 235 amino acid residues was inactive, suggesting that the C-terminal domain is essential for activity/stability. EgPCS transcription in Euglena cells was not modified by Cd(2+), whereas its heterologous expression in ycf-1 yeast cells provided resistance to Cd(2+) stress. Phylogenetic analysis of the N-terminal domain showed that EgPCS is distant from plants and other photosynthetic organisms, suggesting that it evolved independently. Although EgPCS showed typical features of PCSases (constitutive expression; conserved N-terminal domain; kinetic mechanism), it also exhibited distinct characteristics such as preference for Zn-(GS)2 over Cd-(GS)2 as a co-substrate, a monomeric structure, and ability to solely synthesize short-chain PCs, which may be involved in conferring enhanced heavy-metal resistance.

Comparison of photosynthesis recovery dynamics in floating leaves of Trapa natans after inhibition by manganese or molybdenum: effects on Photosystem II

The aquatic plant Trapa natans L. is highly resistant to Mn and moderately resistant to Mo, mainly thanks to its ability to sequestrate the metals by chelation in the vacuole. Excess of Mn and Mo causes somewhat aspecific toxicity symptoms in plants, but the main target of their toxicity seems to be the photosynthetic process. In this work, we aimed at understanding how the effect on photosynthesis caused by Mn (130 μM, full recovery) or Mo (50 μM, partial recovery) in T. natans is linked to changes occurring in the photosynthetic apparatus, with emphasis on Photosystem II (PSII), during a 10 day treatment with these metals. The time-course of net photosynthesis, photosynthetic pigment content, amount of PSII and its peripheral antenna LHCII, and room-temperature fluorescence emission ratios F694/F680 and F700/(F685 + F695) showed that the early inhibiting effect of Mo and Mn (one day exposure) was essentially non-specific with respect to the metal, though more marked in Mo- than in Mn-treated plants. During the subsequent recovery phase, Mo still impaired PSII assembly and, consequently, photosynthesis could not reach the control values. Conversely, in Mn-treated plants the amount of PSII was fully re-established, as was photosynthesis, but the metal induced the accumulation of LHCII. The extent of inhibition and the effectiveness of photosynthesis recovery are proposed to reflect the different ability of T. natans to sequestrate safely excess Mn or Mo in vacuoles.

Morphophysiological analyses of Neochloris oleoabundans (Chlorophyta) grown mixotrophically in a carbon-rich waste product

Neochloris oleoabundans is considered one of the most promising oil-rich microalgae because of its ability to store lipids under nitrogen starvation. However, high biomass densities, required for applications on medium to large scale, are not reached in this condition of growth. As previous studies on other microalgae have shown that mixotrophy allows to obtain higher biomass in comparison to autotrophic cultures, we performed morphophysiological analyses in order to test the mixotrophic growth capability of N. oleoabundans. A carbon-rich manure derived from the apple vinegar production (AWP) was added to the medium. Cells were also cultivated under nutrient starvation (tap water), to observe the expected lipids accumulation, and combining AWP to water, to test the potential of this waste in a low-cost culture system. The results highlighted that AWP in the medium allowed to obtain the highest final cell density. Moreover, starch granules were stored inside chloroplast at the beginning of the experiment. The presence of AWP did not induce variations on light harvesting complex II (LHCII)-photosystem II (PSII) assembly, even if an interesting promotion of pigment synthesis in cells was observed. On the other hand, in starved cells, chloroplast degeneration, pigment content decrease, altered LHCII-PSII assembly and accumulation of high amount of lipid globules were observed, irrespective of the presence of AWP. The results suggest that mixotrophy promotes growth in N. oleoabundans and open up the possibility of using waste products from agri-food industries for this purpose. After growth, cells could be transferred under nutrient starvation to induce lipid accumulation.

Manganese uptake and interactions with cadmium in the hyperaccumulator--Phytolacca Americana L

In the present study, the accumulation of Mn and other metals by Phytolacca Americana L. from contaminated soils in Hunan Province, South China, was investigated. Results showed that the average concentrations of Mn in the leaves and roots reached 2198 and 80.4 mg kg(-1) (dry weight), respectively, with a maximum 13,400 mg kg(-1) in the leaves. A significant correlation was found between Mn concentrations in the plant leaves and those in the corresponding soils. Hydroponic experiments were also conducted to study the Cd uptake ability and interactions between Mn and Cd in the plant. It was found that P. americana hyperaccumulated not only Mn, but also Cd in the leaves. In the presence of Cd, adding Mn to the solution significantly improved the plant growth and reduced the concentrations of Cd in all organs of the plant.

Responses of Trapa natans L. floating laminae to high concentrations of manganese

The present study focuses on the responses of floating laminae of the Mn-tolerant hydrophyte Trapa natans L. to 1 mM Mn and their ability to accumulate the metal. Studies were carried out first on young floating laminae belonging to the second verticil of 30-day-old plants which originated from fruits that had been maintained in a 1 mM Mn-treated environment and again on the young floating laminae after 10 days of further treatment with 1 mM Mn. Mn storing was observed from the first days after germination, but only 10-day-treated laminae showed the capability to hyperaccumulate the element inside specialised cells (>20000 microg/g [dry weight]). Electron microscopy and the Folin-Ciocalteu reaction for phenolics revealed deposits of chelated material inside vacuoles of the first palisade layer and of idioblasts in the spongy tissue. X-ray microanalysis indicated that the deposits were Mn chelated with phenolic compounds. Numerous trichomes were observed at the lower epidermis of 10-day-treated laminae. They were rich in phenolics and characterised by Mn concretions at their base. As they are associated with a high concentration of the metal in culture water and sediments, trichomes may constitute a morphological differentiation for the secretion of Mn-chelating molecules into the culture water, as a probable "avoidance" mechanism. Finally, monitoring of the photosynthetic apparatus showed that photosynthetic function was not impaired, though differences in development occurred.

Phytochelatin-cadmium-sulfide high-molecular-mass complexes of Euglena gracilis

High-molecular-mass PC complexes (PC-HMWCs) constituted by phytochelatins (PCs), cadmium and sulfide are synthesized by several organisms after exposure to cadmium. In this study, PC-HMWCs were isolated from photoheterotrophic Euglena gracilis and purified to homogeneity, resulting in compounds of molecular mass 50-380 kDa depending on the CdCl2 and sulfate concentrations in the culture medium. In contrast with plants and some yeasts, PC-HMWCs from E. gracilis mainly comprise (57-75%) monothiol molecules (Cys, gamma-glutamylcysteine, GSH) and, to a lesser extent (25-43%), PCs. A similar acid-soluble thiol compound composition was found in whole cell extracts. The -SH/Cd2+ and S2-/Cd2+ ratios found in purified PC-HMWCs were 1.5 and 1.8, respectively; the (-SH + S2-)/Cd2+ ratio was 3.2. PC-HMWCs of molecular mass 60 and 100 kDa were also localized inside Percoll-purified chloroplasts, in which cadmium and PCs were mainly compartmentalized. Cadmium and sulfur-rich clusters with similar sulfur/cadmium stoichiometries to those of the purified PC-HMWCs were detected in the chloroplast and throughout the cell by energy dispersive microanalysis and atomic resolution electron microscopy. The presence of PC-HMWCs in primitive photosynthetic eukaryotes such as the protist, E. gracilis, suggests that their function as the final cadmium-storage-inactivation process is widespread. Their particular intracellular localization suggests that chloroplasts may play a major role in the cadmium-resistance mechanism in organisms lacking a plant-like vacuole.

Modulations of the thylakoid system in snow xanthophycean alga cultured in the dark for two months: comparison between microspectrofluorimetric responses and morphological aspects

The response of the plastid was studied, with a special emphasis on thylakoid structure and function, in a snow filamentous xanthophycean alga (Xanthonema sp.) incubated in darkness for two months. Microspectrofluorimetric analyses were performed on single living cells to study the variations in the assembly of the chlorophyll-protein complexes of photosystem II, in comparison with cells grown in light. In parallel, changes in micro- and submicroscopic plastid morphology and in photosynthetic pigment content were monitored. Throughout the experiment, the lamellar architecture of thylakoids in the alga was relatively well preserved, whereas photosystem II underwent disassembly and degradation triggered by prolonged darkness. Conversely, the light-harvesting complex of photosystem II proved to be relatively stable for long periods in darkness. Moreover, a role of the peripheral antennae in determining thylakoid arrangement in xanthophycean algae is implied. Although the responses observed in Xanthonema sp. can be considered in terms of acclimation to darkness, the progressive destabilisation of the light-harvesting complex of photosystem II testifies to incipient ageing of the cells after 35 days.