Comparison of three Chlamydomonas strains which show distinctive oxidative stress tolerance

Herbicides Tolerance in a Pseudomonas Strain Is Associated With Metabolic Plasticity of Antioxidative Enzymes Regardless of Selection

Agriculture uses many food production chains, and herbicides participate in this process by eliminating weeds through different biochemical strategies. However, herbicides can affect non-target organisms such as bacteria, which can suffer damage if there is no efficient control of reactive oxygen species. It is not clear, according to the literature, whether the efficiency of this control needs to be selected by the presence of xenobiotics. Thus, the Pseudomonas sp. CMA 6.9 strain, collected from biofilms in an herbicide packaging washing tank, was selected for its tolerance to pesticides and analyzed for activities of different antioxidative enzymes against the herbicides Boral^®, absent at the isolation site, and Heat^®, present at the site; both herbicides have the same mode of action, the inhibition of the enzyme protoporphyrinogen oxidase. The strain showed tolerance to both herbicides in doses up to 45 times than those applied in agriculture. The toxicity of these herbicides, which is greater for Boral^®, was assessed by means of oxidative stress indicators, growth kinetics, viability, and amounts of peroxide and malondialdehyde. However, the studied strain showed two characteristic antioxidant response systems for each herbicide: glutathione-s-transferase acting to control malondialdehyde in treatments with Boral^®; and catalase, ascorbate peroxidase, and guaiacol peroxidase in the control of peroxide induced by Heat^®. It is possible that this modulation of the activity of different enzymes independent of previous selection characterizes a system of metabolic plasticity that may be more general in the adaptation of microorganisms in soil and water environments subjected to chemical contaminants. This is relevant to the impact of pesticides on the diversity and abundance of microbial species as well as a promising line of metabolic studies in microbial consortia for use in bioremediation.

Isolation and characterization of a stress-responsive gene encoding a CHRD domain-containing protein from a halotolerant green alga

The genetic basis of stress resistance in extremophilic microalgae is not well studied. In this study, a gene of unknown function, the cluster58 or CL58 gene, was identified from the halotolerant green alga Chlamydomonas W80 and characterized. The CL58 gene encodes a protein containing a domain of unknown function, the CHRD domain, and a putative secretory signaling sequence at its N-terminus. The levels of CL58 mRNA increased in response to high copper levels and low temperatures. When the CL58 gene was heterologously expressed as a fusion gene with the NanoLuc luciferase gene in Chlamydomonas reinhardtii, a majority of the NanoLuc activity was detected in the culture medium compared with that in the intracellular fraction. A mutagenic analysis revealed that the putative secretory signaling sequence was sufficient for the secretion of the CL58-NanoLuc fusion protein. In addition, we expressed the protein encoded by the CL58 gene in Escherichia coli; the recombinant, soluble protein was then purified. In summary, we identified a novel gene from C. W80 that appears to encode a stress-responsive, CHRD domain-containing secreted protein.

Effects of alginate oligomer on the expression of cell cycle- and stress-related genes in Chlamydomonas reinhardtii

Enzymatically prepared alginate oligomer (AO) promoted the growth of Chlamydomonas reinhardtii in a concentration-dependent manner. AO at 2.5 mg/mL induced increase in expression levels of cyclin A, cyclin B, and cyclin D in C. reinhardtii. CuSO₄ at 100 μM suppressed the growth of C. reinhardtiin, and AO at 2.5 mg/mL significantly alleviated the toxicity of CuSO₄. Increased intracellular reactive oxygen species level in C. reinhardtii induced by CuSO₄ was reduced by AO. After cultivation with CuSO₄ at 100 μM, expression levels of ascorbate peroxidase and superoxide dismutase in C. reinhardtii were increased, and AO reduced the increased levels of these enzymes. These results suggest that AO exhibits beneficial effects on C. reinhardtii through influencing the expression of various genes not only at normal growth condition but also under CuSO₄ stress.

Is chloroplastic class IIA aldolase a marine enzyme?

Expressed sequence tag analyses revealed that two marine Chlorophyceae green algae, Chlamydomonas sp. W80 and Chlamydomonas sp. HS5, contain genes coding for chloroplastic class IIA aldolase (fructose-1, 6-bisphosphate aldolase: FBA). These genes show robust monophyly with those of the marine Prasinophyceae algae genera Micromonas, Ostreococcus and Bathycoccus, indicating that the acquisition of this gene through horizontal gene transfer by an ancestor of the green algal lineage occurred prior to the divergence of the core chlorophytes (Chlorophyceae and Trebouxiophyceae) and the prasinophytes. The absence of this gene in some freshwater chlorophytes, such as Chlamydomonas reinhardtii, Volvox carteri, Chlorella vulgaris, Chlorella variabilis and Coccomyxa subellipsoidea, can therefore be explained by the loss of this gene somewhere in the evolutionary process. Our survey on the distribution of this gene in genomic and transcriptome databases suggests that this gene occurs almost exclusively in marine algae, with a few exceptions, and as such, we propose that chloroplastic class IIA FBA is a marine environment-adapted enzyme. This hypothesis was also experimentally tested using Chlamydomonas W80, for which we found that the transcript levels of this gene to be significantly lower under low-salt (that is, simulated terrestrial) conditions. Expression analyses of transcriptome data for two algae, Prymnesium parvum and Emiliania huxleyi, taken from the Sequence Read Archive database also indicated that the expression of this gene under terrestrial conditions (low NaCl and low sulfate) is significantly downregulated. Thus, these experimental and transcriptome data provide support for our hypothesis.

Antioxidant and HSP70B responses in Chlamydomonas reinhardtii genotypes with different resistance to oxidative stress

Today, the information from model species that differ in their resistance to oxidative stress and the determination of suitable plant markers for screening stress-resistant genotypes are essential for better understanding of plant stress responses and for selection. Here we aimed to assess the differences in antioxidant and HSP70B responses to paraquat treatment between genotypes susceptible and resistant to oxidative stress. Four genotypes of Chlamydomonas reinhardtii were chosen as a model of plant cells: two susceptible genotypes: wild type and paraquat-sensitive; and two paraquat-resistant genotypes: with high and moderate resistance. Varying responses to paraquat treatment were found depending on the genotype and paraquat concentrations. High paraquat concentrations (>50μM) were shown to be very stressful for all C. reinhardtii genotypes, leading to inhibition of enzyme activity. Only the paraquat-sensitive genotype responded to low-level paraquat treatment with a marked enhancement of SOD, CAT, GST activities. The lack of statistically significant response measured as SOD, CAT, GST activities in WT and resistant genotypes could be considered as an indication of absence of strong oxidative stress. This could relate to higher levels of endogenous SOD and CAT activities characteristic of moderately and highly paraquat-resistant genotypes. The response to lower paraquat concentrations evaluated as HSP70B accumulation was proportional to the level of genotype susceptibility to PQ. New evidence is provided that low-level oxidative stress impacts the antioxidant and HSP70B responses differently depending on the genotype resistance. In light of the still unresolved challenge for identification of reliable characters for screening of genotype resistance/susceptibility to oxidative stress, our study demonstrates that HSP70B accumulation could be used as an early marker for induced oxidative stress in the studied genotypes. The obtained results that the most pronounced differences in the antioxidant and HSP70B response were found between the two susceptible genotypes provoke us to convey future experiments with other susceptible genotypes.

Lack of isocitrate lyase in Chlamydomonas leads to changes in carbon metabolism and in the response to oxidative stress under mixotrophic growth

Isocitrate lyase is a key enzyme of the glyoxylate cycle. This cycle plays an essential role in cell growth on acetate, and is important for gluconeogenesis as it bypasses the two oxidative steps of the tricarboxylic acid (TCA) cycle in which CO₂ is evolved. In this paper, a null icl mutant of the green microalga Chlamydomonas reinhardtii is described. Our data show that isocitrate lyase is required for growth in darkness on acetate (heterotrophic conditions), as well as for efficient growth in the light when acetate is supplied (mixotrophic conditions). Under these latter conditions, reduced acetate assimilation and concomitant reduced respiration occur, and biomass composition analysis reveals an increase in total fatty acid content, including neutral lipids and free fatty acids. Quantitative proteomic analysis by ¹⁴N/¹⁵N labelling was performed, and more than 1600 proteins were identified. These analyses reveal a strong decrease in the amounts of enzymes of the glyoxylate cycle and gluconeogenesis in parallel with a shift of the TCA cycle towards amino acid synthesis, accompanied by an increase in free amino acids. The decrease of the glyoxylate cycle and gluconeogenesis, as well as the decrease in enzymes involved in β-oxidation of fatty acids in the icl mutant are probably major factors that contribute to remodelling of lipids in the icl mutant. These modifications are probably responsible for the elevation of the response to oxidative stress, with significantly augmented levels and activities of superoxide dismutase and ascorbate peroxidase, and increased resistance to paraquat.

Changes of metal-induced toxicity by H2O 2/NO modulators in Scenedesmus quadricauda (Chlorophyceae)

Effect of nitric oxide donor (sodium nitroprusside, SNP, 500 μM) or hydrogen peroxide scavenger (dithiothreitol, DTT, 500 μM) on cadmium (Cd) or copper (Cu) uptake (150 μM solutions) and toxicity using Scenedesmus quadricauda was studied. Combined treatments (Cd or Cu + DTT or SNP) usually ameliorated metal-induced toxicity at the level of pigments, proteins, and mineral nutrients in comparison with respective metal alone. Viability tests (MTT and TTC) showed the lowest values preferentially in Cu treatments, indicating higher toxicity in comparison with Cd. Cd showed low impact on amino acids while strong Cu-induced depletion was mitigated by DTT and SNP. Amount of ROS and NO showed the most pronounced responses in SNP variants being rather reciprocal than parallel and regulated ascorbate peroxidase activity. Blot gel analyses of hsp70 protein did not reveal extensive changes after given exposure period. Phenols were elevated by DTT alone while all Cu treatments revealed depletion. Total Cu content decreased while total Cd content increased in metal + SNP or metal + DTT. Subsequent experiment using lower Cd, SNP or DTT doses (10 and 100 μM) revealed concentration-dependent impact on Cd uptake. Overall, DTT was found to be more suitable for the amelioration of metal-induced toxicity.

Down-regulation of catalase activity allows transient accumulation of a hydrogen peroxide signal in Chlamydomonas reinhardtii

In photosynthetic organisms, excess light is a stress that induces production of reactive oxygen species inside the chloroplasts. As a response, the capacity of antioxidative defence mechanisms increases. However, when cells of Chlamydomonas reinhardtii were shifted from dark to high light, a reversible partial inactivation of catalase activity was observed, which correlated with a transient increase in the level of H2 O2 in the 10 μm range. This concentration range seems to be necessary to activate H2 O2 -dependent signalling pathways stimulating the expression of H2 O2 responsive genes, such as the heat shock protein HSP22C. Catalase knock-down mutants had lost the transient accumulation of H2 O2 , suggesting that a decrease in catalase activity was the key element for establishing a transient H2 O2 burst. Catalase was inactivated by a one-electron event consistent with the reduction of a single cysteine. We propose that under high light intensity, the redox state of the photosynthetic electron transport chain is sensed and transmitted to the cytosol to regulate the catalase activity. This allows a transient accumulation of H2 O2 , inducing a signalling event that is transmitted to the nucleus to modulate the expression of chloroplast-directed protection enzymes.

Phenotypic screening with oleaginous microalgae reveals modulators of lipid productivity

Here we describe the first phenotypic screening with microalgae to study lipid metabolism and to discover organic small molecules as chemical triggers that increase growth and lipid production. A microplate assay has been developed for analysis of intracellular lipids using Nile Red fluorescence in order to screen a collection of diverse bioactive organic molecules (e.g., kinase inhibitors) with four strains of oleaginous microalgae (Nannochloropsis salina, Nannochloropsis oculata, Nannochloris sp., and Phaeodactylum tricornutum). Several small molecules identified in microplate screening increased lipid productivity >200% without decreasing growth and biomass production. Selected compounds were further investigated in the context of larger batch culture experiments (e.g., 500 mL) and demonstrated to increase lipid levels (up to 84%) while maintaining or increasing the specific growth rate. Bioactive molecules such as forskolin and quinacrine were identified as promising probes of microalgae lipid pathways. We have also determined that common antioxidants such as epigallocatechin gallate and butylated hydroxyanisole (BHA) increase lipid productivity and may represent new probes of oxidative signaling pathways for photooxidative protection.

Multiple stressor effects of predation by rotifers and herbicide pollution on different Chlamydomonas strains and potential impacts on population dynamics

Environmental factors can interact with the effects of chemical pollutants on natural systems by inducing multiple stressor effects in individual organisms as well as by altering selection pressure on tolerant strains in heterogeneous populations. Predation is a stressful environmental factor relevant for many species. Therefore, the impact of predation by the rotifer Brachionus calyciflorus on tolerance of eight genetically different strains of the green alga Chlamydomonas reinhardtii to simultaneous exposure to each of the three herbicides (diuron, paraquat, and S-metolachlor) was tested. Interactions of combined stressors were analyzed based on the independent action model; additive, synergistic, and antagonistic effects of the combined exposure could be detected depending on the herbicide and strain tested. If cultures were acclimated (pre-exposed) to one stressor, tolerance to the second stressor could be increased. This indicates that physiological changes can induce cotolerance of predation-exposed algae to herbicides and of herbicide-treated algae to predation depending on the combination of stressors. The strain-specific differences in multiple stressor effects also changed the correlation of strains' tolerances to individual stressors determined during combined and single-stressor exposure. Changes in cotolerance to stressors affect selection pressure and population dynamics during long-term exposure. This shows that predation stress can have adverse effects on the toxicity of chemical pollutants to microalgae on the organism and population levels.