Origin and roles of a novel copper-zinc superoxide dismutase (CuZnSOD) gene from the harmful dinoflagellate Prorocentrum minimum

Two novel glutathione S-transferase (GST) genes in the toxic marine dinoflagellate Alexandrium pacificum and their transcriptional responses to environmental contaminants

The present study identified two novel glutathione S-transferase (GST) genes from the toxic dinoflagellate Alexandrium pacificum and examined their molecular characteristics and transcriptional responses to algicides and environmental contaminants. Bioinformatic analysis revealed that both ApGSTs are cytosolic, belonging to the chi-like class (ApGST1) and an undefined class (ApGST2). The overall expression of ApGSTs showed similar patterns depending on the exposed contaminants, while they were differently regulated by polychlorinated biphenyl (PCB). Copper treatments (CuCl2 and CuSO4) did not significantly induce the expression of ApGSTs. The highest up-regulations of ApGST1 and ApGST2 were under 6-h treatments of 0.10 and 0.50 mg L-1 NaOCl. Interestingly, only ApGST1 increased significantly after 0.10, 0.50, and 1.00 mg L-1 of PCB exposure (6 h). Intracellular reactive oxygen species (ROS) increased considerably under NaOCl; however, it was not significantly higher in the PCB-treated cells. GST activity was increased by NaOCl and PCB treatments, but only PCB caused apoptosis. These results suggest that GSTs are involved in the first line of phase II detoxification, protecting dinoflagellate cells against oxidative damage.

Two novel superoxide dismutase genes (CuZnSOD and MnSOD) in the toxic marine dinoflagellate Alexandrium pacificum and their differential responses to metal stressors

Superoxide dismutase (SOD) is an important antioxidant enzyme that is involved in the first line of defense against reactive oxygen species (ROS) within cells. Herein, we determined two novel CuZnSOD and MnSOD genes from the toxic marine dinoflagellate Alexandrium pacificum (designated as ApCuZnSOD and ApMnSOD) and characterized their structural features and phylogenetic affiliations. In addition, we examined the relative gene expression and ROS levels following exposure to heavy metals. ApCuZnSOD encoded 358 amino acids (aa) with two CuZnSOD-conserved domains. ApMnSOD encoded 203 aa that contained a mitochondrial-targeting signal and a MnSOD signature motif but missed an N-terminal domain. Phylogenetic trees showed that ApCuZnSOD clustered with other dinoflagellates, whereas ApMnSOD formed a clade with green algae and plants. Based on the 72-h median effective concentration (EC₅₀), A. pacificum showed toxic responses in the order of Cu, Ni, Cr, Zn, Cd, and Pb. SOD expression levels dramatically increased after 6 h of Pb (≥6.5 times) and 48 h of Cu treatment (≥3.9 times). These results are consistent with the significant increase in ROS production in the A. pacificum exposed to Pb and Cu. These suggest that the two ApSODs are involved in the antioxidant defense system but respond differentially to individual metals.

Physiological and transcriptomic responses of Chlorella sorokiniana to ciprofloxacin reveal molecular mechanisms for antibiotic removal

Microalgae-based technology is an effective and environmentally friendly method for antibiotics-contaminated wastewater treatment. To assess the tolerance and removal ability of Chlorella sorokiniana to ciprofloxacin (CIP), this study comprehensively revealed the responses of C. sorokiniana to CIP exposure and its degradation processes through physiological and transcriptomic analyses. Although the photosynthetic system was inhibited, the growth of C. sorokiniana was not negatively affected by CIP. Dissolved organic matter was analyzed and indicated that humic-like substances were released to alleviate the stress of CIP. In addition, the maximum removal of CIP was 83.3% under 20 mg L⁻¹ CIP exposure. HPLC-MS/MS and RNA-Seq analyses suggested that CIP could be bioaccumulated and biodegraded by C. sorokiniana through the reactions of hydroxylation, demethylation, ring cleavage, oxidation, dehydrogenation, and decarboxylation with the help of intracellular oxidoreductases, especially cytochrome P450. Collectively, this research shows that C. sorokiniana have a great potential for removing CIP from wastewater.

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Freshwater microalgae Chlorella sorokiniana was exposed to ciprofloxacin

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Microalgae released humic-like substances to alleviate the damage

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The maximum removal of ciprofloxacin was 83.3% under 20 mg L⁻¹ CIP exposure

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Ciprofloxacin could be biodegraded with the help of intracellular oxidoreductases

Molecular cloning and oxidative-stress responses of a novel Phi class glutathione S-transferase (GSTF) gene in the freshwater algae Closterium ehrenbergii

Glutathione S-transferases (GSTs) belong to a family of enzymes involved in diverse biological processes, including detoxification and protection against oxidative damage. Here, we determined the full-length sequence (915 bp) of a novel Phi class cytosolic glutathione S-transferase (GSTF) gene from the green algae Closterium ehrenbergii. We examined the gene structure and expression patterns in response to metals and endocrine disrupting chemicals (EDCs). It was significantly upregulated by metals, but responded differently to EDCs. The highest up-regulation of CeGSTF was registered under 0.1 mg/L CuCl₂ and 0.01 mg/L CuSO₄ treatments. In a 72-h course experiment with treatment of 0.1 mg/L CuCl₂ , CeGSTF was dramatically induced at 6 h, and then gradually decreased with increasing exposure time. This was consistent with the increase in both GST activity and ROS production in copper-treated cells. These results suggest that CeGSTF may be involved in detoxification mechanisms associated with oxidative stress in green algae.

Heavy metal-induced stress in eukaryotic algae-mechanisms of heavy metal toxicity and tolerance with particular emphasis on oxidative stress in exposed cells and the role of antioxidant response

Heavy metals is a collective term describing metals and metalloids with a density higher than 5 g/cm3. Some of them are essential micronutrients; others do not play a positive role in living organisms. Increased anthropogenic emissions of heavy metal ions pose a serious threat to water and land ecosystems. The mechanism of heavy metal toxicity predominantly depends on (1) their high affinity to thiol groups, (2) spatial similarity to biochemical functional groups, (3) competition with essential metal cations, (4) and induction of oxidative stress. The antioxidant response is therefore crucial for providing tolerance to heavy metal-induced stress. This review aims to summarize the knowledge of heavy metal toxicity, oxidative stress and antioxidant response in eukaryotic algae. Types of ROS, their formation sites in photosynthetic cells, and the damage they cause to the cellular components are described at the beginning. Furthermore, heavy metals are characterized in more detail, including their chemical properties, roles they play in living cells, sources of contamination, biochemical mechanisms of toxicity, and stress symptoms. The following subchapters contain the description of low-molecular-weight antioxidants and ROS-detoxifying enzymes, their properties, cellular localization, and the occurrence in algae belonging to different clades, as well as the summary of the results of the experiments concerning antioxidant response in heavy metal-treated eukaryotic algae. Other mechanisms providing tolerance to metal ions are briefly outlined at the end.

Identification of a Metacaspase Gene in the Bloom-Forming Dinoflagellate Prorocentrum minimum and its Putative Function Involved in Programmed Cell Death

Programmed cell death (PCD) in dinoflagellates has been introduced as a new concept that facilitates the demise of harmful algal blooms. Metacaspases (MCAs) play a role in PCD, but their function in dinoflagellates is unclear. Here, we cloned a novel MCA gene (PmMCA) from the harmful dinoflagellate Prorocentrum minimum and examined its molecular characteristics and gene expression during cell death. The gene was encoded in the nuclear genome with two introns. The putative protein contained 288 amino acids and three conserved MCA signature motifs. Phylogenetic analysis showed that PmMCA may have the same ancestor as other dinoflagellates. PmMCA expression and cell apoptosis were significantly induced under copper exposure, considerably affecting cell growth. These results suggest that PmMCA could be involved in PCD triggered by copper stress.

Horizontal Gene Transfer Involving Chloroplasts

Horizontal gene transfer (HGT)- is defined as the acquisition of genetic material from another organism. However, recent findings indicate a possible role of HGT in the acquisition of traits with adaptive significance, suggesting that HGT is an important driving force in the evolution of eukaryotes as well as prokaryotes. It has been noted that, in eukaryotes, HGT is more prevalent than originally thought. Mitochondria and chloroplasts lost a large number of genes after their respective endosymbiotic events occurred. Even after this major content loss, organelle genomes still continue to lose their own genes. Many of these are subsequently acquired by intracellular gene transfer from the original plastid. The aim of our review was to elucidate the role of chloroplasts in the transfer of genes. This review also explores gene transfer involving mitochondrial and nuclear genomes, though recent studies indicate that chloroplast genomes are far more active in HGT as compared to these other two DNA-containing cellular compartments.

Phylogeny and evolution of chloroplast tRNAs in Adoxaceae

Chloroplasts are semiautonomous organelles found in photosynthetic plants. The major functions of chloroplasts include photosynthesis and carbon fixation, which are mainly regulated by its circular genomes. In the highly conserved chloroplast genome, the chloroplast transfer RNA genes (cp tRNA) play important roles in protein translation within chloroplasts. However, the evolution of cp tRNAs remains unclear. Thus, in the present study, we investigated the evolutionary characteristics of chloroplast tRNAs in five Adoxaceae species using 185 tRNA gene sequences. In total, 37 tRNAs encoding 28 anticodons are found in the chloroplast genome in Adoxaceae species. Some consensus sequences are found within the Ψ-stem and anticodon loop of the tRNAs. Some putative novel structures were also identified, including a new stem located in the variable region of tRNATyr in a similar manner to the anticodon stem. Furthermore, phylogenetic and evolutionary analyses indicated that synonymous tRNAs may have evolved from multiple ancestors and frequent tRNA duplications during the evolutionary process may have been primarily caused by positive selection and adaptive evolution. The transition and transversion rates are uneven among different tRNA isotypes. For all tRNAs, the transition rate is greater with a transition/transversion bias of 3.13. Phylogenetic analysis of cp tRNA suggested that the type I introns in different taxa (including eukaryote organisms and cyanobacteria) share the conserved sequences "U-U-x2-C" and "U-x-G-x2-T," thereby indicating the diverse cyanobacterial origins of organelles. This detailed study of cp tRNAs in Adoxaceae may facilitate further investigations of the evolution, phylogeny, structure, and related functions of chloroplast tRNAs.

Transcriptome survey and toxin measurements reveal evolutionary modification and loss of saxitoxin biosynthesis genes in the dinoflagellates Amphidinium carterae and Prorocentrum micans

In the present study, we characterized the potential toxin genes for polyketide synthase (PKS) and saxitoxin (STX) biosynthesis using the transcriptomes of two non-STX producing dinoflagellates Amphidinium carterae and Prorocentrum micans. RNA sequencing revealed 94 and 166 PKS contigs in A. carterae and P. micans, respectively. We first detected type III PKS, which was closely related to bacteria. In addition, dozens of homologs of 20 STX biosynthesis genes were identified. Interestingly, the core STX-synthesizing genes sxtA and sxtB were only found in P. micans, whereas sxtD was detected in A. carterae alone. Bioinformatic analysis showed that the first two core genes (sxtA and sxtG) had a low sequence similarity (37.0-67.6%) and different domain organization compared to those of other toxigenic dinoflagellates, such as Alexandrium pacificum. These might result in the breakdown of the initial reactions in STX production and ultimately the loss of the ability to synthesize the toxins in both dinoflagellates. Our findings suggest that toxin-related PKS and sxt genes are commonly found in non-STX producing dinoflagellates. In addition to their involvement in the synthesis of toxins, our result indicates that genes may also have other molecular metabolic functions.

The herbicide alachlor severely affects photosystem function and photosynthetic gene expression in the marine dinoflagellate Prorocentrum minimum

Alachlor is one of the most widely used herbicides and can remain in agricultural soils and wastewater. The toxicity of alachlor to marine life has been rarely studied; therefore, we evaluated the physiological and transcriptional responses in the marine dinoflagellate Prorocentrum minimum. The herbicide led to considerable decreases in P. minimum cell numbers and pigment contents. The EC₅₀ was determined to be 0.373 mg/L. Photosynthesis efficiency and chlorophyll autofluorescence dramatically decreased with increasing alachlor dose and exposure time. Real-time PCR analysis showed that the photosynthesis-related genes PmpsbA, PmatpB, and PmrbcL were induced the most by alachlor; the transcriptional level of each gene varied with time. PmrbcL expression increased after 30 min of alachlor treatment, whereas PmatpB and PmpsbA increased after 24 h. The PmpsbA expression level was highest (5.0 times compared to control) after 6 h of alachlor treatment. There was no significant change in PmpsaA expression with varying treatment time or concentration. Additionally, there was no notable change in the expression of antioxidant genes PmGST and PmKatG, or in ROS accumulation. These suggest that alachlor may affect microalgal photosystem function, with little oxidative stress, causing severe physiological damage to the cells, and even cell death.

Molecular characterization and expression analysis of copper-zinc superoxide dismutases from the freshwater alga Closterium ehrenbergii under metal stress

Superoxide dismutase (SOD) acts as the first line of defense against reactive oxygen species (ROS) within cells. In the present study, we determined two novel CuZnSOD genes (designated as CeCSD1 and CeCSD2) from the toxicity-testing freshwater algae Closterium ehrenbergii and examined their structural features, phylogenetic relationships, and gene expression under exposure to different metals. Putative CeCSD1 (204 aa, 20.6 kDa) and CeCSD2 (155 aa, 15.3 kDa) proteins had conserved CuZnSOD family motifs and metal (Cu, Zn) binding sites, but different N-terminus structures, that is, CeCSD1 has a signal peptide to chloroplasts. Phylogenetic analysis of each protein revealed that C. ehrenbergii was well clustered with other green algae and plants. Real-time PCR results showed that the gene expression obviously increased with heavy metal exposure. In addition, excess copper considerably increased the SOD activity and ROS generation but decreased the photosynthetic efficiency in treated cells. These results suggest that CeCSDs are involved in the antioxidant defense system and can be regarded as potential biomarkers for monitoring metal contaminants in aquatic environments.

Yellow clay modulates carbohydrate and glutathione responses in the harmful dinoflagellate Cochlodinium polykrikoides and leads to sedimentation

The marine dinoflagellate Cochlodinium polykrikoides is a harmful algal bloom (HAB) species that severely impacts the environment and causes huge economic losses. Yellow clay (YC), considered to be a non-toxic and naturally-occurring material, represents an important step towards the direct control of HABs. In the present study, we evaluated the physiological and biochemical effects of YC on C. polykrikoides after exposures of up to 72 h. We observed little physiological changes in growth rate, chlorophyll a, lipid peroxidation, antioxidant enzymatic activities of superoxide dismutase and catalase, and activity of alkaline phosphatase after exposure to YC. Interestingly, YC significantly increased total carbohydrate and glutathione levels, affecting the physiology of the cells. These results indicate that total carbohydrate content may play an important role in cell-clay aggregation and it could be the main underlying mechanism that mitigates HAB cells via sedimentation.

Molecular cloning and oxidative-stress responses of a novel manganese superoxide dismutase (MnSOD) gene in the dinoflagellate Prorocentrum minimum

Dinoflagellate algae are microeukaryotes that have distinct genomes and gene regulation systems, making them an interesting model for studying protist evolution and genomics. In the present study, we discovered a novel manganese superoxide dismutase (PmMnSOD) gene from the marine dinoflagellate Prorocentrum minimum, examined its molecular characteristics, and evaluated its transcriptional responses to the oxidative stress-inducing contaminants, CuSO₄ and NaOCl. Its cDNA was 1238 bp and contained a dinoflagellate spliced leader sequence, a 906 bp open reading frame (301 amino acids), and a poly (A) tail. The gene was coded on the nuclear genome with one 174 bp intron; signal peptide analysis showed that it might be localized to the mitochondria. Real-time PCR analysis revealed an increase in gene expression of MnSOD and SOD activity when P. minimum cells were separately exposed to CuSO₄ and NaOCl. In addition, both contaminants considerably decreased chlorophyll autofluorescence, and increased intracellular reactive oxygen species. These results suggest that dinoflagellate MnSOD may be involved in protecting cells against oxidative damage.