Basis of genetic adaptation to heavy metal stress in the acidophilic green alga Chlamydomonas acidophila

The cadmium tolerance and bioaccumulation mechanism of Tetratostichococcus sp. P1: insight from transcriptomics analysis

Cadmium (Cd) has become a severe issue in relatively low concentration and attracts expert attention due to its toxicity, accumulation, and biomagnification in living organisms. Cd does not have a biological role and causes serious health issues. Therefore, Cd pollutants should be reduced and removed from the environment. Microalgae have great potential for Cd absorption for waste treatment since they are more environmentally friendly than existing treatment methods and have strong metal sorption selectivity. This study evaluated the tolerance and ability of the microalga Tetratostichococcus sp. P1 to remove Cd ions under acidic conditions and reveal mechanisms based on transcriptomics analysis. The results showed that Tetratostichococcus sp. P1 had a high Cd tolerance that survived under the presence of Cd up to 100 µM, and IC50, the half-maximal inhibitory concentration value, was 57.0 μM, calculated from the change in growth rate based on the chlorophyll content. Long-term Cd exposure affected the algal morphology and photosynthetic pigments of the alga. Tetratostichococcus sp. P1 removed Cd with a maximum uptake of 1.55 mg g-1 dry weight. Transcriptomic analysis revealed the upregulation of the expression of genes related to metal binding, such as metallothionein. Group A, Group B transporters and glutathione, were also found upregulated. While the downregulation of the genes were related to photosynthesis, mitochondria electron transport, ABC-2 transporter, polysaccharide metabolic process, and cell division. This research is the first study on heavy metal bioremediation using Tetratostichococcus sp. P1 and provides a new potential microalga strain for heavy metal removal in wastewater.[Figure: see text]Abbreviations:BP: Biological process; bZIP: Basic Leucine Zipper; CC: Cellular component; ccc1: Ca (II)-sensitive cross complementary 1; Cd: Cadmium; CDF: Cation diffusion facilitator; Chl: Chlorophyll; CTR: Cu TRansporter families; DAGs: Directed acyclic graphs; DEGs: Differentially expressed genes; DVR: Divinyl chlorophyllide, an 8-vinyl-reductase; FPN: FerroportinN; FTIR: Fourier transform infrared; FTR: Fe TRansporter; GO: Gene Ontology; IC50: Growth half maximal inhibitory concentration; ICP: Inductively coupled plasma; MF: molecular function; NRAMPs: Natural resistance-associated aacrophage proteins; OD: Optical density; RPKM: Reads Per Kilobase of Exon Per Million Reads Mapped; VIT1: Vacuolar iron transporter 1 families; ZIPs: Zrt-, Irt-like proteins.

Photosynthetic response of Chlamydomonas reinhardtii and Chlamydomonas sp. 1710 to zinc toxicity

Zinc (Zn) is an essential trace element but can lead to water contamination and ecological deterioration when present in excessive amounts. Therefore, investigating the photosynthetic response of microalgae to Zn stress is of great significance. In this study, we assessed the photosynthetic responses of neutrophilic Chlamydomonas reinhardtii and acidophilic Chlamydomonas sp. 1710 to Zn exposure for 96 h. The specific growth rate (μ), chlorophyll-a (Chl-a) content, and chlorophyll fluorescence parameters were determined. The results demonstrated that Chlamydomonas sp. 1710 was much more tolerant to Zn than C. reinhardtii, with the half-maximal inhibitory concentration (IC50) values of 225.4 mg/L and 23.4  mg/L, respectively. The μ and Chl-a content of C. reinhardtii decreased in the presence of 15  mg/L Zn, whereas those of Chlamydomonas sp. 1710 were unaffected by as high as 100  mg/L Zn. Chlorophyll fluorescence parameters indicated that the regulation of energy dissipation, including non-photochemical quenching, played a crucial role in Zn stress resistance for both Chlamydomonas strains. However, in the case of C. reinhardtii, non-photochemical quenching was inhibited by 5  mg/L Zn in the first 48 h, whereas for Chlamydomonas sp. 1710, it remained unaffected under 100  mg/L Zn. Chlamydomonas sp. 1710 also exhibited a 20 times stronger capacity for regulating the electron transfer rate than C. reinhardtii under Zn stress. The light energy utilization efficiency (α) of Chlamydomonas sp. 1710 had the most highly non-linear correlation with μ, indicating the energy utilization and regulation process of Chlamydomonas sp. 1710 was well protected under Zn stress. Collectively, our findings demonstrate that the photosystem of Chlamydomonas sp. 1710 is much more resilient and tolerant than that of C. reinhardtii under Zn stress.

Mechanistic transcriptome comprehension of Chlamydomonas reinhardtii subjected to black phosphorus

Two-dimensional materials have recently gained significant awareness. A representative of such materials, black phosphorous (BP), earned attention based on its comprehensive application potential. The presented study focuses on the mode of cellular response underlying the BP interaction with Chlamydomonas reinhardtii as an algal model organism. We observed noticeable ROS formation and changes in outer cellular topology after 72 h of incubation at 5 mg/L BP. Transcriptome profiling was employed to examine C. reinhardtii response after exposure to 25 mg/L BP for a deeper understanding of the associated processes. The RNA sequencing has revealed a comprehensive response with abundant transcript downregulation. The mode of action was attributed to cell wall disruption, ROS elevation, and chloroplast disturbance. Besides many other dysregulated genes, the cell response involved the downregulation of GH9 and gametolysin within a cell wall, pointing to a shift to discrete manipulation with resources. The response also included altered expression of the PRDA1 gene associated with redox governance in chloroplasts implying ROS disharmony. Altered expression of the Cre-miR906-3p, Cre-miR910, and Cre-miR914 pointed to those as potential markers in stress response studies.

Improvements in Bioremediation Agents and Their Modified Strains in Mediating Environmental Pollution

Environmental pollution has been a significant concern around the globe as the release of toxic pollutants is associated with carcinogenic, mutagenic, and teratogenic impacts on living organisms. Since microorganisms have the natural potential to degrade toxic metabolites into nontoxic forms, an eco-friendly approach known as bioremediation has been used to tackle toxic-induced pollution. Bioremediation has three fundamental levels, i.e., natural attenuation, bio-augmentation, and biostimulation in which the synthetic biology approach has been lately utilized to enhance the conventional bioremediation techniques. Recently, a more advanced approach of programmable nucleases such as zinc finger nucleases, tale-like effector nucleases, and clustered regularly interspaced short palindromic repeats Cas is being employed to engineer several bacterial, fungal, and algal strains for targeted mutagenesis by knocking in and out specific genes which are involved in reconstructing the metabolic pathways of native microbes. These genetically engineered microorganisms possess heavy metal resistance, greater substrate range, enhanced enzymatic activity, and binding affinity which accelerate the biodegradation of toxic pollutants to environmentally safe levels. This review provides a comprehensive understanding of how we can correlate the novel genetics-based approaches employed to produce genetically engineered microorganisms to enhance the biodegradation of hazardous pollutants, hence, developing a clean and sustainable ecosystem.

Molecular Toxicity Mechanism Induced by the Antibacterial Agent Triclosan in Freshwater Euglena gracilis Based on the Transcriptome

Triclosan (TCS), a commonly used antibacterial preservative, has been demonstrated to have high toxicological potential and adversely affects the water bodies. Since algae are one of the most significant primary producers on the planet, understanding the toxicological processes of TCS is critical for determining its risk in aquatic ecosystems and managing the water environment. The physiological and transcriptome changes in Euglena gracilis were studied in this study after 7 days of TCS treatment. A distinct inhibition ratio for the photosynthetic pigment content in E. gracilis was observed from 2.64% to 37.42% at 0.3-1.2 mg/L, with TCS inhibiting photosynthesis and growth of the algae by up to 38.62%. Superoxide dismutase and glutathione reductase significantly changed after exposure to TCS, compared to the control, indicating that the cellular antioxidant defense responses were induced. Based on transcriptomics, the differentially expressed genes were mainly enriched in biological processes involved in metabolism pathways and microbial metabolism in diverse environments. Integrating transcriptomics and biochemical indicators found that changed reactive oxygen species and antioxidant enzyme activities stimulating algal cell damage and the inhibition of metabolic pathways controlled by the down-regulation of differentially expressed genes were the main toxic mechanisms of TCS exposure to E. gracilis. These findings establish the groundwork for future research into the molecular toxicity to microalgae induced by aquatic pollutants, as well as provide fundamental data and recommendations for TCS ecological risk assessment.

Transcriptomic hallmarks of in vitro TiO2 nanotubes toxicity in Chlamydomonas reinhardtii

Recently, more accessible transcriptomic approaches have provided a new and deeper understanding of environmental toxicity. The present study focuses on the transcriptomic profiles of green microalgae Chlamydomonas reinhardtii exposed to new industrially promising material, TiO₂ nanotubes (NTs), as an example of a widely used one-dimensional nanomaterial. The first algal in vitro assay included 2.5 and 7.5 mg/L TiO₂ NTs, resulting in a dose-dependent negative effect on biological endpoints. At a working concentration of 7.5 mg/L, RNA-sequencing showed a mainly negative effect on the cells. In summary, the results indicated metabolic disruption, ^{such as ATP loss, damage to mitochondria and chloroplasts, loss of solutes due to permeated membranes,} and cell wall damage. Moreover, apoptosis-induced transcripts were detected. Interestingly, reactivation of transposons was observed. In signalling and transcription pathways, including chromatin remodelling and locking, the annotated genes were downregulated.

Advancement in algal bioremediation for organic, inorganic, and emerging pollutants

Rapidly changing bioremediation prospects are key drive to develop sustainable options that can offer extra benefits rather than only environmental remediation. Algal remediating is gaining utmost attention due to its mesmerising sustainable features, removing odour and toxicity, co-remediating numerous common and emerging inorganic and organic pollutants from gaseous and aqueous environments, and yielding biomass for a range of valuable products refining. Moreover, it also improves carbon footprint via carbon-capturing offers a better option than any other non-algal process for several high CO₂-emitting industries. Bio-uptake, bioadsorption, photodegradation, and biodegradation are the main mechanisms to remediate a range of common and emerging pollutants by various algae species. Bioadsorption was a dominant remediation mechanism among others implicating surface properties of pollutants and algal cell walls. Photodegradable pollutants were photodegraded by microalgae by adsorbing photons on the surface and intracellularly via stepwise photodissociation and breakdown. Biodegradation involves the transportation of selective pollutants intracellularly, and enzymes help to convert them into simpler non-toxic forms. Robust models are from the green microalgae group and are dominated by Chlorella species. This article compiles the advancements in microalgae-assisted pollutants remediation and value-addition under sustainable biorefinery prospects. Moreover, filling the knowledge gaps, and recommendations for developing an effective platform for emerging pollutants remediation and realization of commercial-scale algal bioremediation.

Longitudinal physiological and transcriptomic analyses reveal the short term and long term response of Synechocystis sp. PCC6803 to cadmium stress

Due to the bioaccumulation and non-biodegradability of cadmium, Cd can pose a serious threat to ecosystem even at low concentration. Microalgae is widely distributed photosynthetic organisms in nature, which is a promising heavy metal remover and an effective industrial sewage cleaner. However, there are few detailed reports on the short-term and long-term molecular mechanisms of microalgae under Cd stress. In this study, the adsorption behavior (growth curve, Cd removal efficiency, scanning electron microscope, Fourier transform infrared spectroscopy, and dynamic change of extracellular polymeric substances), cytotoxicity (photosynthetic pigment, MDA, GSH, H₂O₂, O₂^-) and stress response mechanism of microalgae were discussed under EC50. RNA-seq detected 1413 DEGs in 4 treatment groups. These genes were related to ribosome, nitrogen metabolism, sulfur transporter, and photosynthesis, and which been proved to be Cd-responsive DEGs. WGCNA (weighted gene co-expression network analysis) revealed two main gene expression patterns, short-term stress (381 genes) and long-term stress (364 genes). The enrichment analysis of DEGs showed that the expression of genes involved in N metabolism, sulfur transporter, and aminoacyl-tRNA biosynthesis were significantly up-regulated. This provided raw material for the synthesis of the important component (cysteine) of metal chelate protein, resistant metalloprotein and transporter (ABC transporter) in the initial stage, which was also the short-term response mechanism. Cd adsorption of the first 15 min was primary dependent on membrane transporter and beforehand accumulated EPS. Simultaneously, the up-regulated glutathione S-transferase (GSTs) family proteins played a role in the initial resistance to exogenous Cd. The damaged photosynthetic system was repaired at the later stage, the expressions of glycolysis and gluconeogenesis were up-regulated, to meet the energy and substances of physiological metabolic activities. The study is the first to provide detailed short-term and long-term genomic information on microalgae responding to Cd stress. Meanwhile, the key genes in this study can be used as potential targets for algae-mediated genetic engineering.

The Chlamydomonas transcription factor MYB1 mediates lipid accumulation under nitrogen depletion

Microalgae accumulate high levels of oil under stress, but the underlying biosynthetic pathways are not fully understood. We sought to identify key regulators of lipid metabolism under stress conditions. We found that the Chlamydomonas reinhardtii gene encoding the MYB-type transcription factor MYB1 is highly induced under stress conditions. Two myb1 mutants accumulated less total fatty acids and storage lipids than their parental strain upon nitrogen (N) depletion. Transcriptome analysis revealed that genes involved in lipid metabolism are highly enriched in the wild-type but not in the myb1-1 mutant after 4 h of N depletion. Among these genes were several involved in the transport of fatty acids from the chloroplast to the endoplasmic reticulum (ER): acyl-ACP thioesterase (FAT1), Fatty Acid EXporters (FAX1, FAX2), and long-chain acyl-CoA synthetase1 (LACS1). Furthermore, overexpression of FAT1 in the chloroplast increased lipid production. These results suggest that, upon N depletion, MYB1 promotes lipid accumulation by facilitating fatty acid transport from the chloroplast to the ER. This study identifies MYB1 as an important positive regulator of lipid accumulation in C. reinhardtii upon N depletion, adding another player to the established regulators of this process, including NITROGEN RESPONSE REGULATOR 1 (NRR1) and TRIACYLGLYCEROL ACCUMULATION REGULATOR 1 (TAR1).

Is Genetic Engineering a Route to Enhance Microalgae-Mediated Bioremediation of Heavy Metal-Containing Effluents?

Contamination of the biosphere by heavy metals has been rising, due to accelerated anthropogenic activities, and is nowadays, a matter of serious global concern. Removal of such inorganic pollutants from aquatic environments via biological processes has earned great popularity, for its cost-effectiveness and high efficiency, compared to conventional physicochemical methods. Among candidate organisms, microalgae offer several competitive advantages; phycoremediation has even been claimed as the next generation of wastewater treatment technologies. Furthermore, integration of microalgae-mediated wastewater treatment and bioenergy production adds favorably to the economic feasibility of the former process-with energy security coming along with environmental sustainability. However, poor biomass productivity under abiotic stress conditions has hindered the large-scale deployment of microalgae. Recent advances encompassing molecular tools for genome editing, together with the advent of multiomics technologies and computational approaches, have permitted the design of tailor-made microalgal cell factories, which encompass multiple beneficial traits, while circumventing those associated with the bioaccumulation of unfavorable chemicals. Previous studies unfolded several routes through which genetic engineering-mediated improvements appear feasible (encompassing sequestration/uptake capacity and specificity for heavy metals); they can be categorized as metal transportation, chelation, or biotransformation, with regulation of metal- and oxidative stress response, as well as cell surface engineering playing a crucial role therein. This review covers the state-of-the-art metal stress mitigation mechanisms prevalent in microalgae, and discusses putative and tested metabolic engineering approaches, aimed at further improvement of those biological processes. Finally, current research gaps and future prospects arising from use of transgenic microalgae for heavy metal phycoremediation are reviewed.

Transcriptome Profiles of Leaves and Roots of Goldenrain Tree (Koelreuteria paniculata Laxm.) in Response to Cadmium Stress

Cadmium (Cd) pollution is a widespread environmental problem. In this study, we explored the transcriptome and biochemical responses of goldenrain tree (Koelreuteria paniculata Laxm.) leaves and roots to Cd stress. Leaf and root growth decreased substantially under Cd stress (50 mg/L CdCl₂), but leaf and root antioxidant mechanisms were significantly activated. In RNA-seq analysis, roots treated with 25 mg/L CdCl₂ featured enriched GO terms in cellular components related to intracellular ribonucleoprotein complex, ribonucleoprotein complex, and macromolecular complex. In leaves under Cd stress, most differentially expressed genes were enriched in the cellular component terms intrinsic component of membrane and membrane part. Weighted gene co-expression network analysis and analysis of module–trait relations revealed candidate genes associated with superoxide dismutase (SOD), peroxidase (POD), and catalase (CAT) activities and malondialdehyde (MDA). Ten transcription factors responded to Cd stress expression, including those in C2H2, MYB, WRKY, and bZIP families. Transcriptomic analysis of goldenrain tree revealed that Cd stress rapidly induced the intracellular ribonucleoprotein complex in the roots and the intrinsic component of membrane in the leaves. The results also indicate directions for further analyses of molecular mechanisms of Cd tolerance and accumulation in goldenrain tree.

Synchrotron Radiation-Fourier Transformed Infrared microspectroscopy (μSR-FTIR) reveals multiple metabolism alterations in microalgae induced by cadmium and mercury

Toxic metals such as cadmium (Cd) and mercury (Hg) represent a threat to photosynthetic organisms of polluted aquatic ecosystems, and knowledge about mechanisms of toxicity is essential for appropriate assessment of environmental risks. We used Synchrotron Radiation-Fourier Transformed Infrared microspectroscopy (μSR-FTIR) to characterise major changes of biomolecules caused by Cd and Hg in the model green microalga Chlamydomonas reinhardtii. μSR-FTIR showed several metabolic alterations in different biochemical groups such as carbohydrates, proteins, and lipids in a time-dose dependent manner, with the strongest changes occurring at concentrations above 10 μM Cd and 15 μM Hg after short-term (24 h) treatments. This occurred in a context where metals triggered intracellular oxidative stress and chloroplast damage, along with autophagy induction by overexpressing AUTOPHAGY-RELATED PROTEIN 8 (ATG8). Thin layer chromatography analysis confirmed that toxic metals promoted remarkable changes in lipid profile, with higher degree of esterified fatty acid unsaturation as detected by gas chromatography coupled with mass spectrometry. Under Cd stress, there was specifically higher unsaturation of free fatty acids, while Hg led to stronger unsaturation in monogalactosyldiacylglycerol. μSR-FTIR spectroscopy proved as a valuable tool to identify biochemical alterations in microalgae, information that could be exploited to optimise approaches for metal decontamination.

Heavy metal detoxification mechanisms by microalgae: Insights from transcriptomics analysis

Heavy metal pollution in ecosystem is a global threat. The associated toxicity and carcinogenic nature of heavy metals/metalloids such as mercury, cadmium, lead, and arsenic are imposing a severe risk to both ecological diversity and human lives. Harnessing the adaptive feature of microalgae for remediating toxic heavy metal has reached a milestone in past few decades. Transcriptomics analyses have provided mechanistic insights to map the dynamics of cellular events under heavy metal stress, thus deciphering the strategic responses of microalgae. Here, the present review comprehensively addresses the elicited molecular responses of microalgae to detoxify the heavy metal stress. The review highlights the intricate role of biochemical components and signaling networks mediating stress responsive transitions of microalgae at physiological level. Furthermore, the differential gene expression signifying the transporters involved in uptake, distribution/sequestration, and efflux of heavy metal has also been reviewed. In a nutshell, this study provided a comprehensive understanding of the molecular mechanisms adopted by microalgae at transcriptome level to nullify the oxidative stress while detoxifying the heavy metals.

An untargeted metabolomic approach for the putative characterization of metabolites from Scenedesmus obliquus in response to cadmium stress

Cadmium (Cd) is a widespread contaminant in aquatic systems and has a variety of toxicological implications on freshwater microorganisms. In this study, the green algae Scenedesmus obliquus was exposed to increasing Cd concentrations that inhibited growth by 20% (12.6 μmol L^-1), 30% (39.8 μmol L^-1) and 40% (83.2 μmol L^-1) and the metabolite profiles of released and cellular biomolecules were explored using an untargeted direct infusion high resolution Fourier transform ion cyclotron resonance mass spectrometry approach. In Cd untreated cultures, intrinsic differences in composition existed between released biomolecules and freeze-dried cells. Based on putatively characterized compound groups, a greater proportion of Cys-GSH isomers and carboxyamides were present in exudates whereas sugar isomers and phosphonic acids comprised most cellular metabolites. In cultures exposed to 83.2 μmol L^-1 Cd, an overall shift in metabolomic response across both released biomolecules and cellular components resulted in an increase of lipid-based esters, and Cys-GSH isomers. These two important metabolites are used in antioxidant defense mechanisms and reactive oxygen species prevention during cellular stress. The diversity of metabolites also decreased as Cd concentrations increased when compared to untreated cultures, suggesting that overall metabolites specialize upon metal stress. We show systemic shifts from sugar and carboxylic isomers to specialized proteins and lipid isomers to help S. obliquus cope with stress. These findings highlight the potential use of this green algae as a potential biosorbent and sheds light into the metabolomics of Cd toxicology and insights into microbial metal adaptation.

The comparison of transcriptomic response of green microalga Chlorella sorokiniana exposure to environmentally relevant concentration of cadmium(II) and 4-n-nonylphenol

The transcriptomic response of green microalga Chlorella sorokiniana exposure to environmentally relevant concentration of cadmium(II) (Cd) and 4-n-nonylphenol (4-n-NP) was compared in the present study. Cd and 4-n-NP exposure showed a similar pattern of dys-regulated pathways. The photosystem was affected due to suppression of chlorophyll biosynthesis via down-regulation of Mg-protoporphyrin IX chelatase subunit ChlD (CHLD) and divinyl chlorophyllide a 8-vinyl-reductase (DVR) in Cd group and via down-regulation of DVR in 4-n-NP group. Furthermore, the reactive oxygen species (ROS) could be induced through down-regulation of solanesyl diphosphate synthase 1 (SPS1) and homogentisate phytyltransferase (HPT) in Cd group and via down-regulation of HPT in 4-n-NP group. Additionally, Cd and 4-n-NP would both cause the dys-regulation of carbohydrate metabolism and protein synthesis. On the other hand, there are some different responses or detoxification mechanism of C. sorokiniana to 4-n-NP stress compared to Cd exposure. The increased ROS would cause the DNA damage and protein destruction in Cd exposure group. Simultaneously, the RNA transcription was dys-regulated and a series of changes in gene expressions were observed. This included lipid metabolism, protein modification, and DNA repair, which involved in response of C. sorokiniana to Cd stress or detoxification of Cd. For 4-n-NP exposure, no effect on lipid metabolism and DNA repair was observed. The nucleotide metabolism including pyrimidine metabolism and purine metabolism was significantly up-regulated in the 4-n-NP exposure group, but not in the Cd exposure group. In addition, 4-n-NP would induce the ubiquitin-mediated proteolysis and proteasomal degradation to diminish the misfolded protein caused by ROS and down-regulation of heat shocking protein 40. In sum, the Cd and 4-n-NP could cause the same toxicological effects via the common pathways and possess similar detoxification mechanism. They also showed different responses in nucleotide metabolism, lipid metabolism, and DNA repair.

Demarcating antioxidant response against aluminum induced oxidative stress in Westiellopsis prolifica Janet 1941

Aluminum metal pollution is considered as a primary limiting factor that reduced crop yield in South Asian subtropical country like India. In national context, Odisha contributes around more than 40% of total ore availability. Moreover, industrial mining and smelting aid are major concern for aluminum metal toxicity in territorial vicinity affecting the soil fertility, ecosystem and human health through food chain. The aluminum metal accumulation limits the soil fertility by antagonistic regulation of photosynthetic and nitrogen fixing microbiota. The increasing concern regarding aluminum pollution enterprise critical investigations for their bioremediation in contamination sites. In this notion, the current study was hypothesized to decrypt the rate limiting factors, their explicit mode of action and intracellular detoxification in a cyanobacterium, i.e., Westiellopsis prolifica isolated from ash pond of NALCO (National Aluminum Company Limited), Angul, Odisha. In the experimental setup, treatment with different concentrations of AlCl₃ (0-0.1 mM) was marked a decline in the growth of the strain due to the adverse regulation of photosynthetic pigments. However, the enforcement of catalase (CAT), ascorbate peroxidase (APX), superoxide dismutase (SOD), guaiacol peroxidase (GPX) and glutathione reductase (GR) was critical for sustaining strain viability under oxidative imbalance. The observation of an increase in the antioxidant enzyme and MDA content was evident to sustain strain viability under such oxidative imbalance. The outcome of the anticipated study was apparent to demonstrate a colossal interlink between Al mediated induction of oxidative stress and their cellular detoxification via intracellular antioxidant enzymes and removal of H₂O₂ accumulation in cyanobacterium, W. prolifica. Statement of novelty Aluminum metal toxicity renders growth of Westiellopsis prolifica via affecting photosynthesis associated pigments. Westiellopsis prolifica deploys antioxidant defense enzymes to combat against aluminum mediated oxidative upset. Intracellular antioxidant enzymes provoke cellular survival of Westiellopsis prolifica under excessive uptake of aluminum in contaminated habitats.

Toxicological responses, bioaccumulation, and metabolic fate of triclosan in Chlamydomonas reinhardtii

Triclosan (TCS) is a broad-spectrum antimicrobial agent that is broadly used in personal care products. It has been shown to cause the contamination of a variety of aquatic environments. Since algae has been the primary producers of aquatic ecosystems, understanding the toxicological mechanisms and the metabolic fate of TCS is vital for assessing its risk in an aquatic environment. In our study, 0.5-4 mg L^-1 TCS treatments for 72 h in a culture of Chlamydomonas reinhardtii (C. reinhardtii) showed progressive inhibition of cell growth and reduced the chlorophyll content. The EC₅₀ value of C. reinhardtii after 72 h was 1.637 mg L^-1, which showed its higher level of resistance to TCS in comparison with other algal species. The exposure to TCS led to oxidative injuries of algae in relation to the increment of malonaldehyde content, cell membrane permeability, and H₂O₂ levels. Furthermore, the oxidative stress from TCS stimulated a series of antioxidant enzyme activities and their gene expressions. Simultaneously, the accumulated TCS in C. reinhardtii arouses the detoxification/degradation-related enzymes and related gene transcriptions. In the medium, approximately 82% of TCS was removed by C. reinhardtii. Importantly, eight TCS metabolites were identified by ultra-performance liquid chromatography-high-resolution mass spectrometry and their relative abundances were measured in a time-course experiment. Six of these metabolites are reported here for the first time. The metabolic pathways of triclosan via C. reinhardtii including reductive dechlorination, hydroxylation, sulfhydrylation, and binding with thiol/cysteine/GSH/glycosyl were manifested to broaden our understanding of the environmental fate of TCS. Graphical Abstract.

Multi-genomic analysis of the cation diffusion facilitator transporters from algae

Metal transport processes are relatively poorly understood in algae in comparison to higher plants and other eukaryotes. A screen of genomes from 33 taxonomically diverse algal species was conducted to identify members of the Cation Diffusion Facilitator (CDF) family of metal ion transporter. All algal genomes contained at least one CDF gene with four species having >10 CDF genes (median of 5 genes per genome), further confirming that this is a ubiquitous gene family. Phylogenetic analysis suggested a CDF gene organisation of five groups, which includes Zn-CDF, Fe/Zn-CDF and Mn-CDF groups, consistent with previous phylogenetic analyses, and two functionally undefined groups. One of these undefined groups was algal specific although excluded chlorophyte and rhodophyte sequences. The majority of sequences (22 out of 26 sequences) from this group had a putative ion binding site motif within transmembrane domain 2 and 5 that was distinct from other CDF proteins, such that alanine or serine replaced the conserved histidine residue. The phylogenetic grouping was supported by sequence cluster analysis. Yeast heterologous expression of CDF proteins from Chlamydomonas reinhardtii indicated Zn²⁺ and Co²⁺ transport function by CrMTP1, and Mn²⁺ transport function by CrMTP2, CrMTP3 and CrMTP4, which validated the phylogenetic prediction. However, the Mn-CDF protein CrMTP3 was also able to provide zinc and cobalt tolerance to the Zn- and Co-sensitive zrc1 cot1 yeast strain. There is wide diversity of CDF transporters within the algae lineage, and some of these genes may be attractive targets for future applications of metal content engineering in plants or microorganisms.

Whole-genome re-sequencing and transcriptome reveal cadmium tolerance related genes and pathways in Chlamydomonas reinhardtii

Cadmium (Cd), a common environmental toxic contaminant, is easily accumulated in living organisms, leading to numerous harmful effects. Chlamydomonas reinhardtii, a unicellular eukaryotic green algae strain, is a very suitable candidate for bioremediation of Cd-contaminated water. However, for the poor resistance to Cd, application of C. reinhardtii was restricted and genes mediating Cd tolerance in C. reinhardtii remain unclear. In this paper, adaptive laboratory evolution was performed with algae constant exposure to Cd over 420-day at environmentally relevant concentrations to select C. reinhardtii strains with high tolerance to Cd. Physiological indicators, such as cell proliferation, photosynthetic pigment contents and photosynthetic activity of photosystem were detected to evaluate the Cd tolerance of selected algae strain ALE0.5. Then, whole-genome re-sequencing and transcriptome were applied to identify the genes related to Cd tolerance. Genes involved in photosynthesis (PSBP1), glutathione metabolism (CHLREDRAFT_167073, GPX5) and calcium transport (CHLREDRAFT_189266, CHLREDRAFT_191203, CHLREDRAFT_187187, CSE1) were related to Cd tolerance in C. reinhardtii. This study provides a basis for obtaining transgenic C. reinhardtii strains with high Cd tolerance used for bioremediation of Cd pollution in the future.

New Prospects for Modified Algae in Heavy Metal Adsorption

Heavy metal pollution is one of the most pervasive environmental problems globally. Novel finely tuned algae have been proposed as a means to improve the efficacy and selectivity of heavy metal biosorption. This article reviews current research on selective algal heavy metal adsorption and critically discusses the performance of novel biosorbents. We emphasize emerging state-of-the-art techniques that customize algae for enhanced performance and selectivity, particularly molecular and chemical extraction techniques as well as nanoparticle (NP) synthesis approaches. The mechanisms and processes for developing novel algal biosorbents are also presented. Finally, we discuss the applications, challenges, and future prospects for modified algae in heavy metal biosorption.

Can Omics Approaches Improve Microalgal Biofuels under Abiotic Stress?

Microalgae hold the promise of an inexpensive and sustainable source of biofuels. The existing microalgal cultivation technologies need significant improvement to outcompete other biofuel sources such as terrestrial plants. Application of 'algomics' approaches under different abiotic stress conditions could be an effective strategy for optimization of microalgal growth and production of high-quality biofuels. In this review, we discuss the roles of omics in understanding genome structure and biocomponents metabolism in various microalgal species to optimize sustainable biofuel production. Application of individual and integrated omics revealed that genes and metabolic pathways of microalgae have been altered under multiple stress conditions, resulting in an increase in biocomponents, providing a research platform for expansion of genetic engineering studies in microalgal strains.

Metabolic adaptation of a Chlamydomonas acidophila strain isolated from acid mine drainage ponds with low eukaryotic diversity

The diversity and biological characteristics of eukaryotic communities within acid mine drainage (AMD) sites is less well studied than for prokaryotic communities. Furthermore, for many eukaryotic extremophiles the potential mechanisms of adaptation are unclear. This study describes an evaluation of eight highly acidic (pH 1.6-3.1) and one moderately acidic (pH 5.6) metal-rich acid mine drainage ponds at a disused copper mine. The severity of AMD pollution on eukaryote biodiversity was examined, and while the most species-rich site was less acidic, biodiversity did not only correlate with pH but also with the concentration of dissolved and particulate metals. Acid-tolerant microalgae were present in all ponds, including the species Chlamydomonas acidophila, abundance of which was high in one very metal-rich and highly acidic (pH 1.6) pond, which had a particularly high PO₄-P concentration. The C. acidophila strain named PM01 had a broad-range pH tolerance and tolerance to high concentrations of Cd, Cu and Zn, with bioaccumulation of these metals within the cell. Comparison of metal tolerance between the isolated strain and other C. acidophila strains previously isolated from different acidic environments found that the new strain exhibited much higher Cu tolerance, suggesting adaptation by C. acidophila PM01 to excess Cu. An analysis of the metabolic profile of the strains in response to increasing concentrations of Cu suggests that this tolerance by PM01 is in part due to metabolic adaptation and changes in protein content and secondary structure.

Biosynthetic Conversion of Ag⁺ to highly Stable Ag⁰ Nanoparticles by Wild Type and Cell Wall Deficient Strains of Chlamydomonas reinhardtii

In the current study, two different strains of the green, freshwater microalga Chlamydomonas reinhardtii bioreduced Ag⁺ to silver nanoparticles (AgNPs), which have applications in biosensors, biomaterials, and therapeutic and diagnostic tools. The bioreduction takes place in cell cultures of C. reinhardtii at ambient temperature and atmospheric pressure, thus eliminating the need for specialized equipment, harmful reducing agents or the generation of toxic byproducts. In addition to the visual changes in the cell culture, the production of AgNPs was confirmed by the characteristic surface plasmon resonance (SPR) band in the range of 415–425 nm using UV-Vis spectrophotometry and further evolution of the SPR peaks were studied by comparing the peak intensity at maximum absorbance over time. X-ray diffraction (XRD) determined that the NPs were Ag⁰. Micrographs from transmission electron microscopy (TEM) revealed that 97 ± 2% AgNPs were <10 nm in diameter. Ag⁺ to AgNP conversion was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The AgNPs were stable over time in the cell culture media, acetone, NaCl and reagent alcohol solutions. This was verified by a negligible change in the features of the SPR band after t > 300 days of storage at 4 °C.

Differential gene expression for carotenoid biosynthesis in a green alga Ulva prolifera based on transcriptome analysis

BACKGROUND

Carotenoids are widely distributed in plants and algae, and their biosynthesis has attracted widespread interest. Carotenoid-related research has mostly focused on model species, and there is a lack of data on the carotenoid biosynthetic pathway in U. prolifera that is the main species leading to green tide, a harmful plague of floating green algae.

RESULTS

The carotenoid content of U. prolifera samples, that is the main species leading to green tide, a harmful plague of floating green algae at different temperatures revealed that its terpenoid was highest in the samples subjected to high temperature at 28 °C (H), followed by the samples subjected to low temperature at 12 °C (L). Its terpenoid was lowest in the samples subjected to medium temperature at 20 °C (M). We conducted transcriptome sequencing (148.5 million raw reads and 49,676 unigenes in total) of samples that were subjected to different temperatures to study the carotenoid biosynthesis of U. prolifera. There were 1125-3164 significant differentially expressed genes between L, M and H incubation temperatures, of which 11-672 genes were upregulated and 453-3102 genes were downregulated. A total of 3164 genes were significantly differentially expressed between H and M, of which 62 genes were upregulated and 3102 genes were downregulated. A total of 2669 significant differentially expressed genes were observed between L and H, of which 11 genes were upregulated and 2658 genes were downregulated. A total of 13 genes were identified to be involved in carotenoid biosynthesis in U. prolifera, and the expression levels of the majority were highest at H and lowest at M of incubation temperature. Both the carotenoid concentrations and the expression of the analysed genes were lowest in the normal temperature group, while low temperature and high temperature seemed to activate the biosynthesis of carotenoids in U. prolifera.

CONCLUSIONS

In this study, transcriptome sequencing provided critical information for understanding the accumulation of carotenoids and will serve as an important reference for the study of other metabolic pathways in U. prolifera.