Heavy metal tolerance in microalgae: Detoxification mechanisms and applications

Revisiting biochemical pathways for lead and cadmium tolerance by domain bacteria, eukarya, and their joint action in bioremediation

With the advent rise is in urbanization and industrialization, heavy metals (HMs) such as lead (Pb) and cadmium (Cd) contamination have increased considerably. It is among the most recalcitrant pollutants majorly affecting the biotic and abiotic components of the ecosystem like human well-being, animals, soil health, crop productivity, and diversity of prokaryotes (bacteria) and eukaryotes (plants, fungi, and algae). At higher concentrations, these metals are toxic for their growth and pose a significant environmental threat, necessitating innovative and sustainable remediation strategies. Bacteria exhibit diverse mechanisms to cope with HM exposure, including biosorption, chelation, and efflux mechanism, while fungi contribute through mycorrhizal associations and hyphal networks. Algae, especially microalgae, demonstrate effective biosorption and bioaccumulation capacities. Plants, as phytoremediators, hyperaccumulate metals, providing a nature-based approach for soil reclamation. Integration of these biological agents in combination presents opportunities for enhanced remediation efficiency. This comprehensive review aims to provide insights into joint action of prokaryotic and eukaryotic interactions in the management of HM stress in the environment.

Impact of inorganic mercury on carotenoids in freshwater algae: Insights from single-cell resonance Raman spectroscopy

The influence of inorganic mercury (Hg(II)) exposure on photosynthetic microorganisms and their pigments remains understudied. Here, we employed resonance Raman (RR) spectroscopy to investigate the responses of two freshwater phytoplankton species, the green alga Chlamydomonas reinhardtii and the diatom Cyclotella meneghiniana to Hg(II) exposure. We selectively recorded the spectral RR signature of carotenoids in intact cells exposed to concentrations of 10 nM and 100 nM of Hg(II), representative for contaminated environment and unexposed control cells. A two-hour exposure of C. reinhardtii resulted in a slight decrease in lutein and β-carotene levels, while total carotenoids RR band broadening, as revealed by the FWHM of the υ1(C=C) stretching mode from averaged RR spectra, suggested conformational changes in pigments. Higher Hg(II) concentration induced more pronounced conformational changes. Similarly, a two-hour exposure of C. meneghiniana resulted in slight decreased level of the fucoxanthin, while diadinoxanthin showed an opposite trend compared to control: when fucoxanthin decreased, diadinoxanthin increased under 10 nM Hg (II) exposure. At higher concentrations, the decrease in fucoxanthin was less pronounced, accompanied by a broadening of the band area, (with FHHM increased), indicating possible conformer occurrence in response to Hg-induced stress. The changes in the main carotenoid species of the two algae are species-specific, Hg(II) concentration-specific, and dependent on exposure time. The calculated spectral differences in absorbances from UV-VIS spectra of methanol extracts from each group supported the main findings obtained by RR, though with caution due to the selective extraction efficiency of the respective carotenoids. This study highlighted for a first time the capability of single-cell RR spectroscopy as a valuable tool for toxicity assessment and for comprehending early-stage alterations in carotenoid metabolism due to toxic metal exposure in vivo.

Growth Characteristics of a Desmodesmus Species from the San Antonio Springs and Its Short-Term Impact on Soil Microbial Dynamics

A new Desmodesmus species was isolated from the largest of the San Antonio Springs, the Blue Hole, in San Antonio, Texas, and characterized for its potential applications in sustainable agriculture. The xenic isolate (XB) was established by enrichment and subcultured to produce the axenic isolate (AxB), which was identified based on morphological features and DNA profiling, confirming its close phylogenetic relationship with Desmodesmus spp. Growth characteristics, biomass composition, and pigment profiles were assessed for both the xenic and axenic isolates along with their growth in saline conditions and a range of seasonal Texas temperatures. Both Desmodesmus XB and Desmodesmus AxB exhibited optimal growth at 25 °C as well as robust growth at 37 °C and in weakly saline media (5 g/kg NaCl). Biomass analysis revealed levels of carbohydrates, proteins, lipids, chlorophylls, and carotenoids comparable to other desmids and pigment profiles supported the Desmodesmus classification. Soil studies demonstrated the persistence of Desmodesmus XB and influence on microbial activity, indicating the potential of this isolate for agricultural applications such as soil remediation.

Utilizing Mixed Cultures of Microalgae to Up-Cycle and Remove Nutrients from Dairy Wastewater

This study explores the novel use of mixed cultures of microalgae-Spirulina platensis, Micractinium, and Chlorella-for nutrient removal from dairy wastewater (DW). Microalgae were isolated from a local wastewater treatment plant and cultivated under various light conditions. The results showed significant biomass production, with mixed cultures achieving the highest biomass (2.51 g/L), followed by Spirulina (1.98 g/L) and Chlorella (1.92 g/L). Supplementing DW (75%) with BG medium (25%) significantly enhanced biomass and pH levels, improving pathogenic bacteria removal. Spirulina and mixed cultures exhibited high nitrogen removal efficiencies of 92.56% and 93.34%, respectively, while Chlorella achieved 86.85% nitrogen and 83.45% phosphorus removal. Although growth rates were lower under phosphorus-limited conditions, the microalgae adapted well to real DW, which is essential for effective algal harvesting. Phosphorus removal efficiencies ranged from 69.56% to 86.67%, with mixed cultures achieving the highest removal. Microbial and coliform removal efficiencies reached 97.81%, with elevated pH levels contributing to significant reductions in fecal E. coli and coliform levels. These findings suggest that integrating microalgae cultivation into DW treatment systems can significantly enhance nutrient and pathogen removal, providing a sustainable solution for wastewater management.

Phytochelatin Synthase: An In Silico Comparative Analysis in Cyanobacteria and Eukaryotic Microalgae

Phytochelatins (PCs) are small cysteine-rich peptides involved in metal detoxification, not genetically encoded but enzymatically synthesized by phytochelatin synthases (PCSs) starting from glutathione. The constitutive PCS expression even in the absence of metal contamination, the wide phylogenetic distribution and the similarity between PCSs and the papain-type cysteine protease catalytic domain suggest a wide range of functions for PCSs. These proteins, widely studied in land plants, have not been fully analyzed in algae and cyanobacteria, although these organisms are the first to cope with heavy-metal stress in aquatic environments and can be exploited for phytoremediation. To fill this gap, we compared the features of the PCS proteins of different cyanobacterial and algal taxa by phylogenetic linkage. The analyzed sequences fall into two main, already known groups of PCS-like proteins. Contrary to previous assumptions, they are not classed as prokaryotic and eukaryotic sequences, but rather as sequences characterized by the alternative presence of asparagine and aspartic/glutamic acid residues in proximity of the catalytic cysteine. The presence of these enzymes with peculiar features suggests differences in their post-translational regulation related to cell/environmental requirements or different cell functions rather than to differences due to their belonging to different phylogenetic taxa.

Settling selection of Chlamydomonas reinhardtii for samarium uptake

Samarium (Sm) is a rare-earth element recently included in the list of critical elements due to its vital role in emerging new technologies. With an increasing demand for Sm, microbial bioremediation may provide a cost-effective and a more ecologically responsible alternative to remove and recover Sm. We capitalized on a previously selected Chlamydomonas reinhardtii strain tolerant to Sm (1.33 × 10-4 M) and acidic pH and carried out settling selection to increase the Sm uptake performance. We observed a rapid response to selection in terms of cellular phenotype. Cellular size decreased and circularity increased in a stepwise manner with every cycle of selection. After four cycles of selection, the derived CSm4 strain was significantly smaller and was capable of sequestrating 41% more Sm per cell (1.7 × 10-05 ± 1.7 × 10-06 ng) and twice as much Sm in terms of wet biomass (4.0 ± 0.4 mg Sm · g-1) compared to the ancestral candidate strain. The majority (~70%) of the Sm was bioaccumulated intracellularly, near acidocalcisomes or autophagic vacuoles as per TEM-EDX microanalyses. However, Sm analyses suggest a stronger response toward bioabsorption resulting from settling selection. Despite working with Sm and pH-tolerant strains, we observed an effect on fitness and photosynthesis inhibition when the strains were grown with Sm. Our results clearly show that phenotypic selection, such as settling selection, can significantly enhance Sm uptake. Laboratory selection of microalgae for rare-earth metal bioaccumulation and sorption can be a promising biotechnological approach.

Response of antioxidant activity, active constituent and rhizosphere microorganisms of Salvia miltiorrhiza to combined application of microbial inoculant, microalgae and biochar under Cu stress

Salvia miltiorrhiza, a widely used medicinal herb renowned for its properties in promoting blood circulation, removing blood stasis and alleviating pain, is currently facing quality degradation due to excessive heavy metal levels, posing a threat to medication safety. In order to investigate the effects of microbial inoculant, microalgae and biochar on the growth of Salvia miltiorrhiza under copper (Cu) stress, as well as its Cu absorption, antioxidant activity, active component contents and rhizosphere microbial community, a pot experiment was conducted. Salvia miltiorrhiza plants were cultivated in the soil containing 400 mg/kg of Cu for six months and treated with microbial inoculant, microalgae and biochar, either individually or in combination. Almost all soil amendment treatments led to an increase in root biomass. Notably, co-application of microbial inoculant and microalgae had the optimal effect with a 63.07 % increase compared to the group treated solely with Cu. Moreover, when microbial inoculant was applied alone or in combination with microalgae, the Cu content in plant roots was reduced by 19.29 % and 25.37 %, respectively, whereas other treatments failed to show a decreasing trend. Intriguingly, Cu stress increased the active component contents in plant roots, and they could also be enhanced beyond non-stress levels when microbial inoculant and microalgae were applied together or in combination with biochar. Analyses of plant antioxidant activity, soil properties and rhizosphere microorganisms indicated that these amendments may alleviate Cu stress by enhancing peroxidase activity, facilitating plant nutrient absorption, and enriching beneficial microorganisms capable of promoting plant growth and mitigating heavy metal-induced damage. This study suggests that the combined application of microbial inoculant and microalgae can reduce Cu levels in Salvia miltiorrhiza while enhancing its quality under Cu stress.

Bioconcentration of Inorganic and Methyl Mercury by Algae Revealed Using Dual-Mass Single-Cell ICP-MS with Double Isotope Tracers

Algae are an entry point for mercury (Hg) into the food web. Bioconcentration of Hg by algae is crucial for its biogeochemical cycling and environmental risk. Herein, considering the cell heterogeneity, we investigated the bioconcentration of coexisting isotope-labeled inorganic (199IHg) and methyl Hg (201MeHg) by six typical freshwater and marine algae using dual-mass single-cell inductively coupled plasma mass spectrometry (scICP-MS). First, a universal pretreatment procedure for the scICP-MS analysis of algae was developed. Using the proposed method, the intra- and interspecies heterogeneities and the kinetics of Hg bioconcentration by algae were revealed at the single-cell level. The heterogeneity in the cellular Hg contents is largely related to cell size. The bioconcentration process reached a dynamic equilibrium involving influx/adsorption and efflux/desorption within hours. Algal density is a key factor affecting the distribution of Hg between algae and ambient water. Cellular Hg contents were negatively correlated with algal density, whereas the volume concentration factors almost remained constant. Accordingly, we developed a model based on single-cell analysis that well describes the density-driven effects of Hg bioconcentration by algae. From a novel single-cell perspective, the findings improve our understanding of algal bioconcentration governed by various biological and environmental factors.

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.

Physiological regulation of microalgae under cadmium stress and response mechanisms of time-series analysis using metabolomics

The investigation of heavy metal wastewater treatment utilizing microalgae adsorption has been extensively demonstrated. However, the response mechanism based on metabolomics to analyze the time-series changes of microalgae under Cd stress has not been described in detail. In this study, SEM/TEM demonstrated that Cd accumulated on the cell surface of microalgae and was bioconcentrated in the cytoplasm, vesicles, and chloroplasts. Carbonyl/quinone/ketone/carboxyl groups (OCO), membrane polysaccharides (OH), and phospholipids (PO) were involved in the interaction of Cd ions, and the chlorophyll content underwent a process of decreasing in the early stage (1.62 mg/g at 48 h) and recovering to the normal level in the late stage, and the contents of MDA, GSH, and SOD were all increased (29.7 nmol/g, 0.23 mg/g, and 30.01 u/106 cells) and then gradually returned to the steady state. The results of EPS content and fluorescent labeling showed that Cd induced the overexpression and synthesis of extracellular polysaccharides and proteins, which is one of the defense mechanisms participating in the reduction of cellular damage by complexed Cd. Metabolomics results indicated that the malate synthesis pathway was activated after Cd-20 h, and the microalgal cells began to shift the metabolic pathway to storage lipid or polysaccharide biosynthesis. In the Calvin cycle, the expression of D-Sedoheptulose 7-phosphate in Cd-20 h_vs_ck and Cd-72 h_vs_Cd-20 h firstly declined and then increased, and the photosynthesis system was suppressed at the beginning, and then gradually returned to normal to maintain the successful development of the dark reaction. The results of time series analysis revealed that the response of microalgae to Cd was categorized into fast response and slow response to regulate cell adsorption and growth metabolism.

Coupling dairy wastewaters for nutritional balancing and water recycling: sustainable heterologous 2-phenylethanol production by engineered cyanobacteria

Microalgae biotechnology is hampered by the high production costs and the massive usage of water during large-volume cultivations. These drawbacks can be softened by the production of high-value compounds and by adopting metabolic engineering strategies to improve their performances and productivity. Today, the most sustainable approach is the exploitation of industrial wastewaters for microalgae cultivation, which couples valuable biomass production with water resource recovery. Among the food processing sectors, the dairy industry generates the largest volume of wastewaters through the manufacturing process. These effluents are typically rich in dissolved organic matter and nutrients, which make it a challenging and expensive waste stream for companies to manage. Nevertheless, these rich wastewaters represent an appealing resource for microalgal biotechnology. In this study, we propose a sustainable approach for high-value compound production from dairy wastewaters through cyanobacteria. This strategy is based on a metabolically engineered strain of the model cyanobacterium Synechococcus elongatus PCC 7942 (already published elsewhere) for 2-phenylethanol (2-PE). 2-PE is a high-value aromatic compound that is widely employed as a fragrance in the food and cosmetics industry thanks to its pleasant floral scent. First, we qualitatively assessed the impact of four dairy effluents on cyanobacterial growth to identify the most promising substrates. Both tank-washing water and the liquid effluent of exhausted sludge resulted as suitable nutrient sources. Thus, we created an ideal buffer system by combining the two wastewaters while simultaneously providing balanced nutrition and completely avoiding the need for fresh water. The combination of 75% liquid effluent of exhausted sludge and 25% tank-washing water with a fine-tuning ammonium supplementation yielded 180 mg L-1 of 2-PE and a biomass concentration of 0.6 gDW L-1 within 10 days. The mixture of 90% exhausted sludge and 10% washing water produced the highest yield of 2-PE (205 mg L-1) and biomass accumulation (0.7 gDW L-1), although in 16 days. Through these treatments, the phosphates were completely consumed, and nitrogen was removed in a range of 74%-77%. Overall, our approach significantly valorized water recycling and the exploitation of valuable wastewaters to circularly produce marketable compounds via microalgae biotechnology, laying a promising groundwork for subsequent implementation and scale-up.

Advances in responses of microalgal-bacterial symbiosis to emerging pollutants in wastewater

Nowadays, emerging pollutants are widely used and exist in wastewater, such as antibiotics, heavy metals, nanoparticle and microplastic. As a green alternative for wastewater treatment, microalgal-bacterial symbiosis has been aware of owning multiple merits of low energy consumption and little greenhouse gas emission. Thus, the responses of microalgal-bacterial symbiosis to emerging pollutants in wastewater treatment have become a hotspot in recent years. In this review paper, the removal performance of microalgal-bacterial symbiosis on organics, nitrogen and phosphorus in wastewater containing emerging pollutants has been summarized. The adaptation mechanisms of microalgal-bacterial symbiosis to emerging pollutants have been analyzed. It is found that antibiotics usually have hormesis effects on microalgal-bacterial symbiosis, and that microalgal-bacterial symbiosis appears to show more capacity to remove tetracycline and sulfamethoxazole, rather than oxytetracycline and enrofloxacin. Generally, microalgal-bacterial symbiosis can adapt to heavy metals at a concentration of less than 1 mg/L, but its capabilities to remove contaminants can be significantly affected at 10 mg/L heavy metals. Further research should focus on the influence of mixed emerging pollutants on microalgal-bacterial symbiosis, and the feasibility of using selected emerging pollutants (e.g., antibiotics) as a carbon source for microalgal-bacterial symbiosis should also be explored. This review is expected to deepen our understandings on emerging pollutants removal from wastewater by microalgal-bacterial symbiosis.