Copper redox state in cells and aquatic organisms: Implication for toxicity

Copper (Cu) redox state has been an important issue in biology and toxicology research, but many research gaps remain to be explored due to the limitations in the detecting techniques. Herein, the regulation of Cu homeostasis, including absorption, translocation, utilization, storage, and elimination behavior is discussed. Cuproptosis, a newly identified type of cell death caused by excessive Cu accumulation, which results in the aggregation of DLAT protein or the loss of Fe-S cluster and finally proteotoxic stress, is reviewed. Several longstanding mysteries of diseases such as Wilson disease and toxic effects, may be attributed to cuproptosis. Furthermore, we review the advanced detection methods and application of Cu(I) and Cu(II), especially the in-situ imaging techniques such as XANES, and chemosensors. Most of the existing studies using these detection techniques focus on the bioaccumulation and toxicity of Cu(I) and Cu(II) in cells and aquatic organisms. Finally, it will be important to identify the roles of Cu(I) and Cu(II) in the growth, development, and diseases of organisms, as well as the relationship between bioaccumulation and toxicity of Cu(I) and Cu(II) in cellular and aquatic toxicology.

The potential of Pavlovophyceae species as a source of valuable carotenoids and polyunsaturated fatty acids for human consumption

Microalgae are a group of microorganisms, mostly photoautotrophs with high CO2 fixation capacity, that have gained increased attention in the last decades due to their ability to produce a wide range of valuable metabolites, such as carotenoids and polyunsaturated fatty acids, for application in food/feed, pharmaceutical, and cosmeceutical industries. Their increasing relevance has highlighted the importance of identifying and culturing new bioactive-rich microalgae species, as well as of a thorough understanding of the growth conditions to optimize the biomass production and master the biochemical composition according to the desired application. Thus, this review intends to describe the main cell processes behind the production of carotenoids and polyunsaturated fatty acids, in order to understand the possible main triggers responsible for the accumulation of those biocompounds. Their economic value and the biological relevance for human consumption are also summarized. In addition, an extensive review of the impact of culture conditions on microalgae growth performance and their biochemical composition is presented, focusing mainly on the studies involving Pavlovophyceae species. A complementary description of the biochemical composition of these microalgae is also presented, highlighting their potential applications as a promising bioresource of compounds for large-scale production and human and animal consumption.

A first estimation of the role of penguin guano on copper cycling and organic speciation in Antarctic coastal waters

Cu is a vital micronutrient, but free Cu ions (Cu2+) in seawater, even at subnanomolar concentrations, can impede phytoplankton growth. Natural Cu complexation with organic ligands regulates Cu acquisition and, in most instances, reduces Cu2+ concentrations below toxic thresholds. Along the Antarctic coast, the sources and sinks of Cu and its associated ligands remain poorly defined. Despite the high productivity in the area, there are no studies on the role of trophic transfer in Cu cycling. This study explores penguin guano release of Cu and Cu ligands and its potential in neutralizing copper toxicity along the Antarctic coast. We collected guano in a Chinstrap penguin nesting location in the West coast of Deception Island and extracted its components into aqueous solution imitating natural processes. Copper concentration in guano was 0.4 mg (dry weight g)-1 constituting a potential toxic threat and showed biomagnification with respect to krill. Surface seawater samples collected from various locations varying in penguin activity, were analyzed to assess the potential influence of guano on the area. Visual examination and elevated levels of Al suggested that a substantial portion of guano was lithogenic. Consequently, only a modest 16 % of the total Cu present in guano could be extracted using mechanical methods. Notably, the concentrations of the extracted organic ligands were approximately 23 times higher than the concentrations of the extracted Cu. This significant presence of ligands effectively nullifies any potential toxicity that could have arisen from free Cu2+ ions. Guano ligands' conditional stability constants were lower than those in surface seawater suggesting phytoplankton exudation was the main ligand source in the area. Overall, guano acts as a key node for Cu cycling in coastal Antarctic waters but its deleterious potential is neutralized by ligands from krill digestion and the high background concentration of phytoplankton exudates.

Effect of dissolved organic matter on copper bioavailability to a coastal dinoflagellate at environmentally relevant concentrations

The speciation and bioavailability of copper (Cu) in the marine environment are affected by the presence of dissolved organic matter (DOM). Previous studies conducted at dissolved Cu concentrations >100 nM confirmed that Cu bioavailability depends on the concentration of labile Cu, as measured by anodic stripping voltammetry (ASV), which aligns with the expectations of the biotic ligand model (BLM). However, ambient Cu concentrations in coastal waters are generally lower, ranging between 1 and 80 nM, and the effect of DOM on the bioavailability of Cu to marine organisms has not been tested within that range of Cu concentrations. The present study aims to assess the impact of two types of DOM, a commercially available fulvic acid, and marine DOM extracted by ultrafiltration, on Cu bioavailability to phytoplankton using short-term 65Cu internalisation by the marine dinoflagellate Prorocentrum micans. Results showed that Cu internalisation decreases with DOM additions as expected according to the BLM and in agreement with ASV measurements of labile Cu, at the highest tested Cu concentration (100 nM). On the contrary, at a lower Cu concentration (20 nM), organic complexes appear to be partially bioavailable, thereby challenging the general applicability of the BLM model at environmentally relevant concentrations in coastal areas.

Physiological adaptation of the diatom Pseudo-nitzschia delicatissima under copper starvation

In the open ocean and particularly in iron (Fe)-limited environment, copper (Cu) deficiency might limit the growth of phytoplankton species. Cu is an essential trace metal used in electron-transfer reactions, such as respiration and photosynthesis, when bound to specific enzymes. Some phytoplankton species, such as the diatom Pseudo-nitzschia spp. can cope with Cu starvation through adaptative strategies. In this study, we investigated the physiological strategies of the marine diatom P. delicatissima against Cu starvation. Compared to the control, Cu starvation inhibited growth by 35%, but did not induce any excess mortality. Despite the bottleneck measured in the electron flow of the photosynthetic chain, cells of P. delicatissima conserved their photosynthesis ability. This photosynthesis maintenance was accompanied by structural changes of membranes, where pigments and lipid composition were strongly modified. Diatoms also strongly modified their metabolism, by redirecting their C allocation to energy storage under the form of triglycerides. By maintaining essential metabolic functions and storing energy under the form of lipids, these physiological adaptations might be a strategy enabling this diatom to later bloom under the return of favorable nutritional condition.

Phytoplankton community shift in response to experimental Cu addition at the elevated CO2 levels (Arabian Sea, winter monsoon)

Understanding phytoplankton community shifts under multiple stressors is becoming increasingly important. Among other combinations of stressors, the impact of trace metal toxicity on marine phytoplankton under the ocean acidification scenario is an important aspect to address. Such multiple stressor studies are rare from the Arabian Sea, one of the highest productive oceanic provinces within the North Indian Ocean. We studied the interactive impacts of copper (Cu) and CO₂ enrichment on two natural phytoplankton communities from the eastern and central Arabian Sea. Low dissolved silicate (DSi < 2 µM) favoured smaller diatoms (e.g. Nitzschia sp.) and non-diatom (Phaeocystis). CO₂ enrichment caused both positive (Nitzschia sp. and Phaeocystis sp.) and negative (Cylindrotheca closterium, Navicula sp., Pseudo-nitzschia sp., Alexandrium sp., and Gymnodinium sp.) growth impacts. The addition of Cu under the ambient CO₂ level (A-CO₂) hindered cell division in most of the species, whereas Chla contents were nearly unaffected. Interestingly, CO₂ enrichment seemed to alleviate Cu toxicity in some species (Nitzschia sp., Cylindrotheca closterium, Guinardia flaccida, and Phaeocystis) and increased their growth rates. This could be related to the cellular Cu demand and energy budget at elevated CO₂ levels. Dinoflagellates were more sensitive to Cu supply compared to diatoms and prymnesiophytes and could be related to the unavailability of prey. Such community shifts in response to the projected ocean acidification, oligotrophy, and Cu pollution may impact trophic transfer and carbon cycling in this region.

Copper Requirement and Acquisition by Marine Microalgae

Copper is a critical metal nutrient required by marine microalgae but may be toxic when supplied in excess. Maintaining an optimal intracellular Cu content is thus fundamentally necessary for microalgae and relies on cellular regulatory metabolisms and the process of Cu uptake that buffers the variation in environmental Cu availability. In this article the current progress in understanding the Cu requirements and acquisition mechanisms of marine microalgae is reviewed. Cu requirement by microalgae is primarily determined by the amount of Cu-dependent enzymes involved in cellular metabolisms and can be adjusted by Cu-sparing pathways. Decrease in metabolic Cu quotas caused a decline in the abundance of cuproenzymes and the dependent cellular metabolisms, and an induction of Cu acquisition pathways. Conventional models of Cu uptake describe the dependence of Cu uptake rate on free Cu²⁺ ions or kinetically labile species. A reductive, high-affinity Cu uptake system in marine microalgae is identified which enables cells to directly utilize organically complexed Cu, highlighting the importance of cell surface Cu reduction in the marine Cu cycle. This review provides new insights into Cu uptake models that may update the existing knowledge of Cu availability in the ocean.

Copper Effect on Microalgae: Toxicity and Bioremediation Strategies

Microalgae are increasingly recognised as suitable microorganisms for heavy metal (HM) removal, since they are able to adsorb them onto their cell wall and, in some cases, compartmentalise them inside organelles. However, at relatively high HM concentrations, they could also show signs of stress, such as organelle impairments and increased activities of antioxidant enzymes. The main aim of this review is to report on the mechanisms adopted by microalgae to counteract detrimental effects of high copper (Cu) concentrations, and on the microalgal potential for Cu bioremediation of aquatic environments. Studying the delicate balance between beneficial and detrimental effects of Cu on microalgae is of particular relevance as this metal is widely present in aquatic environments facing industrial discharges. This metal often induces chloroplast functioning impairment, generation of reactive oxygen species (ROS) and growth rate reduction in a dose-dependent manner. However, microalgae also possess proteins and small molecules with protective role against Cu and, in general, metal stress, which increase their resistance towards these pollutants. Our critical literature analysis reveals that microalgae can be suitable indicators of Cu pollution in aquatic environments, and could also be considered as components of eco-sustainable devices for HM bioremediation in association with other organisms.

Light Stimulates Copper-Limited Growth of an Oceanic Diatom by Increasing Cellular Copper(II) Reduction─A Rate-Determining Step in Copper Uptake

Uptake of Cu by Thalassiosira oceanica requires that Cu(II) is reduced to Cu(I) prior to transport across the cell membrane. The reduction step is mediated biochemically by cellular reductases active with a broad range of Cu chemical species. Here, we report on the cellular Cu(II) reduction and Cu(I) uptake of a diatom under saturating and subsaturating irradiance. An increase in growth irradiance, from 50 to 400 μmol photons m^-2 s^-1, increased the rate of extracellular Cu(II) reduction and steady-state Cu uptake. Under these conditions, Cu-limited cells acquired Cu more efficiently and maintained faster rates of growth than Cu-limited cells in low light. Pseudo-first-order reaction rate constants were about 70-fold faster for Cu(I) uptake than for Cu(II) reduction so that reduction was the rate-determining step in Cu acquisition. Accordingly, steady-state Cu uptake rates predicted from the reduction rate constants agreed well with measured rates of Cu uptake obtained from cultures growing at low nanomolar Cu concentrations. Transcript abundance of putative Cu(II) reductases followed a similar pattern to cupric reductase activity, increasing in Cu-limited cells and with increasing growth irradiance. The results are significant in showing Cu(II) reduction as the rate-determining step in Cu uptake: they suggest that biologically mediated Cu(II) reduction may be an important part of the Cu cycle in surface waters of the open sea.

Trophic transfer of copper decreases the condition index in Crassostrea gigas spat in concomitance with a change in the microalgal fatty acid profile and enhanced oyster energy demand

Due to new usages and sources, copper (Cu) concentrations are increasing in the Arcachon Basin, an important shellfish production area in France. In the present paper, the trophic transfer of Cu was studied between a microalga, Tetraselmis suecica, and Crassostrea gigas (Pacific oyster) spat. An experimental approach was developed to assess Cu exposure, transfer and toxicity on both phytoplankton and spat. Exposure of microalgal cultures to Cu for 7-8 days (3.1 ± 0.1, 15.7 ± 0.2 and 50.4 ± 1.0 μg Cu·L^-1 for the control, Cu15 and Cu50 conditions, respectively) led to concentrations in microalgae (28.3 ± 0.9 and 110.7 ± 11.9 mg Cu·kg dry weight^-1 for Cu15 and Cu50, respectively) close to those measured in the field. Despite Cu accumulation, the physiology of the microalgae remained poorly affected. Exposed cultures could only be discriminated from controls by a higher relative content in intracellular reactive oxygen species, and a lower relative content in lipids together with a reduced metabolic activity. By contrast, the fatty acid profile of microalgae was modified, with a particularly relevant lower content of the essential polyunsaturated fatty acid 22:6n-3 (docosahexaenoic acid [DHA]). Following 21 days of spat feeding with Cu15 and Cu50 microalgal cultures, trophic transfer of Cu was observed with a high initial Cu concentration in spat tissues. No effect was observed on oxidative stress endpoints. Cu exposure was responsible for a decrease in the spat condition index, an outcome that could be related to an insufficient DHA supply and extra energy demand as suggested by the overexpression of genes involved in energy metabolism, ATP synthesis and glycogen catabolism.

Metal stresses modify soluble proteomes and toxin profiles in two Mediterranean strains of the distributed dinoflagellate Alexandrium pacificum

HABs involving Alexandrium pacificum have been reported in metal-contaminated ecosystems, suggesting that this distributed species adapts to and/or can tolerate the effects of metals. Modifications in soluble proteomes and PST contents were characterized in two Mediterranean A. pacificum strains exposed to mono- or polymetallic stresses (zinc, lead, copper, cadmium). These strains were isolated from two anthropized locations: Santa Giusta Lagoon (Italy, SG C10-3) and the Tarragona seaport (Spain, TAR C5-4F). In both strains, metals primarily downregulated key photosynthesis proteins. Metals also upregulated other proteins involved in photosynthesis (PCP in both strains), the oxidative stress response (HSP 60, proteasome and SOD in SG C10-3; HSP 70 in TAR C5-4F), energy metabolism (AdK in TAR C5-4F), neoglucogenesis/glycolysis (GAPDH and PEP synthase in SG C10-3) and protein modification (PP in TAR C5-4F). These proteins, possibly involved in adaptive proteomic responses, may explain the development of these A. pacificum strains in metal-contaminated ecosystems. The two strains showed different proteomic responses to metals, with SG C10-3 upregulating more proteins, particularly PCP. Among the PSTs, regardless of the metal and the strain studied, C2 and GTX4 predominated, followed by GTX5. Under the polymetallic cocktail, (i) total PSTs, C2 and GTX4 reached the highest levels in SG C10-3 only, and (ii) total PSTs, C2, GTX5 and neoSTX were higher in SG C10-3 than in TAR C5-4F, whereas in SG C10-3 under copper stress, total PSTs, GTX5, GTX1 and C1 were higher than in the controls, revealing variability in PST biosynthesis between the two strains. Total PSTs, C2, GTX4 and GTX1 showed significant positive correlations with PCP, indicating that PST production may be positively related to photosynthesis. Our results showed that the A. pacificum strains adapt their proteomic and physiological responses to metals, which may contribute to their ecological success in highly anthropized areas.

Proteomic analysis of metabolic pathways supports chloroplast-mitochondria cross-talk in a Cu-limited diatom

Diatoms are one of the most successful phytoplankton groups in our oceans, being responsible for over 20% of the Earth's photosynthetic productivity. Their chimeric genomes have genes derived from red algae, green algae, bacteria, and heterotrophs, resulting in multiple isoenzymes targeted to different cellular compartments with the potential for differential regulation under nutrient limitation. The resulting interactions between metabolic pathways are not yet fully understood. We previously showed how acclimation to Cu limitation enhanced susceptibility to overreduction of the photosynthetic electron transport chain and its reorganization to favor photoprotection over light harvesting in the oceanic diatom Thalassiosira oceanica (Hippmann et al., 2017, 10.1371/journal.pone.0181753). In order to gain a better understanding of the overall metabolic changes that help alleviate the stress of Cu limitation, we have further analyzed the comprehensive proteomic datasets generated in that study to identify differentially expressed proteins involved in carbon, nitrogen, and oxidative stress-related metabolic pathways. Metabolic pathway analysis showed integrated responses to Cu limitation. The upregulation of ferredoxin (Fdx) was correlated with upregulation of plastidial Fdx-dependent isoenzymes involved in nitrogen assimilation as well as enzymes involved in glutathione synthesis, thus suggesting an integration of nitrogen uptake and metabolism with photosynthesis and oxidative stress resistance. The differential expression of glycolytic isoenzymes located in the chloroplast and mitochondria may enable them to channel both excess electrons and/or ATP between these compartments. An additional support for chloroplast-mitochondrial cross-talk is the increased expression of chloroplast and mitochondrial proteins involved in the proposed malate shunt under Cu limitation.

New Insights into the Evolution of the Electron Transfer from Cytochrome f to Photosystem I in the Green and Red Branches of Photosynthetic Eukaryotes

In cyanobacteria and most green algae of the eukaryotic green lineage, the copper-protein plastocyanin (Pc) alternatively replaces the heme-protein cytochrome c6 (Cc6) as the soluble electron carrier from cytochrome f (Cf) to photosystem I (PSI). The functional and structural equivalence of 'green' Pc and Cc6 has been well established, representing an example of convergent evolution of two unrelated proteins. However, plants only produce Pc, despite having evolved from green algae. On the other hand, Cc6 is the only soluble donor available in most species of the red lineage of photosynthetic organisms, which includes, among others, red algae and diatoms. Interestingly, Pc genes have been identified in oceanic diatoms, probably acquired by horizontal gene transfer from green algae. However, the mechanisms that regulate the expression of a functional Pc in diatoms are still unclear. In the green eukaryotic lineage, the transfer of electrons from Cf to PSI has been characterized in depth. The conclusion is that in the green lineage, this process involves strong electrostatic interactions between partners, which ensure a high affinity and an efficient electron transfer (ET) at the cost of limiting the turnover of the process. In the red lineage, recent kinetic and structural modeling data suggest a different strategy, based on weaker electrostatic interactions between partners, with lower affinity and less efficient ET, but favoring instead the protein exchange and the turnover of the process. Finally, in diatoms the interaction of the acquired green-type Pc with both Cf and PSI may not yet be optimized.

Transcriptomes of an oceanic diatom reveal the initial and final stages of acclimation to copper deficiency

Copper (Cu) concentration is greatly reduced in the open sea so that phytoplankton must adjust their uptake systems and acclimate to sustain growth. Acclimation to low Cu involves changes to the photosynthetic apparatus and specific biochemical reactions that use Cu, but little is known how Cu affects cellular metabolic networks. Here we report results of whole transcriptome analysis of a plastocyanin-containing diatom, Thalassiosira oceanica 1005, during its initial stages of acclimation and after long-term adaptation in Cu-deficient seawater. Gene expression profiles, used to identify Cu-regulated metabolic pathways, show downregulation of anabolic and energy-yielding reactions in Cu-limited cells. These include the light reactions of photosynthesis, carbon fixation, nitrogen assimilation and glycolysis. Reduction of these pathways is consistent with reduced growth requirements for C and N caused by slower rates of photosynthetic electron transport. Upregulation of oxidative stress defence systems persists in adapted cells, suggesting cellular damage by increased reactive oxygen species (ROS) occurs even after acclimation. Copper deficiency also alters fatty acid metabolism, possibly in response to an increase in lipid peroxidation and membrane damage driven by ROS. During the initial stages of Cu-limitation the majority of differentially regulated genes are associated with photosynthetic metabolism, highlighting the chloroplast as the primary target of low Cu availability. The results provide insights into the mechanisms of acclimation and adaptation of T. oceanica to Cu deficiency.

The heterologous expression of a plastocyanin in the diatom Phaeodactylum tricornutum improves cell growth under iron-deficient conditions

We have investigated if the heterologous expression of a functional green alga plastocyanin in the diatom Phaeodactylum tricornutum can improve photosynthetic activity and cell growth. Previous in vitro assays showed that a single-mutant of the plastocyanin from the green algae Chlamydomonas reinhardtii is effective in reducing P. tricornutum photosystem I. In this study, in vivo assays with P. tricornutum strains expressing this plastocyanin indicate that even the relatively low intracellular concentrations of holo-plastocyanin detected (≈4 μM) are enough to promote an increased growth (up to 60%) under iron-deficient conditions as compared with the WT strain, measured as higher cell densities, content in pigments and active photosystem I, global photosynthetic rates per cell, and even cell volume. In addition, the presence of plastocyanin as an additional photosynthetic electron carrier seems to decrease the over-reduction of the plastoquinone pool. Consequently, it promotes an improvement in the maximum quantum yield of both photosystem II and I, together with a decrease in the acceptor side photoinhibition of photosystem II-also associated to a reduced oxidative stress-a decrease in the peroxidation of membrane lipids in the choroplast, and a lower degree of limitation on the donor side of photosystem I. Thus the heterologous plastocyanin appears to act as a functional electron carrier, alternative to the native cytochrome c₆ , under iron-limiting conditions.