The effects of copper on the photosynthetic response of Phaeocystis cordata

Phosphorus limitation combined with aluminum triggers synergistic responses on the freshwater microalgae Raphidocelis subcapitata (Chlorophyceae)

In the environment, algae are exposed to several stressors such as limitation of essential nutrients and excess of toxic substances. It is well known the importance of phosphorus (P) supply for healthy metabolism of algae and impacts at this level can affect the whole aquatic trophic chain. Aluminum (Al) is the most abundant metal on Earth and it is toxic to different trophic levels. Processes related to P and Al assimilation still need to be clarified and little is known about the responses of microalgae exposed to the two stressors simultaneously. We evaluated the effects of environmental concentrations of Al and P limitation, isolated and in combination, on growth, pigment production and photosynthesis of the freshwater microalga Raphidocelis subcapitata. Both stressors affected cell density, chlorophyll a, carotenoids, and maximum quantum yield. Al did not affect any other evaluated parameter, while P limitation affected parameters related to the dissipation of heat by algae and the maximum electron transport rate, decreasing the saturation irradiance. In the combination of both stressors, all parameters evaluated were affected in a synergistic way, i.e., the results were more harmful than expected considering the responses to isolated stressors. Our results indicate that photoprotection mechanisms of algae were efficient in the presence of both stressors, avoiding damages to the photosynthetic apparatus. In addition, our data highlight the higher susceptibility of R. subcapitata to Al in P-limited conditions.

Does cobalt antagonize P limitation effects on photosynthetic parameters on the freshwater microalgae Raphidocelis subcapitata (Chlorophyceae), or does P limitation acclimation antagonize cobalt effects? More questions than answers

Phosphorus (P; macronutrient) and cobalt (Co; micronutrient) are essential for algal healthy metabolism. While P provides energy, Co is a co-factor of several enzymes and component of B12 vitamin. However, in concentrations higher or lower than required, P and Co alter algal metabolism, impacting physiological processes (e.g., growth and photosynthesis), usually in a harmful way. In the environment, algae are exposed to multiple stressors simultaneously and studies evaluating the algal response to the combination of macronutrient limitation and micronutrient excess are still scarce. We assessed the effects of P limitation and Co excess, isolated and combined, in Raphidocelis subcapitata (Chlorophyceae), in terms of growth, pigments production, and photosynthetic parameters. Except for the photochemical quenching (qP) and the efficiency in light capture (α) under P limitation, all parameters were affected by both stressors, isolated and combined. Under P limitation, chlorophyll a was the most sensitive parameter; while excess of Co affected most the photoprotective mechanisms of algae, altering the non-photochemical quenchings qN and NPQ, influencing the light use and dissipation of heat by algae. The combination of two stressors resulted in a significant decrease in algal growth, with synergistic responses in growth and pigments production, and antagonism in the photosynthetic parameters. We suggest that algal metabolism was altered during P limitation acclimation and the excess of Co was used in a beneficial way by P-limited algae in photosynthesis, resulting in the well-functioning of the photosynthetic apparatus in the combination of both stressors. However, more studies are needed to understand which mechanisms are involved in this adaptation which resulted in antagonism in photosynthetic processes and synergism in growth and pigments production.

Physiological responses of Humboldt current system diatoms to Fe and Cu co-limitation

Diatoms account for ∼20% of global primary production, often limited by the availability of Fe and other trace nutrients such as Cu. The present study examined the role of both metals in the physiology of two diatoms isolated from the Humboldt Currents System, the centric Chaetoceros c.f. dicipiens and the pennate Nitzschia c.f. draveillensis. Under Fe limitation, a decrease in specific growth rates and sizes of both species was observed, especially in Chaetoceros. However, regarding different photosynthetic parameters, Nitzschia was more impacted. The increase in Cu concentrations improved the physiology of both diatoms, mostly of Chaetoceros. When grown in mixed cultures and under co-limiting conditions, both species remained competive due to morphological advantages (i.e., lower cell size). These results may suggest that the increase of Cu under Fe limitation benefited C. c.f. dicipiens over N. c.f. draveillensis.

Biomass, photosynthetic activity, and biomolecule composition in Chlorella fusca (Chlorophyta) cultured in a raceway pond operated under greenhouse conditions

Raceways are widely used as microalgae culture systems due to their low cost, but they are not the best option for biomass yield. Understanding in situ photosynthetic performance can be a first step to increase their biomass productivity. This study aimed at comparing the real time photosynthetic activity in a greenhouse raceway culture (250L) with discrete measurements under laboratory conditions. We evaluated the photophysiology and biochemical composition of Chlorella fusca culture up to 120h. In situ photosynthetic activity was continuously monitored and compared to discrete ex situ measurements; biochemical compounds were measured daily. The results showed a final biomass density of 0.45gL^-1 (5 days - 120h) and an increase of the electron transport rate (ETR) up to 48h but decreased thereafter. When the relative ETR was estimated considering the absorption coefficient (a) positive correlations of this parameter with photosynthetic capacity, cell density, biomass, biocompounds and antioxidant activity were obtained, whereas no correlation was detected without considering a. In situ photosynthesis monitoring showed higher absolute maximal ETR (10 - 160 μmol m^-3s^-1) than discrete ex situ measurements. We demonstrated the importance of considering the light absorption coefficient for expressing photosynthetic capacity and showed that C. fusca can produce, in the short-term, bioactive compounds that are correlated to photosynthetic conditions.

Response of Coccomyxa cimbrica sp.nov. to Increasing Doses of Cu(II) as a Function of Time: Comparison between Exposure in a Microfluidic Device or with Standard Protocols

In this study, we explore how the in vitro conditions chosen to cultivate and observe the long-term (up to 72 h) toxic effect of Cu(II) on the freshwater microalga Coccomyxa cimbrica sp.nov. can affect the dose response in time. We test three different cultivation protocols: (i) under static conditions in sealed glass cells, (ii) in a microfluidic device, where the sample is constantly circulated with a peristaltic pump, and (iii) under continuous agitation in plastic falcons on an orbital shaker. The advantage and novelty of this study resides in the fact that each condition can mimic different environmental conditions that alga cells can find in nature. The effect of increasing dose of Cu(II) as a function of time (24, 48, and 72 h) is monitored following chlorophyll a fluorescence intensity from single cells. Fluorescence lifetime imaging experiments are also explored to gain information on the changes induced by Cu(II) in the photosynthetic cycle of this microalga.

Study of the growth and biochemical composition of 20 species of cyanobacteria cultured in cylindrical photobioreactors

BACKGROUND

Cyanobacteria are prokaryotic organisms with wide morphological and metabolic diversity. By means of photosynthesis, they convert inorganic compounds into biomolecules, which may have commercial interest. In this work, we evaluated 20 cyanobacterial strains regarding their physiological aspects such as growth, photosynthesis and biochemical composition, some of which are revealed here for the first time. The organisms were cultivated in cylindrical photobioreactors (CPBR) for 144 h and the biomass was obtained. The light inside cultures was constant throughout experimental time and maintained at the saturation irradiance (Ik) of each species. Culture pH was maintained within 7.8 and 8.4 by automatic CO₂ bubbling. Growth rate, dry biomass, chlorophyll a, carotenoids, phycocyanin, proteins, carbohydrates, lipids, polyhydroxyalkanoate (PHA) and antioxidant activity were determined.

RESULTS

The proportionality of the biochemical composition varied among species, as well as the growth rates. Leptolyngbya sp. and Nostoc sp. (CCIBt3249) showed growth rates in the range of 0.7-0.8 d^-1, followed by Rhabdorderma sp. (~ 0.6 d^-1), and Phormidium sp. (~ 0.5 d^-1). High carotenoid content was obtained in Rhabdoderma sp. (4.0 μg mL^-1) and phycocyanin in Leptolyngbya sp. (60 μg mL^-1). Higher total proteins were found in the genus Geitlerinema (75% DW), carbohydrates in Microcystis navacekii (30% DW) and lipids in Phormidium sp. (15% DW). Furthermore, Aphanocapsa holsatica showed the highest antioxidant activity (65%) and Sphaerocavum brasiliense, Microcystis aeruginosa, Nostoc sp. (CCIBt3249) and A. holsatica higher levels of PHA (~ 2% DW).

CONCLUSIONS

This study reports on the biochemical composition of cyanobacteria that can impact the biotechnology of their production, highlighting potential strains with high productivity of specific biomolecules.

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.

Environmentally relevant copper concentrations stimulate photosynthesis in Monoraphidium sp

Microalgae require copper (Cu) in trace levels for their growth and metabolism, it is a vital component of certain metalloproteins. Although this element has been widely studied concerning microalgae physiology, the effects of environmentally relevant levels have been less studied. We studied the photosynthesis and growth of the Chlorophyte Monoraphidium sp. exposed to Cu ranging from low (1.7 nM) to high (589.0 nM) free Cu ions (Cu²⁺) concentrations. The growth rate was unaffected by Cu concentrations in the range of 1.7-7.4 nM Cu²⁺, but decreased beyond it. The relative maximum electron transport rate (rETR_m), saturation irradiance (E_k), photochemical quenching (qP and qL), and PSII operating efficiency [Formula: see text] were stimulated in the 3.4-7.4 nM Cu²⁺ range, concentrations slightly higher than the control, whereas non-photochemical quenching (NPQ) gradually increased with increasing Cu²⁺. The photosystem II antenna size [Sigma (II)₄₄₀] increased under high Cu (589.0 nM), which resulted in a decrease in the quinone A (Q_A) reduction time (tau). In contrast, the Q_A re-oxidation time was unaffected by Cu exposure. These findings show that a slight increase in Cu stimulated photosynthesis in Monoraphidium sp., whereas high Cu reduced photosynthesis and increased the dissipation of captured light energy. This research is a contribution to the understanding of the dynamic photo-physiological responses of Monoraphidium sp. to Cu ions.

Differential responses of Pleurochrysis sp. (Haptophyta) to the effect of copper and light intensity

The effect of light, copper ions, copper oxide nanoparticles on the change in the structural, functional, cytometric, fluorescent parameters of coccolithophore Pleurochrysis sp. was investigated. The culture Pleurochrysis sp. was represented by two cell forms: (1) covered with coccoliths; and (2) not covered, the ratio of which depends from growth conditions. An increase in light from 20 to 650μEm-2 s-1 led to a decrease in the concentration of cells covered with coccoliths from 90 to 35%. With an increase in light, the decrease in the values of variable chlorophyll a fluorescence was observed, a decrease in the chlorophyll concentration was noted, and an increase in cell volumes and their granularity due to coccoliths 'overproduction' was recorded. A tolerance of Pleurochrysis sp. to the effect of copper was registered, both in the ionic form and in the form of a nanopowder. This is probably due to the morphological (presence of coccoliths) and physiological (ligand production) peculiarities of species. Copper did not affect the ratio of cells covered with coccoliths; its value was about 85%. Growth inhibition, a 2-fold decrease in the intracellular chlorophyll content, a decrease in F v /F m , and a pronounced cell coagulation were recorded at the maximum Cu2+ concentration (625μgL-1 ). The mechanical effect was registered of CuO nanoparticles on the surface of Pleurochrysis sp. coccosphere, which results in the emergence of destroyed and deformed coccoliths. A hypothesis is proposed considering the protective function of coccoliths acting as a barrier when the cells are exposed to nanoparticles and copper ions.

Adaptation of a marine diatom to ocean acidification increases its sensitivity to toxic metal exposure

Most previous studies investigating the interplay of ocean acidification (OA) and heavy metal on marine phytoplankton were only conducted in short-term, which may provide conservative estimates of the adaptive capacity of them. Here, we examined the physiological responses of long-term (~900 generations) OA-adapted and non-adapted populations of the diatom Phaeodactylum tricornutum to different concentrations of the two heavy metals Cd and Cu. Our results showed that long-term OA selected populations exhibited significantly lower growth and reduced photosynthetic activity than ambient CO₂ selected populations at relatively high heavy metal levels. Those findings suggest that the adaptations to high CO₂ results in an increased sensitivity of the marine diatom to toxic metal exposure. This study provides evidence for the costs and the cascading consequences associated with the adaptation of phytoplankton to elevated CO₂ conditions, and improves our understanding of the complex interactions of future OA and heavy metal pollution in marine waters.

Differential Effects of Atrazine on Chlorophyceae Species and Association with Morphology, Photosynthesis, Chlorophyll Content, and Glutathione-S-Transferase Activity

Atrazine is a herbicide widely used in the control of weeds in crops such as corn, sugar cane, and sorghum. It is often found in aquatic environments, where it can potentially endanger nontarget organisms such as microalgae. The present study evaluated atrazine toxicity to seven different species of Chlorophyceae and the tolerance of the species to the herbicide was related to morphological, photosynthetic, chlorophyll-a content and the activity of the glutathione-S-transferase enzyme (GST). The comparison of median effect concentration (EC50) values for growth inhibition indicates higher toxicity of atrazine for Pseudopediastrum boryanum and Desmodesmus communis, intermediate toxicity for Ankistrodesmus densus, Chlamydomonas puliminiorfes, and Raphidocelis subcapitata, and lower toxicity for Kirchneriella lunaris and Ankistrodesmus falcatus (EC50: 38, 42, 66, 103, 248, 1004, and 1585 µg L^-1 atrazine, respectively). Principal component analysis (PCA) with algal characteristics suggested that the atrazine-sensitive algae P. boryanum and D. communis were positively associated with photosynthetic levels and negatively associated with GST activity and chlorophyll-a concentration. The PCA also suggested that the atrazine-tolerant algae A. falcatus and K. lunaris were positively associated with morphological parameters, where the larger the cell size, the more tolerant. Although it is difficult to associate a single characteristic of algae as the key factor determining the tolerance to atrazine, results presented in this work indicate that the cell area, the photosynthetic parameters (mainly saturating irradiance), chlorophyll-a content, and the biotransformation by GST in combination may be potential predictors for the differential tolerance of Chlorophyceae species to the herbicide. Environ Toxicol Chem 2022;41:1675-1685. © 2022 SETAC.

Bioprospecting and selection of tolerant strains and productive analyses of microalgae grown in vinasse

In order to contribute to the biotechnology of microalgae cultivated in vinasse, we carried out the bioprospection of tolerant species and synthesized biomolecules of the total biomass (microalgae and bacteria), recovered from cultures. To use vinasse as a culture medium for the microalgae, waste was centrifuged and used in concentrations from 5 to 50%. Daily cell densities, growth rates, and EC50 values were obtained. After defining the best pair of vinasse concentration/microalgae strain, dry biomass, and composition (proteins and carbohydrates) were determined in 96 h cultures, considering the associated community (bacteria and yeast). The microalgae tested were Chlamydomonas sp., Chlorella sorokiniana, Chlorella vulgaris, Desmodesmus spinosus, Haematococcus pluvialis, Monoraphidium sp., Scenedesmus quadricauda, and Tetraselmis gracilis. The results showed that although the microalgal growth rates in vinasse were similar to controls in BG11, the cells in vinasse had higher biovolumes, dry biomass, and total proteins. The species H. pluvialis, S. quadricauda, and T. gracilis showed the best productivity parameters in vinasse, despite lower growth rates than the other species. Using low concentrations of centrifuged vinasse as a culture medium, only 22% of biological contaminants were present, thus most of the processed biomass was mainly composed of microalgae. Thus, Chlamydomonas sp., D. spinosus, S. quadricauda, and H. pluvialis microalgae have attributes such as resistance and biomolecules that make them candidates for further optimization in production systems, combining the environmental benefits of using waste with the production of biomolecules and/or biomass of commercial interest.

Microalgal Activity and Nutrient Uptake from Wastewater Enhanced by Nanoscale Zerovalent Iron: Performance and Molecular Mechanism

Microalgae-based bioremediation presents an alternative to traditional biological wastewater treatment. However, its efficiency is still challenging due to low microalgal activities and growth rate in wastewater. Iron plays an important role in microbial metabolism and is effective to stimulate microbial growth. In this study, a novel approach was proposed to simultaneously promote microalgal activity and nutrient uptake from wastewater using nanoscale zerovalent iron (nZVI), and the underlying molecular mechanism was explored. Compared to the control, 0.05 mg/L of nZVI significantly enhanced biomass production by 113.3% as well as NH₄⁺-N and PO₄^3--P uptake rates by 32.2% and 75.0%, respectively. These observations were attributed to the enhanced metabolic pathways and intracellular regulations. Specifically, nZVI alleviated the cellular oxidative stress via decreased peroxisome biogenesis as indicated by reduced reactive oxygen species, enzymes, and genes involved. nZVI promoted ammonium assimilation, phosphate metabolism, carbon fixation, and energy generation. Moreover, nZVI regulated the biosynthesis and conversions of intracellular biocomposition, leading to increased carotenoid, carbohydrate, and lipid productions and decreased protein and fatty acid yields. The above metabolisms were supported by the regulations of differentially expressed genes involved. This study provided an nZVI-based approach and molecular mechanism for enhancing microalgal activities and nutrient uptake from wastewater.

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.

Environmental concentrations of copper nanoparticles affect vital functions in Ankistrodesmus densus

Nanoparticles (NPs) have unique properties, leading to their widespread application in industry, consequently increasing their concentration in aquatic ecosystems. Although environmentally significant concentrations are still low, they tend to increase because of the intense use, posing into risk microalgae communities. Microalgae are primary producers that support food chains in aquatic ecosystems; thus factors that interfere with their physiology can be propagated throughout the food web. The present research investigated the effects of copper nanoparticles (Cu-NPs) in the physiology of a cosmopolitan green microalgae, Ankistrodesmus densus. Here, we focused on environmental NPs levels, so an ample Cu-NPs range was used, 0.3-635 μg L^-1. Considering that NPs dissolve into the medium releasing their constituent material, free Cu²⁺ ions were determined and considered as surrogate for NPs concentration, which varied from 2.1 × 10^-9 to 8.4 × 10^-9 mol L^-1. The experiment was based in 72 h Cu-NPs exposure, and to access the physiology of A. densus, we monitored population growth, photochemistry of photosynthesis and the content of cell biomolecules (total proteins, carbohydrates and lipids). The results showed that 2.1 × 10^-9 mol L^-1 free Cu²⁺ was enough to decrease growth rate, but 2.5x higher Cu was necessary to affect the photosynthetic parameters. Inorganic carbon fixation rate calculated by absolute electron transport rates was affected. Considering cell biomolecules, total proteins accumulated at 6.5 × 10^-9 and kept increasing up to 8.4 × 10^-9 mol L^-1 free Cu²⁺. Because this was not related to biomass formation, we suggest a possible association with cell detoxification mechanisms. The most clear finding that emerged from this study is that environmental Cu-NPs concentrations affect vital functions in the green microalgae A. densus. An implication of this is the possibility of facing problems related to a increase of NPs in aquatic ecosystems in the near future.

Effect of phosphorus on the toxicity of zinc to the microalga Raphidocelis subcapitata

The aim of this study was to evaluate the effect of phosphorus (P) on the toxicity of zinc (Zn) for the alga Raphidocelis subcapitata. P was provided in three concentrations: 2.3 x 10-4 mol L-1, 2.3 x 10-6 mol L-1 and 1.0 x 10-6 mol L-1. Algal cells were acclimated to the specific P concentrations before the start of the experiment. The chemical equilibrium software MINEQL+ 4.61 was employed to calculate the Zn2+ concentration. After acclimated, the algal cells were inoculated into media containing different Zn concentrations (0.09 x 10-6 mol L-1 to 9.08 x 10-6 mol L-1). The study showed that besides the reduction in algal growth rates, phosphorus had an important influence on the toxicity of zinc for microalga. The inhibitory Zn2+ concentration values for R. subcapitata were 2.74 x 10-6 mol L-1, 0.58 x 10-6 mol L-1 and 0.24 x 10-6 mol L-1 for the microalgae acclimated at P concentrations of 2.3 x 10-4 mol L-1, 2.3 x 10-6 mol L-1 and 1.0 x 10-6 mol L-1, respectively. Ecotoxicological studies should consider the interaction between metal concentrations and varying P values to provide realistic data of what occurs in phytoplankton communities in environments.

Photosynthetic, morphological and biochemical biomarkers as tools to investigate copper oxide nanoparticle toxicity to a freshwater chlorophyceae

Copper oxide nanoparticles (CuO NP) have been produced on a large scale due to their economically interesting thermophysical properties. This heightens the concern about risks they may pose on their release into the environment, possibly affecting non-target organisms. Microalga are important organisms in ecotoxicological studies as they are at the base of the aquatic food chain, but information about their biochemical and photosynthetic changes in response CuO NP are still scarce. We studied the effects of CuO NP in Raphidocelis subcapitata using morphological, photosynthetic and biochemical biomarkers. Our results showed that the NP affected microalgal population growth with 0.70 mg Cu L^-1 IC_50-96 h (inhibition concentration). Based on predicted environmental concentrations of Cu NPs in aquatic environments, our results indicate potential risks of the NP to microalgae. Algal cell size, granularity and photosynthetic efficiencies were affected by the CuO NP at 0.97 and 11.74 mg Cu L^-1. Furthermore, lipid metabolism was affected mostly at the highest NP concentration, but at environmentally relevant values (0.012 and 0.065 mg Cu L^-1) the production of sterols (structural lipids) and triacylglycerols (reserve lipid) increased. Moreover, we found evidence of cell membrane impairment at the highest CuO NP concentration, and, as a photosynthetic response, the oxygen evolving complex was its main site of action. To the best of our knowledge, this is the first study to date to investigate microalgal lipid composition during CuO NP exposure, showing that it is a sensitive diagnostic tool. This research demonstrated that CuO NP may affect the physiology of R. subcapitata, and because they were observed in a primary producer, we foresee consequences to higher trophic levels in aquatic communities.

The effects of copper on photosynthesis and biomolecules yield in Chlorolobion braunii

Our knowledge of the effects of copper on microalgal physiology is largely based on studies conducted with high copper concentrations; much less is known when environmentally relevant copper levels come into question. Here, we evaluated the physiology of Chlorolobion braunii exposed to free copper ion concentrations between 5.7 × 10^-9 and 5.0 × 10^-6  mol · L^-1 , thus including environmentally relevant values. Population growth and maximum photosynthetic quantum yield of PSII were determined daily during the 96 h laboratory controlled experiment. Exponentially-growing cells (48 h) were analyzed for effective quantum yield and rapid light curves (RLC), and total lipids, proteins, carbohydrates, chlorophyll a and carotenoids were determined. The results showed that growth rates and population density decreased gradually as copper increased in experiment, but the photosynthetic parameters (maximum and effective quantum yields) and photochemical quenching (qP) decreased only at the highest free copper concentration tested (5.0 × 10^-6 mol · L^-1 ); nonphotochemical quenching (NPQ) increased gradually with copper increase. The RLC parameters Ek and rETRmax were inversely proportional to copper concentration, while α and Im decreased only at 5.0 × 10^-6 mol · L^-1 . The effects of copper in biomolecules yield (mg · L^-1 ) varied depending on the biomolecule. Lipid yield increased at free copper concentration as low as 2.5 × 10^-8 mol · L^-1 , but proteins and carbohydrates were constant throughout.

Physiological responses of Chlorella sorokiniana to copper nanoparticles

Copper (Cu) nanomaterials have been increasingly researched and produced for many different consumer products. They have high reactivity and bactericidal properties, making them important in antifouling paints, which are thus directly introduced into aquatic ecosystems. However, studies are scarce on the behavior of Cu nanoparticles (Cu-NPs) in natural aquatic systems and their interactions with primary producers such as microalgae. We investigated the effects of NPs on some physiological responses of the freshwater phytoplankton Chlorella sorokiniana. The cells were exposed to nominal concentrations ranging from 2.50 to 635.00 μg L^-1 Cu-NPs for 96 h under laboratory-controlled conditions. The cultures were monitored daily for population growth and maximum photosynthetic quantum yield. Total lipids, proteins, and carbohydrates were quantified at 72 h of Cu-NP exposure. The results showed a positive correlation between nominal Cu-NPs and Cu in the biomass (0.97 correlation coefficient) and that this was inversely proportional to total carbohydrates, with a -0.64 correlation coefficient. At the higher end of the Cu-NP concentrations tested, higher total proteins and reduced growth rates were obtained in comparison with controls; we suggest that metal-binding proteins/antioxidants and nonstructural proteins were preferentially produced under these conditions. Our results contribute to an understanding of the interaction between Cu-NPs and a cosmopolitan phytoplankton, C. sorokiniana, and we emphasize that the disposal and use of Cu-NPs requires monitoring because even at environmentally relevant concentrations, the composition of the algae was affected. Environ Toxicol Chem 2019;38:387-395. © 2018 SETAC.

An investigation onto Cd toxicity to freshwater microalga Chlorella sorokiniana in mixotrophy and photoautotrophy: A Bayesian approach

Aquatic ecosystems are composed by a myriad of dissolved organic materials that can be assimilated by microalgae, while they can perform photosynthesis, this is refereed as mixotrophy. However, ecotoxicological tests usually consider only the photoautotrophic metabolism. This research investigated the ecotoxicological differences between photoautotrophy and mixotrophy in Chlorella sorokiniana exposed to cadmium (Cd). Chlorophyll a, photosynthetic efficiency (Fv/Fm), cell viability, biochemical composition and pH were used to monitor possible toxic effects at 72 h cultures. Glucose (1 g.L^-1) was used as organic carbon source. To evaluate the probability of the photoautotrophic culture being more affected by Cd than the mixotrophic one, Bayesian statistical analysis was performed. The photoautotrophic cultures were more affected by Cd than the mixotrophic ones, with reduction of all evaluated parameters, except for protein concentration. However, in mixotrophic cultures, no changes in protein concentration and proteins:carbohydrates ratio were observed, and chlorophyll a, Fv/Fm and cell viability were only affected at the high Cd concentrations (range ln -11.5 to -9.4). However, both mixotrophy and photoautotrophy had the same probability of having the carbohydrates concentration affected by Cd. We conclude that the microalgae in mixotrophy were more resistant to the Cd than in photoautotrophy. In addition, we showed that under photoautotrophy Fv/Fm decreased linearly as Cd concentration increased, but in mixotrophy no effect was observed up to 10^-5 molL^-1 Cd, after which it decreased. We rationale that the reduced photosynthetic capacity under mixotrophy can end up reducing the release of oxygen gas, which can compromise the entire aquatic ecosystem.

Higher biomolecules yield in phytoplankton under copper exposure

Copper is an important metal for industry, and its toxic threshold in natural ecosystems has increased since the industrial revolution. As an essential nutrient, it is required in minute amounts, being toxic in slightly increased concentrations, causing great biochemical transformation in microalgae. This study aimed at investigating the physiology of Scenedesmus quadricauda, a cosmopolitan species, exposed to copper concentrations including those that trigger intracellular biochemical modifications. The Cu exposure concentrations tested ranged from 0.1 to 25 µM, thus including environmentally important levels. Microalgae cultures were kept under controlled environmental conditions and monitored daily for cell density, in vivo chlorophyll a, and photosynthetic quantum yield (Φ_M). After 24 h growth, free Cu²⁺ ions were determined, and after 96 h, cellular Cu concentration, total carbohydrates, proteins, lipids, and cell volume were determined. The results showed that both free Cu²⁺ ions and cellular Cu increased with Cu increase in culture medium. Microalgae cell abundance and in vivo chlorophyll a were mostly affected at 2.5 µM Cu exposure (3.8 pg Cu cell^-1) and above. Approximately 31% decrease of photosynthetic quantum yield was obtained at the highest Cu exposure concentration (25 µM; 25 pg Cu cell^-1) in comparison with the control. However, at environmentally relevant copper concentrations (0.5 µM Cu; 0.4 pg Cu cell^-1) cell volume increased in comparison with the control. Considering biomolecules accumulation per unit cell volume, the highest carbohydrates and proteins yield was obtained at 1.0 µM Cu (1.1 pg Cu cell^-1), while for lipids higher Cu was necessary (2.5 µM Cu; 3.8 pg Cu cell^-1). This study is a contribution to the understanding of the effects of environmentally significant copper concentrations in the physiology of S. quadricauda, as well as to biotechnological approach to increase biomolecule yield in microalgae production.

Cu and Cd affect distinctly the physiology of a cosmopolitan tropical freshwater phytoplankton

Copper and Cd are natural constituents of freshwater ecosystems, both cycling influenced by microbial communities. The present research examined the impacts of environmentally relevant concentrations of Cu and Cd on the growth, viability, cell size, chlorophyll a (Chl a) content and photochemical efficiency of the tropical freshwater phytoplankton Chlorolobion braunii. Cell growth was significantly impaired by Cu and Cd, with EC₅₀ occurring at 33.6 and 1.6µM, respectively. At sublethal levels (< EC₅₀), cell death was already induced at 5µM Cu and 1µMCd. Average cell volume significantly increased as metal concentrations increased, as did the Chl a content per cell, although the Chl a content per unit volume decreased. Copper did not affect both the photosystem II (PSII) maximum quantum yield (Φ_M) or the operational quantum yield (Φ_E), while Cd significantly impacted Φ_E, with EC₅₀ occurring at 18.4µM. Different responses for Cu and Cd were obtained whether the photochemical fluorescence quenching (Qp) or non-photochemical quenching (Qn) were considered. Qp decreased after Cd addition, but was not altered after Cu addition. Qn values significantly increased after the addition of either metal. Non-photochemical quenching due to heat dissipation (NPQ) significantly increased in response to both metals, but it was more pronounced in the case of Cd. Overall, Cd was more toxic to C. braunii than Cu.

Contrasting effects of copper limitation on the photosynthetic apparatus in two strains of the open ocean diatom Thalassiosira oceanica

There is an intricate interaction between iron (Fe) and copper (Cu) physiology in diatoms. However, strategies to cope with low Cu are largely unknown. This study unveils the comprehensive restructuring of the photosynthetic apparatus in the diatom Thalassiosira oceanica (CCMP1003) in response to low Cu, at the physiological and proteomic level. The restructuring results in a shift from light harvesting for photochemistry—and ultimately for carbon fixation—to photoprotection, reducing carbon fixation and oxygen evolution. The observed decreases in the physiological parameters F_v/F_m, carbon fixation, and oxygen evolution, concomitant with increases in the antennae absorption cross section (σ_PSII), non-photochemical quenching (NPQ) and the conversion factor (φ_e:C/η_PSII) are in agreement with well documented cellular responses to low Fe. However, the underlying proteomic changes due to low Cu are very different from those elicited by low Fe. Low Cu induces a significant four-fold reduction in the Cu-containing photosynthetic electron carrier plastocyanin. The decrease in plastocyanin causes a bottleneck within the photosynthetic electron transport chain (ETC), ultimately leading to substantial stoichiometric changes. Namely, 2-fold reduction in both cytochrome b₆f complex (cytb₆f) and photosystem II (PSII), no change in the Fe-rich PSI and a 40- and 2-fold increase in proteins potentially involved in detoxification of reactive oxygen species (ferredoxin and ferredoxin:NADP⁺ reductase, respectively). Furthermore, we identify 48 light harvesting complex (LHC) proteins in the publicly available genome of T. oceanica and provide proteomic evidence for 33 of these. The change in the LHC composition within the antennae in response to low Cu underlines the shift from photochemistry to photoprotection in T. oceanica (CCMP1003). Interestingly, we also reveal very significant intra-specific strain differences. Another strain of T. oceanica (CCMP 1005) requires significantly higher Cu concentrations to sustain both its maximal and minimal growth rate compared to CCMP 1003. Under low Cu, CCMP 1005 decreases its growth rate, cell size, Chla and total protein per cell. We argue that the reduction in protein per cell is the main strategy to decrease its cellular Cu requirement, as none of the other parameters tested are affected. Differences between the two strains, as well as differences between the well documented responses to low Fe and those presented here in response to low Cu are discussed.

Copper affects biochemical and physiological responses of Selenastrum gracile (Reinsch)

Copper is an essential metal for several physiological and metabolic processes, but a narrow range regulate its effect in phytoplankton cells. It can affect the production of biomolecules and be toxic at concentrations slightly above those required, e.g. decreasing photosynthesis and increasing respiration. The aims of this study were to analyse the changes in growth and chlorophyll a synthesis, and in biochemistry (total carbohydrates, proteins, lipids and fatty acids) of the freshwater microalga Selenastrum gracile after exposure to copper. Exponentially growing cells were exposed to 5 concentrations of free copper ions (Cu²⁺) ranging from 0.7 (control) to 13 × 10^-8 M for up to 120 h. Free Cu²⁺ ion concentrations were calculated through the chemical equilibrium model MINEQL⁺. We observed that copper was responsible for a decrease in cell density and an increase in total protein and lipid production, but no effect on total carbohydrates was detected. The increase in phospholipids and sterols and a decrease in saturated fatty acids under copper exposure suggest a change in conformation of the cell membrane, by decreasing its fluidity. We suggest this serves the cell as a system to avoid the internalization of metal, thereby acting as a detoxifying mechanism.

Influence of phosphorus on copper toxicity to Selenastrum gracile (Reinsch) Korshikov

Microalgae need a variety of nutrients for optimal growth and health. However, this rarely occurs in nature, and if nutrient proportions vary, biochemical changes can occur in phytoplankton community. This may result in modifications of zooplankton food quality, affecting aquatic food chains. Our aim was to investigate the toxicity of copper (Cu) to Selenastrum gracile, a common freshwater Chlorophyceae, at different physiological status induced by varying phosphorus (P) concentration in culture medium. Phosphorus was investigated at 2.3×10(-4), 1.1×10(-4), 2.3×10(-5), 4.6×10(-6) and 2.3×10(-6) mol L(-1) and Cu at six concentrations, ranging from 6.9×10(-9) mol L(-1) to 1.0×10(-7) mol L(-1) free Cu(2+) ions. To guarantee the cells would be in a physiological status that reflected the external P concentration, they were previously acclimated up to constant growth rate at each P concentration. Phosphorus acclimated cells were then exposed to Cu and toxicity was evaluated through population density, growth rates and chlorophyll a content. Free Cu(2+) ions concentrations were calculated through the chemical equilibrium model MINEQL(+). The results showed that higher Cu toxicity was obtained in P-limited than in P-replete cells, and that chlorophyll a/cell was higher in P-limited cells and excess Cu than in P-replete cells. This confirms the importance of microalgae nutritional status to withstand the negative effects of the trace metal.

Simulating CO₂ leakages from CCS to determine Zn toxicity using the marine microalgae Pleurochrysis roscoffensis

Due to the current climate change and ocean acidification, a new technology for CO2 mitigation has been proposed, the Carbon dioxide Capture and Storage (CCS). However, there is an ecological risk associated with potential CO2 leakages from the sub-seabed storages sites. To evaluate the effects related to CO2 leakages, laboratory-scales experiments were performed using the marine microalgae Pleurochrysis roscoffensis. Five Zn concentrations were tested at different pHs to study Zn toxicity under acidified conditions. Seawater was collected and submitted to acidification by means of CO2 injection and by HCl addition. Results showed differences between both acidification techniques: while microalgae growth was enhanced by CO2 supply, reaching the optimal growth at pH 6.5 and full inhibition at pH 5.5, HCl acidification growth was inhibited at pH 6.5. Although small concentrations of Zn were positive for P. roscoffensis growth, Zn toxicity increased at lower pHs, and more severely on samples acidified with HCl. The conclusions obtained in this work are useful to address the potential effects on the marine ecosystem related to changes in metal bioavailability during CO2 leakages scenarios.

Combined nitrogen limitation and cadmium stress stimulate total carbohydrates, lipids, protein and amino acid accumulation in Chlorella vulgaris (Trebouxiophyceae)

Metals have interactive effects on the uptake and metabolism of nutrients in microalgae. However, the effect of trace metal toxicity on amino acid composition of Chlorella vulgaris as a function of varying nitrogen concentrations is not known. In this research, C. vulgaris was used to investigate the influence of cadmium (10(-7) and 2.0×10(-8)molL(-1) Cd) under varying nitrogen (2.9×10(-6), 1.1×10(-5) and 1.1×10(-3)molL(-1)N) concentrations on its growth rate, biomass and biochemical composition. Total carbohydrates, total proteins, total lipids, as well as individual amino acid proportions were determined. The combination of Cd stress and N limitation significantly inhibited growth rate and cell density of C. vulgaris. However, increasing N limitation and Cd stress stimulated higher dry weight and chlorophyll a production per cell. Furthermore, biomolecules like total proteins, carbohydrates and lipids increased with increasing N limitation and Cd stress. Ketogenic and glucogenic amino acids were accumulated under the stress conditions investigated in the present study. Amino acids involved in metal chelation like proline, histidine and glutamine were significantly increased after exposure to combined Cd stress and N limitation. We conclude that N limitation and Cd stress affects the physiology of C. vulgaris by not only decreasing its growth but also stimulating biomolecule production.

Algal photosynthetic responses to toxic metals and herbicides assessed by chlorophyll a fluorescence

Chlorophyll a fluorescence is established as a rapid, non-intrusive technique to monitor photosynthetic performance of plants and algae, as well as to analyze their protective responses. Apart from its utility in determining the physiological status of photosynthesizers in the natural environment, chlorophyll a fluorescence-based methods are applied in ecophysiological and toxicological studies to examine the effect of environmental changes and pollutants on plants and algae (microalgae and seaweeds). Pollutants or environmental changes cause alteration of the photosynthetic capacity which could be evaluated by fluorescence kinetics. Hence, evaluating key fluorescence parameters and assessing photosynthetic performances would provide an insight regarding the probable causes of changes in photosynthetic performances. This technique quintessentially provides non-invasive determination of changes in the photosynthetic apparatus prior to the appearance of visible damage. It is reliable, economically feasible, time-saving, highly sensitive, versatile, accurate, non-invasive and portable; thereby comprising an excellent alternative for detecting pollution. The present review demonstrates the applicability of chlorophyll a fluorescence in determining photochemical responses of algae exposed to environmental toxicants (such as toxic metals and herbicides).

Metal mobility and toxicity to microalgae associated with acidification of sediments: CO2 and acid comparison

The injection and storage of CO2 into marine geological formations has been suggested as a mitigation measure to prevent global warming. However, storage leaks are possible resulting in several effects in the ecosystem. Laboratory-scale experiments were performed to evaluate the effects of CO2 leakage on the fate of metals and on the growth of the microalgae Phaeodactylum tricornutum. Metal contaminated sediments were collected and submitted to acidification by means of CO2 injection or by adding HCl. Sediments elutriate were prepared to perform toxicity tests. The results showed that sediment acidification enhanced the release of metals to elutriates. Iron and zinc were the metals most influenced by this process and their concentration increased greatly with pH decreases. Diatom growth was inhibited by both processes: acidification and the presence of metals. Data obtained is this study is useful to calculate the potential risk of CCS activities to the marine environment.

Cylindrospermopsis raciborskii (Cyanobacteria) exudates: chemical characterization and complexation capacity for Cu, Zn, Cd and Pb

Cylindrospermopsis raciborskii is a cosmopolitan and potentially toxic planktonic Cyanobacteria that produces and exudes copious amounts of dissolved organic materials. This organism dominates the eutrophic reservoir Barra Bonita (Brazil), where it normally blooms throughout the year. This investigation focused on the characterization of such exudates analyzing their capacity to complex copper, zinc, lead and cadmium through the determination of ligand concentration (CL) and conditional stability constant (logK'ML), as well as elemental composition (C, H, N and S), the content of carbohydrates, proteins, lipids and dissolved organic carbon (DOC). The dissolved organic material was fractionated into 3 molecular weights (>30 kDa; 30-10 kDa; 10-3 kDa) and each fraction was analyzed. The results showed that in the >30 kDa and 30-10 kDa fractions carbohydrates dominate over proteins and lipids. Different CL and logK'ML were obtained for the different molecular weight fractions of the excreted organic materials, suggesting high diversity of ligands. In the >30 kDa, there were more complexing sites (CL) for Cu, but higher affinity (K') for Zn. In the 30-10 kDa fraction, the higher CL was for Cd, but the greatest affinities were for Cu and Zn. In the 10-3 kDa fraction, higher CL was obtained for Cd and Zn, while Cu and Cd had the highest strengths of association. In the environment, such diversity of ligands and strengths of association can result in a displacement of metals weakly bound to the EOM, and increase metal buffering capacity of the environment, supporting higher metal inputs before toxic effects are detected in the biota.

Growth and biochemical composition of Chlorella vulgaris in different growth media

The need for clean and low-cost algae production demands for investigations on algal physiological response under different growth conditions. In this research, we investigated the growth, biomass production and biochemical composition of Chlorella vulgaris using semi-continuous cultures employing three growth media (LC Oligo, Chu 10 and WC media). The highest cell density was obtained in LC Oligo, while the lowest in Chu medium. Chlorophyll a, carbohydrate and protein concentrations and yield were highest in Chu and LC Oligo media. Lipid class analysis showed that hydrocarbons (HC), sterol esthers (SE), free fatty acids (FFA), aliphatic alcohols (ALC), acetone mobile polar lipids (AMPL) and phospholipids (PL) concentrations and yields were highest in the Chu medium. Triglyceride (TAG) and sterol (ST) concentrations were highest in the LC Oligo medium. The results suggested that for cost effective cultivation, LC Oligo medium is the best choice among those studied, as it saved the cost of buying vitamins and EDTA associated with the other growth media, while at the same time resulted in the best growth performance and biomass production.

Paralytic shellfish toxins inhibit copper uptake in Chlamydomonas reinhardtii

Paralytic shellfish toxins are secondary metabolites produced by several species of dinoflagellates and cyanobacteria. Known targets of these toxins, which typically occur at detrimental concentrations during harmful algal blooms, include voltage-gated ion channels in humans and other mammals. However, the effects of the toxins on the co-occurring phytoplankton community remain unknown. The present study examined the molecular mechanisms of the model photosynthetic alga Chlamydomonas reinhardtii in response to saxitoxin exposure as a means of gaining insight into the phytoplankton community response to a bloom. Previous work with yeast indicated that saxitoxin inhibited copper uptake, so experiments were designed to examine whether saxitoxin exhibited a similar mode of action in algae. Expression profiling following exposure to saxitoxin or a copper chelator produced similar profiles in copper homeostasis genes, notably induction of the cytochrome c6 (CYC6) and copper transporter (COPT1, CTR1) genes. Cytochrome c6 is used as an alternative to plastocyanin under conditions of copper deficiency, and immunofluorescence data showed this protein to be present in a significantly greater proportion of saxitoxin-exposed cells compared to controls. Live-cell imaging with a copper-sensor probe for intracellular labile Cu(I) confirmed that saxitoxin blocked copper uptake. Extrapolations of these data to phytoplankton metabolic processes along with the copper transporter as a molecular target of saxitoxin based on existing structural models are discussed.

Toxicant induced changes on delayed fluorescence decay kinetics of cyanobacteria and green algae: a rapid and sensitive biotest

Algal tests have developed into routine tools for testing toxicity of pollutants in aquatic environments. Meanwhile, in addition to algal growth rates, an increasing number of fluorescence based methods are used for rapid and sensitive toxicity measures. The present study stresses the suitability of delayed fluorescence (DF) as a promising parameter for biotests. DF is based on the recombination fluorescence at the reaction centre of photosystem II, which is emitted only by photosynthetically active cells. We analyzed the effects of three chemicals (3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), 3,5 Dichlorophenol (3,5 DCP) and copper) on the shape of the DF decay kinetics for potential use in phytoplankton toxicity tests. The short incubation tests were done with four phytoplankton species, with special emphasis on the cyanobacterium Microcystis aeruginosa. All species exhibited a high sensitivity to DCMU, but cyanobacteria were more affected by copper and less by 3,5 DCP than the tested green algae. Analyses of changes in the DF decay curve in response to the added chemicals indicated the feasibility of the DF decay approach as a rapid and sensitive testing tool.

Lipid composition of Chlorella vulgaris (Trebouxiophyceae) as a function of different cadmium and phosphate concentrations

Fatty acids are the fundamental structural components of membrane lipids, and the degree of saturation of the long hydrocarbon chains in microalgae contributes to regulation of growth, biomass production and reproduction of aquatic consumers. This research aimed at evaluating the effects of cadmium (2×10(-8); 10(-7) mol L(-1) Cd) on lipid class and fatty acid composition of the microalga Chlorella vulgaris under varying phosphate (PO(4)(3-)) concentrations (6.0×10(-7) to 2.3×10(-4) mol L(-1)). Under PO(4)(3-) limitation and Cd stress, the storage lipid class triacylglycerol (TAG) was the most accumulated among the lipid classes. Fatty acid composition revealed that the degree of saturation increased with increasing Cd stress and PO(4)(3-) limitation. Decreasing PO(4)(3-) and increasing Cd concentrations resulted in higher saturated fatty acid (SAFA) and monounsaturated FA (MUFA) concentrations. Total polyunsaturated FA (PUFA) and ω3 PUFA, and PUFA:SAFA ratios were higher in the control (2.3×10(-4) mol L(-1) PO(4)(3-)) cells than in either PO(4)(3-) limitation or Cd stress, or in the combination of both stresses. Contrasting with all the other PUFAs, 18:2n - 6 increased as PO(4)(3-) limitation increased. A significant positive relationship of PUFAs, acetone mobile polar lipids (AMPL) and phospholipids (PL) with phosphate concentration in the culture media was obtained, while TAG concentrations had a positive association with total MUFA and SAFA. Total SAFA, 14:0, 18:1n - 9 and 18:2n - 6 were positively correlated with Cd and negatively with PO(4)(3-) concentrations. The microalga responded to combined PO(4)(3-) limitation and Cd exposure by increasing its total lipid production and significantly altering its lipid composition. The FA 18:2n - 6 may be considered a stress biomarker for PO(4)(3-) limitation and Cd stress in C. vulgaris.