Gene regulation in the marine diatom Thalassiosira pseudonana upon exposure to polycyclic aromatic hydrocarbons (PAHs)

Selection of aquatic microbiota exposed to the herbicides flufenacet and metazachlor

Herbicides are important, ubiquitous environmental contaminants, but little is known about their interaction with bacterial aquatic communities. Here, we sampled a protected natural freshwater habitat and characterised its microbiome in interaction with herbicides. We evolved the freshwater microbiomes in a microcosm assay of exposure (28 days) to flufenacet and metazachlor at environmental concentrations of 0.5, 5 and 50 μg L-1 . Inhibitory effects of herbicides were exemplarily assessed in cultured bacteria from the same pond (Pseudomonas alcaligenes, Paenibacillus amylolyticus and Microbacterium hominis). Findings were compared to long-term concentrations as provided by local authorities. Here, environmental concentrations reached up to 11 μg L-1 (flufenacet) and 76 μg L-1 (metazachlor). Bacteria were inhibited at minimum inhibitory concentrations far above these values; however, concentrations of 50 μg L-1 of flufenacet resulted in measurable growth impairment. While most herbicide-exposed microcosm assays did not differ from controls, Acidobacteria were selected at high environmental concentrations of herbicides. Alpha-diversity (e.g., taxonomic richness on phylum level) was reduced when aquatic microbiomes were exposed to 50 μg metazachlor or flufenacet. One environmental strain of P. alcaligenes showed resistance to high concentrations of flufenacet (50 g L-1 ). In total, this study reveals that ecologic imbalance due to herbicide use significantly impacts aquatic microbiomes.

Trophodynamics and bioaccumulation of polycyclic aromatic hydrocarbons (PAHs) in marine food web from Laizhou Bay, China

Here, we collected 16 species (n = 298) from Laizhou Bay, China to investigate the trophodynamics, bioaccumulation and cancer risks of polycyclic aromatic hydrocarbons (PAHs). Results demonstrated that naphthalene was the most abundant PAH, followed by phenanthrene and fluorene in the marine organisms. The sum of 16 PAHs concentrations (Ʃ16PAHs) ranked with algae (19,435 ng·g-1 lipid weight, lw) > benthonic animals (6599 ng·g-1 lw) > fish (1760 ng·g-1 lw). Combustion and oil spill are two primary sources, contributing 60.3 % and 39.7 % of Ʃ16PAHs, respectively. High values of log BAF were found for 4-6 rings PAHs. Algae and benthonic animals showed a high ability to accumulate 2-4 rings PAHs and 5-6 rings PAHs, respectively. A biodilution pattern for PAHs was found in the marine food web. The carcinogenic risks of some benthos and fish were higher than 1 × 10-6, threatening resident health by consumption of these seafoods.

The Ability of Airborne Microalgae and Cyanobacteria to Survive and Transfer the Carcinogenic Benzo(a)pyrene in Coastal Regions

Air pollution has been a significant problem threatening human health for years. One commonly reported air pollutant is benzo(a)pyrene, a dangerous compound with carcinogenic properties. Values which exceed normative values for benzo(a)pyrene concentration in the air are often noted in many regions of the world. Studies on the worldwide spread of COVID-19 since 2020, as well as avian flu, measles, and SARS, have proven that viruses and bacteria are more dangerous to human health when they occur in polluted air. Regarding cyanobacteria and microalgae, little is known about their relationship with benzo(a)pyrene. The question is whether these microorganisms can pose a threat when present in poor quality air. We initially assessed whether cyanobacteria and microalgae isolated from the atmosphere are sensitive to changes in PAH concentrations and whether they can accumulate or degrade PAHs. The presence of B(a)P has significantly affected both the quantity of cyanobacteria and microalgae cells as well as their chlorophyll a (chl a) content and their ability to fluorescence. For many cyanobacteria and microalgae, an increase in cell numbers was observed after the addition of B(a)P. Therefore, even slight air pollution with benzo(a)pyrene is likely to facilitate the growth of airborne cyanobacteria and microalgae. The results provided an assessment of the organisms that are most susceptible to cellular stress following exposure to benzo(a)pyrene, as well as the potential consequences for the environment. Additionally, the results indicated that green algae have the greatest potential for degrading PAHs, making their use a promising bioremediation approach. Kirchneriella sp. demonstrated the highest average degradation of B(a)P, with the above-mentioned research indicating it can even degrade up to 80% of B(a)P. The other studied green algae exhibited a lower, yet still significant, B(a)P degradation rate exceeding 50% when compared to cyanobacteria and diatoms.

Effects of polycyclic aromatic hydrocarbons on marine and freshwater microalgae - A review

The first synthetic review of the PAHs effects on microalgae in experimental studies and aquatic ecosystems is provided. Phytoplankton and phytobenthos from marine and freshwaters show a wide range of sensitivities to PAHs, and can accumulate, transfer and degrade PAHs. Different toxicological endpoints including growth, chlorophyll a, in vivo fluorescence yield, membrane integrity, lipid content, anti-oxidant responses and gene expression are reported for both freshwater and marine microalgal species exposed to PAHs in culture and in natural assemblages. Photosynthesis, the key process carried out by microalgae appears to be the most impacted by PAH exposure. The effect of PAHs is both dose- and species-dependent and influenced by environmental factors such as UV radiation, temperature, and salinity. Under natural conditions, PAHs are typically present in mixtures and the toxic effects induced by single PAHs are not necessarily extrapolated to mixtures. Natural microalgal communities appear more sensitive to PAH contamination than microalgae in monospecific culture. To further refine the ecological risks linked to PAH exposure, species-sensitivity distributions (SSD) were analyzed based on published EC₅₀s (half-maximal effective concentrations during exposure). HC₅ (harmful concentration for 5% of the species assessed) was derived from SSD to provide a toxicity ranking for each of nine PAHs. The most water-soluble PAHs naphthalene (HC₅ = 650 µg/L), acenaphthene (HC₅ = 274 µg/L), and fluorene (HC₅ = 76.8 µg/L) are the least toxic to microalgae, whereas benzo[a]pyrene (HC₅ = 0.834 µg/L) appeared as the more toxic. No relationship between EC₅₀ and cell biovolume was established, which does not support assumptions that larger microalgal cells are less sensitive to PAHs, and calls for further experimental evidence. The global PAHs HC₅ for marine species was on average higher than for freshwater species (26.3 and 1.09 µg/L, respectively), suggesting a greater tolerance of marine phytoplankton towards PAHs. Nevertheless, an important number of experimental exposure concentrations and reported toxicity thresholds are above known PAHs solubility in water. The precise and accurate assessment of PAHs toxicity to microalgae will continue to benefit from more rigorously designed experimental studies, including control of exposure duration and biometric data on test microalgae.

Holistic approach to chemical and microbiological quality of aquatic ecosystems impacted by wastewater effluent discharges

Wastewater treatment plants (WWTPs) collect wastewater from various sources and use different treatment processes to reduce the load of pollutants in the environment. Since the removal of many chemical pollutants and bacteria by WWTPs is incomplete, they constitute a potential source of contaminants. The continuous release of contaminants through WWTP effluents can compromise the health of the aquatic ecosystems, even if they occur at very low concentrations. The main objective of this work was to characterize, over a period of four months, the treatment steps starting from income to the effluent and 5 km downstream to the receiving river. In this context, the efficiency removal of chemical pollutants (e.g. hormones and pharmaceuticals, including antibiotics) and bacteria was assessed in a WWTP case study by using a holistic approach. It embraces different chemical and biological-based methods, such as pharmaceutical analysis by HPLC-MSMS, growth rate inhibition in algae, ligand binding estrogen receptor assay, microbial community study by 16S and shotgun sequencing along with relative quantification of resistance genes by quantitative polymerase chain reaction. Although both, chemical and biological-based methods showed a significant reduction of the pollutant burden in effluent and surface waters compared to the influent of the WWTP, no complete removal of pollutants, pathogens and antibiotic resistance genes was observed.

Effects of a Light Crude Oil Spill on a Tropical Coastal Phytoplankton Community

Oiling scenarios following spills vary in concentration and usually can affect large coastal areas. Consequently, this research evaluated different crude oil concentrations (10, 40, and 80 mg L^-1) on the nearshore phytoplanktonic community in the southern Gulf of Mexico. This experiment was carried out for ten days using eight units of 2500 L each; factors monitored included shifts in phytoplankton composition, physicochemical parameters and the culturable bacterial abundance of heterotrophic and hydrocarbonoclastic groups. The temperature, salinity, and nutrient concentrations measured were within the ranges previously reported for Yucatan Peninsula waters. The total hydrocarbon concentration (TPH) in the control at T0 indicated the presence of hydrocarbons (PAHs 0.80 μg L^-1, aliphatics 7.83 μg L^-1 and UCM 184.09 μg L^-1). At T0, the phytoplankton community showed a similar assemblage structure and composition in all treatments. At T10, the community composition remained heterogeneous in the control, in agreement with previous reports for the area. However, for oiled treatments, Bacillariophyceae dominated at T10. Hydrocarbonoclastic bacteria were associated with oiled treatments throughout the experiment, while heterotrophic bacteria were associated with control conditions. Our results agreed with previous works at the taxonomic level showing the presence of Bacillariophyceae and Dinophyceae in oil-related treatments, where these groups showed the major interactions in co-occurrence networks. In contrast, Chlorophyceae showed the key node in the co-occurrence network for the control. This study aims to contribute to knowledge on phytoplankton community shifts during a crude oil spill in subtropical oligotrophic regions.

Influence of nutrient status on the response of the diatom Phaeodactylum tricornutum to oil and dispersant

Phytoplankton play a central role in our ecosystems, they are responsible for nearly 50 percent of the global primary productivity and major drivers of macro-elemental cycles in the ocean. Phytoplankton are constantly subjected to stressors, some natural such as nutrient limitation and some manmade such as oil spills. With increasing oil exploration activities in coastal zones in the Gulf of Mexico and elsewhere, an oil spill during nutrient-limited conditions for phytoplankton growth is highly likely. We performed a multifactorial study exposing the diatom Phaeodactylum tricornutum (UTEX 646) to oil and/or dispersants under nitrogen and silica limitation as well as co-limitation of both nutrients. Our study found that treatments with nitrogen limitation (-N and–N-Si) showed overall lower growth and chlorophyll a, lower photosynthetic antennae size, lower maximum photosynthetic efficiency, lower protein in exopolymeric substance (EPS), but higher connectivity between photosystems compared to non-nitrogen limited treatments (-Si and +N+Si) in almost all the conditions with oil and/or dispersants. However, certain combinations of nutrient limitation and oil and/or dispersant differed from this trend indicating strong interactive effects. When analyzed for significant interactive effects, the–N treatment impact on cellular growth in oil and oil plus dispersant conditions; and oil and oil plus dispersant conditions on cellular growth in–N-Si and–N treatments were found to be significant. Overall, we demonstrate that nitrogen limitation can affect the oil resistant trait of P. tricornutum, and oil with and without dispersants can have interactive effects with nutrient limitation on this diatom.

Diatom microalgae as smart nanocontainers for biosensing wastewater pollutants: recent trends and innovations

Microalgae have been recognized as one of the most efficient microorganisms to remediate industrial effluents. Among microalgae diatoms are silica shelled unicellular eukaryotes, found in all types of water bodies and flourish very well even in wastewater. They have their silica cell wall made up of nano arrayed pores arranged in a uniform fashion. Therefore, they act as smart nanocontainers to adsorb various trace metals, dyes, polymers, and drugs which are hazardous to human as well to aquatic life. The beautiful nanoarchitecture in diatoms allows them to easily bind to ligands of choice to form a nanocomposite structure with the pollutants which can be a chemical or biological component. Such naturally available diatom nanomaterials are economical and highly sensitive compared to manmade artificial silica nanomaterials to help in facile removal of the toxic pollutants from wastewater. This review is thus focused on employing diatoms to remediate various pollutants such as heavy metals, dyes, hydrocarbons detected in the wastewater. It also includes different microalgae as biosensors for determination of pollutants in effluents and the perspectives for nanotechnological applications in the field of remediating pollutants through microalgae. The review also discusses in length the hurdles and perspectives of employing microalgae in wastewater remediation.

Molecular mechanism of oil induced growth inhibition in diatoms using Thalassiosira pseudonana as the model species

The 2010 Deepwater Horizon oil-spill exposed the microbes of Gulf of Mexico to unprecedented amount of oil. Conclusive evidence of the underlying molecular mechanism(s) on the negative effects of oil exposure on certain phytoplankton species such as Thalassiosira pseudonana is still lacking, curtailing our understanding of how oil spills alter community composition. We performed experiments on model diatom T. pseudonana to understand the mechanisms underpinning observed reduced growth and photosynthesis rates during oil exposure. Results show severe impairment to processes upstream of photosynthesis, such as light absorption, with proteins associated with the light harvesting complex damaged while the pigments were unaffected. Proteins associated with photosynthetic electron transport were also damaged, severely affecting photosynthetic apparatus and depriving cells of energy and carbon for growth. Negative growth effects were alleviated when an organic carbon source was provided. Further investigation through proteomics combined with pathway enrichment analysis confirmed the above findings, while highlighting other negatively affected processes such as those associated with ferroxidase complex, high-affinity iron-permease complex, and multiple transmembrane transport. We also show that oxidative stress is not the primary route of negative effects, rather secondary. Overall, this study provides a mechanistic understanding of the cellular damage that occurs during oil exposure to T. pseudonana.

Modelling of possible tanker accident oil spills in the Istanbul Strait in order to demonstrate the dispersion and toxic effects of oil pollution

Countries located on the Black Sea coast perform most of their sea trade through the Istanbul Strait (IS). Approximately 50,000 ships pass through the IS each year, with crude oil tankers making up the majority. Thus, the aim of the study is to determine the acute toxic effect of oil pollution that may occur as a result of crude oil tanker accidents in the IS. By utilising data related to accidents that have occurred in the IS, locations of concentrated tanker accidents, or "hot spots," were determined by Kernel Density Analysis. Subsequently, the distribution of potential leaks following an oil tanker accident, within these hot spots, is modelled with GNOME software. Finally, acute toxicity caused by oil pollution in the marine ecosystem is determined by Aliivibrio fischeri luminescent bacteria toxicity test. In this research, 5 hot spots are identified, where the maximum calculated amount of oil that can reach the coastline after 72 h is 3096 metric tons. Similarly, oil pollution can affect a total coastline of 30-35 km. Furthermore, it was determined that after the oil was diluted in seawater, at a ratio of 1:200,000, the toxic effects decrease (EC₅₀ above 100 mg/L), yet the chronic effects may still continue. The results of this study may serve as a reference for coastal state authorities to develop emergency response plans. Having this valuable knowledge of where high-risk accidents are most concentrated, where the accidents occur intensely, which areas can be affected by the pollution, the duration of the pollution effects, and the distance between the areas, will help determine the number of intervention stations to be installed, their locations, and equipment to be installed to the stations.

Capacity of the potentially toxic diatoms Pseudo-nitzschia mannii and Pseudo-nitzschia hasleana to tolerate polycyclic aromatic hydrocarbons

This study investigates the effects of polycyclic aromatic hydrocarbons (PAHs) on two potentially toxic Pseudo-nitzschia hasleana and P. mannii, isolated from a PAH contaminated marine environment. Both species, maintained in non-axenic cultures, have been exposed during 144 h to increasing concentrations of a 15 PAHs mixture. Analysis of the domoic acid, showed very low concentrations. Dose-response curves for growth and photosynthesis inhibition were determined. Both species have maintained their growth until the end of incubation even at the highest concentration tested (120 µg l^-1), Nevertheless, P mannii showed faster growth and seemed to be more tolerant than P. hasleana. To reduce PAH toxicity, both species have enhanced their biovolume, with a higher increase for P. mannii relative to P hasleana. Both species were also capable of bio-concentrating PAHs and were able to degrade them probably in synergy with their associated bacteria. The highest biodegradation was observed for P. mannii, which could harbored more efficient hydrocarbon-degrading bacteria. This study provides the first evidence that PAHs can control the growth and physiology of potentially toxic diatoms. Future studies should investigate the bacterial community associated with Pseudo-nitzschia species, as responses to pollutants or to other environmental stressors could be strongly influence by associated bacteria.

Marine phytoplankton responses to oil and dispersant exposures: Knowledge gained since the Deepwater Horizon oil spill

The Deepwater Horizon oil spill of 2010 brought the ecology and health of the Gulf of Mexico to the forefront of the public's and scientific community's attention. Not only did we need a better understanding of how this oil spill impacted the Gulf of Mexico ecosystem, but we also needed to apply this knowledge to help assess impacts from perturbations in the region and guide future response actions. Phytoplankton represent the base of the food web in oceanic systems. As such, alterations of the phytoplankton community propagate to upper trophic levels. This review brings together new insights into the influence of oil and dispersant on phytoplankton. We bring together laboratory, mesocosm and field experiments, including insights into novel observations of harmful algal bloom (HAB) forming species and zooplankton as well as bacteria-phytoplankton interactions. We finish by addressing knowledge gaps and highlighting key topics for research in novel areas.

Influence of fuel oil on Platymonas helgolandica: An acute toxicity evaluation to amino acids

It is highly likely that the toxicity of water accommodated fractions (WAF) will influence marine microalgae, and consequently lead to potential risk for the marine ecological environment. However, it was often neglected whether WAF can influence the transformation of relative compounds in organisms. The metabolism of amino acids (AAs) can be used to track physiological changes in microalgae because amino acids are the basis of proteins and enzymes. In this study, using marine Chlorophyta Platymonas helgolandica as the test organism, the effects of different concentrations of WAF on AA compositions and stable carbon isotope ratios (δ¹³C) of individual AAs of Platymonas helgolandica were investigated. The results showed that the WAF of #180 fuel oil had an obvious suppressing effect on the growth and chlorophyll a content of microalgae. The growth inhibitory rate at 96 h was 80.66% at a WAF concentration of 0.50 mg L^-1 compared with the control. Furthermore, seven among the 16 AAs, including alanine, cysteine, proline, aspartic acid, lysine, histidine and tyrosine, had relatively high abundance. Under the glycolysis pathway, the cysteine abundance was higher than control, meaning that the biosynthesized pathway of alanine through cysteine as a precursor could be damaged. Phosphoenolpyruvate (PEP) was an important synthesis precursor of alanine (leucine) and aromatic AA family (Phenylalanine and tyrosine), and played an important role in δ¹³C_AAs fractionation under the WAF stress. Under the TCA pathway, to protect cell metabolism activities under WAF stress, the δ¹³C value of threonine and proline abundance in microalgae with the increase in WAF stress. Therefore, δ¹³C_AAs fractionation can be used as a novel method for toxicity evaluation of WAF on future.

Efficiency of benthic diatom-associated bacteria in the removal of benzo(a)pyrene and fluoranthene

We investigated the efficiency of a benthic diatom-associated bacteria in removing benzo(a)pyrene (BaP) and fluoranthene (Flt). The diatom, isolated from a PAH-contaminated sediment of the Bizerte Lagoon (Tunisia), was exposed in axenic and non-axenic cultures to PAHs over 7 days. The diversity of the associated bacteria, both attached (AB) and free-living bacteria (FB), was analyzed by the 16S rRNA amplicon sequencing. The diatom, which maintained continuous growth under PAH treatments, was able to accumulate BaP and Flt, with different efficiencies between axenic and non-axenic cultures. Biodegradation, which constituted the main process for PAH elimination, was enhanced in the presence of bacteria, indicating the co-metabolic synergy of microalgae and associated bacteria in removing BaP and Flt. Diatom and bacteria showed different capacities in the degradation of BaP and Flt. Nitzschia sp. harbored bacterial communities with a distinct composition between attached and free-living bacteria. The AB fraction exhibited higher diversity and abundance relative to FB, while the FB fraction contained genera with the known ability of PAH degradation, such as Marivita, Erythrobacter, and Alcaligenes. Moreover, strains of Staphylococcus and Micrococcus, isolated from the FB community, showed the capacity to grow in the presence of crude oil. These results suggest that a "benthic Nitzschia sp.-associated hydrocarbon-degrading bacteria" consortium can be applied in the bioremediation of PAH-contaminated sites.

Cell-by-cell estimation of PAH sorption and subsequent toxicity in marine phytoplankton

Polycyclic Aromatic Hydrocarbons (PAHs) have elicited increasing concern due to their ubiquitous occurrence in coastal marine environments and resultant toxicity in organisms. Due to their lipophilic nature, PAHs tend to accumulate in phytoplankton cells and thus subsequently transfer to other compartments of the marine ecosystem. The intrinsic fluorescence properties of PAHs in the ultraviolet (UV)/blue spectral range have recently been exploited to investigate their uptake modes, localization, and aggregation in various biological tissues. Here, we quantitatively evaluate the sorption of two model PAHs (phenanthrene and pyrene) in three marine phytoplankton species (Chaetoceros tenuissimus, Thalassiosira sp. and Proteomonas sp.) using a combined approach of UV excitation flow cytometry and fluorescence microscopy. Over a 48-h exposure to a gradient of PAHs, Thalassiosira sp. showed the highest proportion of PAH-sorbed cells (29% and 97% of total abundance for phenanthrene and pyrene, respectively), which may be attributed to its relatively high total lipid content (33.87 percent dry weight). Moreover, cell-specific pulse amplitude modulation (PAM) microscope fluorometry revealed that PAH sorption significantly reduced the photosynthetic quantum efficiency (F_v/F_m) of individual phytoplankton cells. We describe a rapid and precise hybrid method for the detection of sorption of PAHs on phytoplankton cells. Our results emphasize the ecologically relevant sub-lethal effects of PAHs in phytoplankton at the cellular level, even at concentrations where no growth inhibition was apparent. This work is the first study to address the cell-specific impacts of fluorescent toxicants in a more relevant toxicant-sorbed subpopulation; these cell-specific impacts have to date been unidentified in traditional population-based phytoplankton toxicity assays.

Current issues confounding the rapid toxicological assessment of oil spills

Oil spills of varying magnitude occur every year, each presenting a unique challenge to the local ecosystem. The complex, changeable nature of oil makes standardised risk assessment difficult. Our review of the state of science regarding oil's unique complexity; biological impact of oil spills and use of rapid assessment tools, including commercial toxicity kits and bioassays, allows us to explore the current issues preventing effective, rapid risk assessment of oils. We found that despite the advantages to monitoring programmes of using well validated standardised tests, which investigate impacts across trophic levels at environmentally relevant concentrations, only a small percentage of the available tests are specialised for use within the marine environment, or validated for the assessment of crude oil toxicity. We discuss the use of rapid tests at low trophic levels in addition to relevant sublethal toxicity assays to allow the characterisation of oil, dispersant and oil and dispersant mixture toxicity. We identify novel, passive dosing techniques as a practical and reproducible means of improving the accuracy and maintenance of nominal concentrations. Future work should explore the possibility of linking this tiered testing system with ecosystem models to allow the prediction and risk assessment of the entire ecosystem.

Growth dynamics and domoic acid production of Pseudo-nitzschia sp. in response to oil and dispersant exposure

The diatom genus Pseudo-nitzschia is a common component of phytoplankton communities in the Gulf of Mexico and is potentially toxic as some species produce the potent neurotoxin domoic acid. The impact of oil and chemical dispersants on Pseudo-nitzschia spp. and domoic acid production have not yet been studied; preliminary findings from a mesocosm experiment suggest this genus may be particularly resilient. A toxicological study was conducted using a colony of Pseudo-nitzschia sp. isolated from a station off the coast of Louisiana in the Gulf of Mexico. The cultures were exposed to a water accommodated fraction (WAF) of oil and a diluted chemically enhanced WAF (DCEWAF) which was a mix of oil and dispersant (20:1). Exposure to WAF induced a lag phase but did not inhibit growth rates once in exponential growth. Cultures grown in DCEWAF did not experience a lag phase but had significantly lower growth rates than the Control and WAF cultures. The cellular quota of domoic acid was higher in cultures treated with DCEWAF and WAF relative to their control values, and half of the domoic acid had leaked out of the cells into the surrounding seawater in the DCEWAF cultures while all the domoic acid remained inside the cells in WAF-treated cultures. These results suggest that the presence of oil could lead to toxic blooms, but that the application of dispersant could decrease bioaccumulation of domoic acid through the food web.

Effect of oil spill on compound-specific stable carbon isotope composition of amino acid in Nitzschia closterium and Heterosigma akashiwo

Marine microalgae, the most important primary producers in marine ecosystems, are susceptible to toxicants and usually selected as the promising organisms for chemical risk testing. The stable carbon isotope ratios (δ¹³C) of amino acids (AAs) were measured to determine if compound-specific stable isotope analysis could be used to provide insight into the toxic effects of oil spill on the marine microalgae. The water accommodated fractions (WAF) of #180 fuel oil were selected as the toxic targets with different concentrations in acute (96 h) toxic tests. Naphthalene, phenanthrene, anthracene, and fluoranthene were detected as the predominant congeners of polycyclic aromatic hydrocarbons in the WAF, and may be the main toxic components. At the same WAF concentration, the δ¹³C values of leucine in Nitzschia closterium increased with the culture time, whereas decreased in Heterosigma akashiwo. However, with the increasing WAF concentrations, the δ¹³C values of glutamic acid exhibited the increasing trend in both of the two kinds of microalgae at the same culture time. The similar phenomenon was also observed for histidine in H. akashiwo, while opposite in N. closterium, but both enriching in δ¹³C compared to the controlled experiments without the WAF. Under the different culture times and WAF concentrations, the δ¹³C values of alanine and methionine showed unexpected trend. These findings indicated that the synthesis pathways of AA in microalgae cells were affected by the WAF, thus leading to the carbon isotopic fractionation in AAs. The present study provide a novel approach to explore the toxicity mechanism of primary producers under oil spill stress, and monitor and evaluate the marine ecological risk of oil spill pollution.

Response of natural phytoplankton communities exposed to crude oil and chemical dispersants during a mesocosm experiment

During the 2010 Deepwater Horizon oil spill, the chemical dispersant Corexit was applied over vast areas of the Gulf of Mexico. Marine phytoplankton play a key role in aggregate formation through the production of extracellular polymeric materials (EPS), an important step in the biological carbon pump. This study examined the impacts of oil and dispersants on the composition and physiology of natural marine phytoplankton communities from the Gulf of Mexico during a 72-hour mesocosm experiment and consequences to carbon export. The communities were treated using the water accommodated fraction (WAF) of oil, which was produced by adding Macondo surrogate oil to natural seawater and mixed for 24 h in the dark. A chemically enhanced WAF (CEWAF) was made in a similar manner, but using a mixture of oil and the dispersant Corexit in a 20:1 ratio as well as a diluted CEWAF (DCEWAF). Phytoplankton communities exposed to WAF showed no significant changes in PSII quantum yield (F_v/F_m) or electron transfer rates (ETR_max) compared to Control communities. In contrast, both F_v/F_m and ETR_max declined rapidly in communities treated with either CEWAF or DCEWAF. Analysis of other photophysiological parameters showed that photosystem II (PSII) antenna size and PSII connectivity factor were not altered by exposure to DCEWAF, suggesting that processes downstream of PSII were affected. The eukaryote community composition in each experimental tank was characterized at the end of the 72 h exposure time using 18S rRNA sequencing. Diatoms dominated the communities in both the control and WAF treatments (52 and 56% relative abundance respectively), while in CEWAF and DCEWAF treatments were dominated by heterotrophic Euglenozoa (51 and 84% respectively). Diatoms made up the largest relative contribution to the autotrophic eukaryote community in all treatments. EPS concentration was four times higher in CEWAF tanks compared to other treatments. Changes in particle size distributions (a proxy for aggregates) over time indicated that a higher degree of particle aggregation occurred in both the CEWAF and DCEWAF treatments than the WAF or Controls. Our results demonstrate that chemically dispersed oil has more negative impacts on photophysiology, phytoplankton community structure and aggregation dynamics than oil alone, with potential implications for export processes that affect the distribution and turnover of carbon and oil in the water column.

Structural and functional responses of coastal marine phytoplankton communities to PAH mixtures

The toxicity of polycyclic aromatic hydrocarbons (PAHs) mixtures was evaluated on natural phytoplankton communities sampled from lagoons of Bizerte (South-western Mediterranean Sea) and Thau (North-western Mediterranean Sea). PAHs induced short-term dose and ecosystem-dependant decreases in photosynthetic potential. Chlorophyll a was negatively affected by increasing PAHs concentrations, together with dramatic changes in phytoplankton community composition. Size classes were strongly affected in the Bizerte compare to the Thau lagoon, with a decrease in nano- and microphytoplankton densities compare to picophytoplankton. In both locations, the diatom Entomoneis paludosa appeared favoured under PAH exposure as evidenced by increase in cell density, whereas autotrophic flagellates and dinophytes were strongly reduced. Smaller cells were more tolerant to exposure to highest PAHs concentrations, with persistent picophytoplankton carbon biomass at the end of the incubations. Apparent recovery of photosynthetic potential, accompanied with a regrowth of chlorophyll a under the lowest PAH doses, coincided with a significantly altered community composition in both lagoons. Furthermore, sensitivity to PAHs was not related to the phytoplankton cell size, and toxicity-induced modification of top-down control by grazers during the experiment cannot be excluded.

Polycyclic Aromatic Hydrocarbons (PAHs) in inland aquatic ecosystems: Perils and remedies through biosensors and bioremediation

Polycyclic Aromatic Hydrocarbons (PAHs) are among the most ubiquitous environmental pollutants of high global concern. PAHs belong to a diverse family of hydrocarbons with over one hundred compounds known, each containing at least two aromatic rings in their structure. Due to hydrophobic nature, PAHs tend to accumulate in the aquatic sediments, leading to bioaccumulation and elevated concentrations over time. In addition to their well-manifested mutagenic and carcinogenic effects in humans, they pose severe detrimental effects to aquatic life. The high eco-toxicity of PAHs has attracted a number of reviews, each dealing specifically with individual aspects of this global pollutant. However, efficient management of PAHs warrants a holistic approach that combines a thorough understanding of their physico-chemical properties, modes of environmental distribution and bioaccumulation, efficient detection, and bioremediation strategies. Currently, there is a lack of a comprehensive study that amalgamates all these aspects together. The current review, for the first time, overcomes this constraint, through providing a high level comprehensive understanding of the complexities faced during PAH management, while also recommending future directions through potentially viable solutions. Importantly, effective management of PAHs strongly relies upon reliable detection tools, which are currently non-existent, or at the very best inefficient, and therefore have a strong prospect of future development. Notably, the currently available biosensor technologies for PAH monitoring have not so far been compiled together, and therefore a significant focus of this article is on biosensor technologies that are critical for timely detection and efficient management of PAHs. This review is focussed on inland aquatic ecosystems with an emphasis on fish biodiversity, as fish remains a major source of food and livelihood for a large proportion of the global population. This thought provoking study is likely to instigate new collaborative approaches for protecting aquatic biodiversity from PAHs-induced eco-toxicity.

Morphological, physiological and molecular responses of Nitzschia palea under cadmium stress

The impact of cadmium on the diatom Nitzschia palea (Kützing) W. Smith 1856 was studied by examining the relation between valve deformities and response through biological processes and genetic expression. Cultures of N. palea were exposed to two Cd treatments (C₁ = 2.4 ± 0.6 and C₂ = 42.6 ± 4.2 µg Cd/L) along with a control (C₀ = 0 µg Cd/L) for 28 days. Cadmium bioaccumulation, diatoms growth, photosynthetic efficiencies, valve deformities and genetic expression were investigated during the course of the experiment. Cadmium exposure had significant effects on bioaccumulation, growth, valve deformities and genetic expression. Maximal effects for all studied endpoints were recorded after 7 days of exposure for the C₂ treatment, which corresponded to the sampling time and condition with maximum cadmium bioaccumulation. Abnormal raphe formations (deviation from its lateral position) were significantly more abundant in the C₂ treatment compared to the control. Molecular responses were related to cadmium level based on the number of genes impacted, intensity of the response and the frequency of observations. The expression of genes involved in the regulation of mitochondrial metabolism, photosynthesis, oxidative stress and silica metabolism was affected by cadmium exposure.

Lipid metabolism and pro-oxidant/antioxidant balance of Halamphora oceanica from the Gulf of Mexico exposed to water accommodated fraction of Maya crude oil

Diatoms play key roles in primary production and carbon fixation at a global scale and in some cases these species live on marine ecosystems impacted by crude oil (CO) spills. Halamphora oceanica, a new diatom species from the Southwest of the Gulf of Mexico was isolated and cultured in the laboratory and was exposed to water accommodated fraction (WAF) of different Maya CO loads at 0.01, 0.1, 1 and 10g/L by 96h. A battery of biomarkers involved in oxidative stress (O₂•, H₂O₂, TBARS, ROOH, RC=O, SOD, CAT, GPx), biotransformation and conjugation (total CYP450 activity and GST) moreover fatty acid (FA) metabolism (FA levels, fatty-acid synthase and acyl-CoA oxidase) were measured. Obtained results suggest that increases of PAHs in the medium (below to EC₅₀) acts as external forces able to turn-on regulatory mechanisms on H. oceanica involved in both, on the PAHs uptake and changing its aerobic metabolism to anaerobic metabolism. However, the growth of this microalgae species evaluated as chlorophyll "a" and pheophytin levels increased as the WAF concentration indicating that PAHs and other hydrosoluble hydrocarbons were used as carbon and energy sources by unidentified enzymes not evaluated in the current study. Our hypothesis was also corroborated by IBRv2. In the current study, we suppose the change from aerobic to anaerobic metabolism as a strategy for Halamphora oceanica survival exposed to petroleum hydrocarbons.

Photosynthetic and transcriptional responses of the marine diatom Thalassiosira pseudonana to the combined effect of temperature stress and copper exposure

A 96-h exposure experiment was conducted to elucidate the toxicity responses of the marine diatom Thalassiosira pseudonana upon exposure to different temperatures and copper (Cu) concentrations. Three Cu treatments (seawater control; 200μg/L Cu, EC₅₀ for the yield at 25°C; and 1000μg/L Cu, EC₅₀ for growth inhibition at 25°C) were conducted against four temperatures (10°C, 15°C, 25°C and 30°C). Growth rate and photosynthetic responses showed a significant interacting thermal-chemical effect with strong synergistic responses observed at 30°C treatments. Expression of heat shock protein (hsp) was positively modulated by increasing temperatures. Hsp 90, hsp90-2 and sit1 (related to silica shell formation) were highly expressed at 30°C under 1000μg/L Cu, while the genes encoding light harvesting proteins (3HfcpA and 3HfcpB) and silaffin precursor sil3 were significantly up-regulated at 15°C under 200μg/L Cu. Our results indicated an increase Cu toxicity to T. pseudonana under high temperature and Cu dose.

Physicochemical characteristics and toxicity of surface-modified zinc oxide nanoparticles to freshwater and marine microalgae

Because of wide applications of surface-modified zinc oxide nanoparticles (ZnO-NPs) in commercial sunscreens and their easiness of being released into water, concerns have been raised over their potential effects on aquatic organisms. This study compared physicochemical properties of silane-coated and uncoated ZnO-NPs to elucidate their toxic potencies toward three freshwater and three marine microalgae. Surfaces of ZnO-NPs (20 nm) were modified by coating with 3-aminopropyltrimethoxysilane (A-ZnO-NPs) that provides the particles with a more hydrophilic surface, or dodecyltrichlorosilane (D-ZnO-NPs) that turns the particles to hydrophobic. Uncoated ZnO-NPs formed larger aggregates and released more Zn²⁺ than did either of the two coated ZnO-NPs. The three nanoparticles formed larger aggregates but released less Zn²⁺ at pH 8 than at pH 7. Although sensitivities varied among algal species, A-ZnO-NPs and uncoated ZnO-NPs were more potent at inhibiting growth of algal cells than were D-ZnO-NPs after 96-h exposure to ZnO, uncoated ZnO-NPs, each of the coated ZnO-NPs or ZnSO₄ at 10 concentrations ranging from 0.1 to 100 mg/L. The marine diatom Thalassiosira pseudonana exposed to ZnO-NPs, A-ZnO-NPs or D-ZnO-NPs resulted in differential expressions of genes, suggesting that each of the coatings resulted in ZnO-NPs acting through different mechanisms of toxic action.

Identification and molecular characterization of cytochrome P450 (CYP450) family genes in the marine ciliate Euplotes crassus: The effect of benzo[a]pyrene and beta-naphthoflavone

Marine ciliate Euplotes crassus, a single-cell eukaryote, and has been considered as a model organism for monitoring of environmental pollutions in sediments. Cytochrome P450 (CYP450) monooxygenase are phase I enzyme involved in detoxification of environmental pollutants, such as polycyclic aromatic hydrocarbons (PAHs). However, little information on CYP450 family genes in ciliate is available. In the present study, acute toxicity of PAH, benzo[a]pyrene (B[a]P) and PAH-like model compound, beta-naphthoflavone (β-NF), was investigated; full-length cDNA sequences and genomic structure of five CYP450 genes (CYP5680A1, CYP5681A1, CYP5681B1, CYP5682A1, and CYP5683A1) were analyzed; and finally their activities and transcriptional changes were measured after exposure to PAHs for 48h. According to the results, B[a]P exposure showed a negative effect on E. crassus survival, whereas β-NF exposure showed no significant effect. The 8h-LC₅₀ value of B[a]P was determined to be 2.449μM (95%-C.L., 7.726-3.619μM). Five genes belonging to the CYP450 family had conserved domains and clustered with those of ciliate group, as revealed in phylogenetic analysis. CYP activity did not change after exposure to B[a]P, whereas it was slightly, but significantly, induced after exposure to β-NF. The mRNA expression of five CYP450 genes was significantly modulated in a concentration- and time-dependent manner after exposure to both the chemicals. Our findings suggest that CYP450 genes in E. crassus may be involved in detoxification of B[a]P and β-NF. This study would give a better understanding about the mode of action of B[a]P and β-NF in marine ciliates at the molecular level.

Accumulation of petroleum hydrocarbons in intracellular lipid bodies of the freshwater diatom Synedra acus subsp. radians

The accumulation of hydrophobic compounds by phytoplankton plays a crucial role in the biogeochemical cycle of persistent organic pollutants (POPs) in aquatic environments. We studied the accumulation of polycyclic aromatic hydrocarbons (PAHs) in the freshwater diatom Synedra acus subsp. radians during its cultivation with crude oil hydrocarbons, using epifluorescent and laser confocal microscopy as well as gas chromatography-mass spectrometry (GC/MS) analysis. Our results revealed that in the presence of crude oil or an extract of a crude oil/n-hexane solution (light oil), S. acus subsp. radians accumulated PAHs in its lipid bodies. During cultivation in the presence of a crude oil/n-hexane solution, the cells selectively accumulated C12-C18 alkanes, with a preference for C15 and C16 homologues. The length of n-alkane hydrocarbon chains accumulated in cells was similar to the acyl chains of fatty acids of the diatom. We therefore suggest that the insertion of n-alkanes into the membrane lipid bilayer promotes the transmembrane transport of PAH in diatoms. Our results confirm the hypothesis that diatoms play a role in the elimination of hydrophobic hydrocarbons from aquatic systems.

Impact of silver nanoparticles on marine diatom Skeletonema costatum

When silver nanoparticles (AgNPs) are used commercially at a large scale, they infiltrate the environment at a rapid pace. However, the impact of large quantities of AgNPs on aquatic ecosystems is still largely unknown. In aquatic ecosystems, the phytoplanktons have a vital ecological function and, therefore, the potential impact of AgNPs on the microalgae community has elicited substantial concern. Therefore, in this study, the impacts of AgNPs on a marine diatom, the Skeletonema costatum, are investigated, with a focus on their photosynthesis and associated mechanisms. Exposure to AgNPs at a concentration of 0.5 mg l(-1) significantly induces excess intracellular reactive oxygen species (ROS, 122%) and reduces 28% of their cell viability. More importantly, exposure to AgNPs reduces the algal chlorophyll-a content. Scanning electron microscopy (SEM) was conducted, which revealed that AgNPs obstruct the light absorption of algae because they adhere to their surface. The maximum photochemical efficiency of photosystem II (Fv/Fm) demonstrates that exposure to AgNPs significantly inhibits the conversion of light energy into photosynthetic electron transport. Moreover, the genes of the photosystem II reaction center protein (D1) are significantly down-regulated (P < 0.05) upon exposure to 5 mg l(-1) AgNPs. These results suggest that the physical adhesion and effects of shading of AgNPs on algae might affect their light energy delivery system and damage the crucial protein function of PSII. The photosynthesis inhibition effect of AgNPs is largely different from Ag(+) . This study shows that AgNPs at higher concentrations might have serious consequences for the succession of the phytoplankton communities and aquatic ecosystem equilibrium. Copyright © 2016 John Wiley & Sons, Ltd.

The positive relationship between ocean acidification and pollution

Ocean acidification and pollution coexist to exert combined effects on the functions and services of marine ecosystems. Ocean acidification can increase the biotoxicity of heavy metals by altering their speciation and bioavailability. Marine pollutants, such as heavy metals and oils, could decrease the photosynthesis rate and increase the respiration rate of marine organisms as a result of biotoxicity and eutrophication, facilitating ocean acidification to varying degrees. Here we review the complex interactions between ocean acidification and pollution in the context of linkage of multiple stressors to marine ecosystems. The synthesized information shows that pollution-affected respiration acidifies coastal oceans more than the uptake of anthropogenic carbon dioxide. Coastal regions are more vulnerable to the negative impact of ocean acidification due to large influxes of pollutants from terrestrial ecosystems. Ocean acidification and pollution facilitate each other, and thus coastal environmental protection from pollution has a large potential for mitigating acidification risk.

Screening the toxicity and biodegradability of petroleum hydrocarbons by a rapid colorimetric method

Crude oil and petroleum products have a wide variety of hazardous components with high toxicity and low biodegradability. Certain dyes change their colors by intercepting electron transfer reactions during the transformation processes. This study applied resazurin and 2,6-dichlorophenol-indophenol indicators for a rapid screening biodegradation capability and toxicity response to various petroleum products such as motor oil, diesel, gasoline, and phenol. Colorimetry tests were performed in test tubes, and the absorbance values were measured over time. We observed different discoloration profiles after degradation tests using Bacillus subtilis inoculum. Phytotoxicity assays were also performed to compare colorimetric screening assays with a conventional toxicity testing with plants (seed germination). The results indicated that biotransformation of oils can increase its overall toxicity. Intermediate byproducts can be formed through biodegradation and thereby increase the toxicity of oils. The assessment of acute toxicity has shown that phenol is extremely toxic to petroleum-biodegrading microbial communities. Low molecular-weight gasoline was considered biodegradable, but it also exhibited a high acute toxic effect, mainly due to its high solubility and the presence of more volatile compounds that can penetrate cells and potentially damage cellular structures.

Different transcriptional responses of heat shock protein 20 in the marine diatom Ditylum brightwellii exposed to metals and endocrine-disrupting chemicals

Diatoms are sensitive indicators of water quality, and hence used for environmental hazard assessments; however, their toxicogenomic studies have been insufficiently attempted. In the present study, we determined the cDNA sequence of heat shock protein 20 (Hsp20) gene from the diatom Ditylum brightwellii, and examined the transcriptional responses of the gene after exposing it to environmental stressors such as thermal shock, metals, and endocrine-disrupting chemicals (EDCs). The open reading frame (ORF) of DbHsp20 was 531 bp long, encoding 177 amino acid residues (19.49 kDa) with a conserved C-terminal and α-crystallin domain. The genomic region of DbHsp20 did not contain introns. Phylogeny of eukaryotic Hsp20s showed D. brightwellii was closely related to other diatoms. With regard to the gene expressional profile, real-time PCR results showed that the gene was significantly upregulated (P < 0.001) under thermal stress, with the highest change of 3.2-fold. Metals' (copper and nickel) treatments showed that it was induced after a certain point of treated concentration. On the contrary, EDCs did not display noticeable change on the expression of DbHsp20. These results suggest that the diatom Hsp20 basically responds to thermal stress, but may differentially respond to toxic substances such as metals and organic compounds such as EDCs.

Mixtures of chemical pollutants at European legislation safety concentrations: how safe are they?

The risk posed by complex chemical mixtures in the environment to wildlife and humans is increasingly debated, but has been rarely tested under environmentally relevant scenarios. To address this issue, two mixtures of 14 or 19 substances of concern (pesticides, pharmaceuticals, heavy metals, polyaromatic hydrocarbons, a surfactant, and a plasticizer), each present at its safety limit concentration imposed by the European legislation, were prepared and tested for their toxic effects. The effects of the mixtures were assessed in 35 bioassays, based on 11 organisms representing different trophic levels. A consortium of 16 laboratories was involved in performing the bioassays. The mixtures elicited quantifiable toxic effects on some of the test systems employed, including i) changes in marine microbial composition, ii) microalgae toxicity, iii) immobilization in the crustacean Daphnia magna, iv) fish embryo toxicity, v) impaired frog embryo development, and vi) increased expression on oxidative stress-linked reporter genes. Estrogenic activity close to regulatory safety limit concentrations was uncovered by receptor-binding assays. The results highlight the need of precautionary actions on the assessment of chemical mixtures even in cases where individual toxicants are present at seemingly harmless concentrations.

Photosynthetic and molecular responses of the marine diatom Thalassiosira pseudonana to triphenyltin exposure

This study aimed to investigate the responses of the marine diatom Thalassiosira pseudonana upon waterborne exposure to triphenyltin chloride (TPTCl) through determining their photosynthetic response, growth performance, and expressions of genes and proteins. Based on the growth inhibition test, the 96-h IC50 (i.e., median inhibition concentration) was found to be 1.09 μg/L (95% confidence interval (CI): 0.89-1.34 μg/L). According to photosynthetic parameters, the 96-h EC50s (i.e., median effect concentrations) were estimated at 1.54 μg/L (95% CI: 1.40-1.69 μg/L) and 1.51 μg/L (95% CI: 1.44-1.58 μg/L) for the maximum quantum yield of photosystem II (PSII) photochemistry (ΦPo) and the effective quantum yield of photochemical energy conversion in PSII (Φ2), respectively. Non-photochemical quenching in the algae was increased at low concentrations of TPTCl (0.5-1.0 μg/L) but it decreased gradually when the TPTCl concentration further increased from 1.0 to 2.5 μg/L. Results of gene expressions showed that lipid metabolism related genes were not influenced by TPTCl at 0.5 or 1.0 μg/L, while silica shell formation genes were down-regulated at 0.5 μg/L. Photosynthesis related genes were up-regulated at 0.5 μg/L TPTCl but were down-regulated at 1.0 μg/L TPTCl. Proteomics analysis revealed that relatively less proteins could be detected after exposure to 1.0 μg/L TPTCl (only about 50-60 spots) compared with that observed in the 0.5μg/L TPTCl treatment and two control groups (each with about 290-300 protein spots). At 0.5 μg/L TPTCl, five proteins were differentially expressed when compared with the seawater control and solvent control, and most of these proteins are involved in defence function to protect the biological systems from reactive oxygen species that generated by TPTCl. These proteins include oxygen-evolving enhancer protein 1 precursor, fucoxanthin chlorophyll a/c protein - LI818 clade, and mitochondrial manganese superoxide dismutase, which can function to maintain the capacity of PSII and stabilize the photosynthesis efficiency as reflected by the unchanged ΦPo and Φ2 values at 0.5 μg/L TPTCl. In contrast, the excess toxicity that caused by TPTCl at the high concentration (1.0 μg/L TPTCl) might directly damage the proteins, inhibit their expression, and/or cause the suppression of metabolism as indicated by the down-regulation of most studied proteins and genes, which could ultimately inhibit the photosynthesis and growth of the algae. Overall, this study comprehensively elucidated the toxicity effects of TPT on T. pseudonana, and partially revealed the molecular toxic mechanisms and corresponding defence responses in this model algal species.

António D, Rossi F, Lettieri T. Detection of silver nanoparticles inside marine diatom Thalassiosira pseudonana by electron microscopy and focused ion beam

In the following article an electron/ion microscopy study will be presented which investigates the uptake of silver nanoparticles (AgNPs) by the marine diatom Thalassiosira pseudonana, a primary producer aquatic species. This organism has a characteristic silica exoskeleton that may represent a barrier for the uptake of some chemical pollutants, including nanoparticles (NPs), but that presents a technical challenge when attempting to use electron-microscopy (EM) methods to study NP uptake. Here we present a convenient method to detect the NPs interacting with the diatom cell. It is based on a fixation procedure involving critical point drying which, without prior slicing of the cell, allows its inspection using transmission electron microscopy. Employing a combination of electron and ion microscopy techniques to selectively cut the cell where the NPs were detected, we are able to demonstrate and visualize for the first time the presence of AgNPs inside the cell membrane.

Can crude oil toxicity on phytoplankton be predicted based on toxicity data on benzo(a)pyrene and naphthalene?

Polycyclic aromatic hydrocarbons (PAHs), which are major components of crude oil, are responsible in large part for the toxicity of crude oil to phytoplankton. This study addressed the following question. Can reliable predictions of the aquatic toxicity of crude oil, a multi-component mixture, be described from toxicity data on individual PAH compounds? Naphthalene, the most abundant PAH compound, and benzo(a)pyrene, a highly toxic PAH compound, were selected as model compounds to quantify toxicity of crude oil on two phytoplankton species, Ditylum brightwellii and Heterocapsa triquetra, by analyzing the effects of different concentrations of these PAHs on growth rate. EC50 values suggested that the diatom D. brightwellii was more vulnerable to both toxicants than the dinoflagellate H. triquetra. However, a previous study, which investigated the impact of crude oil on the same two species, had opposite results. The differences in response from these phytoplankton species to naphthalene and benzo(a)pyrene toxicity compared to their response to crude oil suggest that they may not be solely used as surrogates to assess crude oil toxicity on phytoplankton.

EU-wide monitoring survey on emerging polar organic contaminants in wastewater treatment plant effluents

In the year 2010, effluents from 90 European wastewater treatment plants (WWTPs) were analyzed for 156 polar organic chemical contaminants. The analyses were complemented by effect-based monitoring approaches aiming at estrogenicity and dioxin-like toxicity analyzed by in vitro reporter gene bioassays, and yeast and diatom culture acute toxicity optical bioassays. Analyses of organic substances were performed by solid-phase extraction (SPE) or liquid-liquid extraction (LLE) followed by liquid chromatography tandem mass spectrometry (LC-MS-MS) or gas chromatography high-resolution mass spectrometry (GC-HRMS). Target microcontaminants were pharmaceuticals and personal care products (PPCPs), veterinary (antibiotic) drugs, perfluoroalkyl substances (PFASs), organophosphate ester flame retardants, pesticides (and some metabolites), industrial chemicals such as benzotriazoles (corrosion inhibitors), iodinated x-ray contrast agents, and gadolinium magnetic resonance imaging agents; in addition biological endpoints were measured. The obtained results show the presence of 125 substances (80% of the target compounds) in European wastewater effluents, in concentrations ranging from low nanograms to milligrams per liter. These results allow for an estimation to be made of a European median level for the chemicals investigated in WWTP effluents. The most relevant compounds in the effluent waters with the highest median concentration levels were the artificial sweeteners acesulfame and sucralose, benzotriazoles (corrosion inhibitors), several organophosphate ester flame retardants and plasticizers (e.g. tris(2-chloroisopropyl)phosphate; TCPP), pharmaceutical compounds such as carbamazepine, tramadol, telmisartan, venlafaxine, irbesartan, fluconazole, oxazepam, fexofenadine, diclofenac, citalopram, codeine, bisoprolol, eprosartan, the antibiotics trimethoprim, ciprofloxacine, sulfamethoxazole, and clindamycine, the insect repellent N,N'-diethyltoluamide (DEET), the pesticides MCPA and mecoprop, perfluoroalkyl substances (such as PFOS and PFOA), caffeine, and gadolinium.

Using transcriptomic profiles in the diatom Phaeodactylum tricornutum to identify and prioritize stressors

The transcriptomic profile of the marine diatom, Phaeodactylum tricornutum, exposed to several ecologically relevant stressors, was used to develop toxicity identification evaluation (TIE)-like gene expression assays. Algal growth inhibition was measured by flow cytometry to determine exposure concentrations that elicited a sublethal toxic response. P. tricornutum was exposed to concentrations of copper (2 μg L⁻¹), cadmium (5 μg L⁻¹), silver (20 μg L⁻¹), simazine (75 μg L⁻¹), the water accommodated fraction (WAF) of weathered crude oil (5 mg L⁻¹), 50 μg L⁻¹ ammonia, a decreased salinity treatment (15‰), and a mixture exposure of ammonia, decreased salinity and cadmium (10 μg L⁻¹). Analysis of the gene expression via microarray indicated that unique transcriptomic signals were generated for each of the individual treatments. Transcriptomic profiles of ammonia and the mixture treatment overlapped substantially. Photosynthesis related transcripts were altered in the simazine (herbicide) treatment. A transcript involved in degrading hydrocarbons, dioxygenase, had increased abundance after crude oil exposure. Overall, transcriptomic responses in the different treatments were associated with stress responses, membrane transport, transcription and translation and could be linked to contaminant mode of action. The transcriptomic profiles were used to design real-time (quantitative) polymerase chain reaction (qPCR) assays that would link changes in transcript abundance to a particular stressor in a TIE-based approach. At least one transcript for each contaminant tested (copper, cadmium, silver, salinity and ammonia) responded exclusively to that contaminant. With further development of additional transcriptomic markers for each contaminant, this new approach has potential to enhance traditional toxicology bioassays by providing additional lines of evidence to identify biologically relevant stressors within a contaminated ecosystem based on changes in the transcriptomic profile.

Thiobencarb herbicide reduces growth, photosynthetic activity, and amount of Rieske iron-sulfur protein in the diatom Thalassiosira pseudonana

We investigated the effects of the herbicide thiobencarb on the growth, photosynthetic activity, and expression profile of photosynthesis-related proteins in the marine diatom Thalassiosira pseudonana. Growth rate was suppressed by 50% at a thiobencarb concentration of 1.26 mg/L. Growth and photosystem II activity (Fv /Fm ratio) were drastically decreased at 5 mg/L, at which the expression levels of 13 proteins increased significantly and those of 11 proteins decreased significantly. Among these proteins, the level of the Rieske iron-sulfur protein was decreased to less than half of the control level. This protein is an essential component of the cytochrome b6 f complex in the photosynthetic electron transport chain. Although the mechanism by which thiobencarb decreased the Rieske iron-sulfur protein level is not clear, these results suggest that growth was inhibited by interruption of the photosynthetic electron transport chain by thiobencarb.

Estimating the capability of microalgae to physiological acclimatization and genetic adaptation to petroleum and diesel oil contamination

There is increasing scientific interest in how phytoplankton reacts to petroleum contamination, since crude oil and its derivatives are generating extensive contamination of aquatic environments. However, toxic effects of short-term petroleum exposure are more widely known than the adaptation of phytoplankton to long-term petroleum exposure. An analysis of short-term and long-term effects of petroleum exposure was done using experimental populations of freshwater (Scenedesmus intermedius and Microcystis aeruginosa) and marine (Dunaliella tertiolecta) microalgae isolated from pristine sites without crude oil product contamination. These strains were exposed to increased levels of petroleum and diesel oil. Short-term exposure to petroleum or diesel oil revealed a rapid inhibition of photosynthetic performance and cell proliferation in freshwater and marine phytoplankton species. A broad degree of inter-specific variation in lethal contamination level was observed. When different strains were exposed to petroleum or diesel oil over the long-term, the cultures showed massive destruction of the sensitive cells. Nonetheless, after further incubation, some cultures were able to grow again due to cells that were resistant to the toxins. By means of a fluctuation analysis, discrimination between cells that had become resistant due to physiological acclimatization and resistant cells arising from rare spontaneous mutations was accomplished. In addition, an analysis was done as to the maximum capacity of adaptation to a gradual contamination process. An experimental ratchet protocol was used, which maintains a strong selection pressure in a temporal scale up to several months over very large experimental populations of microalgae. Microalgae are able to survive to petroleum contamination as a result of physiological acclimatization without genetic changes. However, when petroleum concentration exceeds the physiological limits, survival depends exclusively on the occurrence on mutations that confer resistance and subsequent selection of these mutants. Finally, it is certain that further mutations and selection will ultimately determine adaptation of microalgae to the environmental forcing.

Comparison of toxicity and transcriptomic profiles in a diatom exposed to oil, dispersants, dispersed oil

Dispersants are commonly used to mitigate the impact of oil spills, however, the ecological cost associated with their use is uncertain. The toxicity of weathered oil, dispersed weathered oil, and the hydrocarbon-based dispersant Slickgone NS(®), to the diatom Phaeodactylum tricornutum has been examined using standardized toxicity tests. The assumption that most toxicity occurs via narcosis was tested by measuring membrane damage in diatoms after exposure to one of the petroleum products. The mode of toxic action was determined using microarray-based gene expression profiling in diatoms after exposure to one of the petroleum products. The diatoms were found to be much more sensitive to dispersants than to the water accommodated fraction (WAF), and more sensitive to the chemically enhanced WAF (CEWAF) than to either the WAF itself or the dispersants. Exposure to dispersants and CEWAF caused membrane damage, while exposure to WAF did not. The gene expression profiles resulting from exposure to all three petroleum mixtures were highly similar, suggesting a similar mode of action for these compounds. The observed toxicity bore no relationship to PAH concentrations in the water column or to total petroleum hydrocarbon (TPH), suggesting that an undescribed component of the oil was causing toxicity. Taken together, these results suggest that the use of dispersants to clean up oil spills will dramatically increase the oil toxicity to diatoms, and may have implications for ecological processes such as the timing of blooms necessary for recruitment.

Low concentrations of polycyclic aromatic hydrocarbons promote the growth of Microcystis aeruginosa

There is an increasing need to describe the growth characteristics of cyanobacteria exposed to polycyclic aromatic hydrocarbons (PAHs) because the presence of PAHs in lakes is known to affect the growth of this kind of microorganisms. In this work, the effects of low concentrations of PAHs on Microcystis aeruginosa (M. aeruginosa) were investigated. M. aeruginosa were cultivated in the medium with a mixture of PAHs (0.486 mg L(-1) naphthalene, 0.049 mg L(-1) phenanthrene, and 0.0015 mg L(-1) pyrene) and different concentrations of nitrogen and phosphorus. During 31 d of incubation, profiles of cell number and chlorophyll-a content were determined. The results indicated that when the concentration of an individual PAH was below its no observed effect concentration (NOEC), the exposure of M. aeruginosa to a mixture of PAHs markedly promoted cell density after 7d of culture. Low concentrations of nutrients in the medium improved the growth of M. aeruginosa in the presence of PAHs. When concentrations of both phosphorus and nitrogen were 50% lower than those of the control, the specific growth rate of M. aeruginosa increased by 100% when exposed to PAHs, and the generation time decreased from 10.5 to 5.3d. The chlorophyll-a content in medium also increased from 2.23 to 3.18 μg mL(-1), which was attributed to an increase in cell numbers.

Effects of silver nanoparticles in diatom Thalassiosira pseudonana and cyanobacterium Synechococcus sp

The aim of the present study was to investigate the effect of silver nanoparticles (AgNP) of different sizes toward two primary producer aquatic species. Thalassiosira pseudonana and Synechococcus sp. have been selected as representative models for the lower trophic organisms in marine and freshwater habitats, respectively. Time-dependent cellular growth was measured upon exposure to both AgNP and silver nitrate (AgNO(3)). In addition, AgNP behavior in freshwater and marine waters has been followed by CPS disc centrifuge, in the time frame of AgNP exposure studies, and the kinetic release of silver from AgNP of different sizes was measured by dialysis and inductively coupled plasma mass spectrometry (ICP-MS). The combination and interpretation of all these data suggest that a shared effect of AgNP and released silver was responsible for the toxicity in both organisms. Furthermore, the toxic effects induced by AgNP exposure in the present study seem to result from a mixture of parameters including aggregated state, size of the AgNP, stability of the preparation, and speciation of the released silver.

Transcriptomics responses in marine diatom Thalassiosira pseudonana exposed to the polycyclic aromatic hydrocarbon benzo[a]pyrene

Diatoms are unicellular, photosynthetic, eukaryotic algae with a ubiquitous distribution in water environments and they play an important role in the carbon cycle. Molecular or morphological changes in these species under ecological stress conditions are expected to serve as early indicators of toxicity and can point to a global impact on the entire ecosystem. Thalassiosira pseudonana, a marine diatom and the first with a fully sequenced genome has been selected as an aquatic model organism for ecotoxicological studies using molecular tools. A customized DNA microarray containing probes for the available gene sequences has been developed and tested to analyze the effects of a common pollutant, benzo(a)pyrene (BaP), at a sub-lethal concentration. This approach in diatoms has helped to elucidate pathway/metabolic processes involved in the mode of action of this pollutant, including lipid metabolism, silicon metabolism and stress response. A dose-response of BaP on diatoms has been made and the effect of this compound on the expression of selected genes was assessed by quantitative real time-PCR. Up-regulation of the long-chain acyl-CoA synthetase and the anti-apoptotic transmembrane Bax inhibitor, as well as down-regulation of silicon transporter 1 and a heat shock factor was confirmed at lower concentrations of BaP, but not the heat-shock protein 20. The study has allowed the identification of molecular biomarkers to BaP to be later on integrated into environmental monitoring for water quality assessment.

Transcriptional response of hepatic largemouth bass (Micropterus salmoides) mRNA upon exposure to environmental contaminants

Microarrays enable gene transcript expression changes in near-whole genomes to be assessed in response to environmental stimuli. We utilized oligonucleotide microarrays and subsequent gene set enrichment analysis (GSEA) to assess patterns of gene expression changes in male largemouth bass (Micropterus salmoides) hepatic tissues after a 96 h exposure to common environmental contaminants. Fish were exposed to atrazine, cadmium chloride, PCB 126, phenanthrene and toxaphene via intraperitoneal injection with target body burdens of 3.0, 0.00067, 2.5, 50 and 100 µg g(-1), respectively. This was conducted in an effort to identify potential biomarkers of exposure. The expressions of 4, 126, 118, 137 and 58 mRNA transcripts were significantly (P ≤ 0.001, fold change ≥2×) affected by exposure to atrazine, cadmium chloride, PCB 126, phenanthrene and toxaphene exposures, respectively. GSEA revealed that none, four, five, five and three biological function gene ontology categories were significantly influenced by exposure to these chemicals, respectively. We observed that cadmium chloride elicited ethanol metabolism responses, and along with PCB 126 and phenanthrene affected transcripts associated with protein biosynthesis. PCB 126, phenanthrene and toxaphene also influenced one-carbon compound metabolism while PCB 126 and phenanthrene affected mRNA transcription and mRNA export from the nucleus and may have induced an antiestrogenic response. Atrazine was found to alter the expression of few hepatic transcripts. This work has highlighted several biological processes of interest that may be helpful in the development of gene transcript biomarkers of chemical exposure in fish.