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

Interference of two typical polycyclic aromatic hydrocarbons on the induced anti-grazing defense of Tetradesmus obliquus

Anthropogenic emissions of polycyclic aromatic hydrocarbons (PAHs) cause severe ecological impacts by contaminating natural water bodies, affecting various biological groups, and altering interspecies relationships and ecological functions. This study examined the effects of two typical PAHs, phenanthrene (Phe) and naphthalene (Nap), on the anti-grazing defense mechanisms of Tetradesmus obliquus, a primary producer in freshwater food chains. Four non-lethal concentrations (0.01, 0.1, 1, and 10 mg L-1) of Phe and Nap were tested and the population growth, photosynthetic capacity, pigment content, and morphological defense of T. obliquus were analyzed. The results indicated that Phe and Nap inhibited both the growth rate and formation of defensive colonies of T. obliquus induced by Daphnia grazing cues, and the inhibition ratio increased with concentration. Phe and Nap significantly shortened the defense colony formation time of T. obliquus. Phe and Nap significantly suppressed photosynthesis in the early stages; however, the photosynthetic efficiency recovered over time. These findings highlight the high sensitivity of grazing-induced colony formation in T. obliquus to Phe and Nap at non-lethal concentrations, which could affect the interactions between phytoplankton and zooplankton in aquatic ecosystems. Our study underscores the influence of Phe and Nap on the defense mechanisms of phytoplankton and the consequential effects on ecological interactions within freshwater ecosystems, providing insight into the complex impacts of pollutants on phytoplankton-zooplankton relationships. Therefore, it is necessary to consider interspecific interactions when assessing the potential negative effects of environmental pollutants on aquatic ecosystems.

Risk assessment of trace metals and polycyclic aromatic hydrocarbons in seawater of typical bays in the Bohai Sea

The ecological risks of trace metals (Cu, Zn, As, Cd, Pb, and Hg) and PAHs in seawater from three typical bays of the Bohai Sea (the Liaodong Bay, Bohai Bay, and Laizhou Bay) were comprehensively assessed by recompiling 637 sites. Results highlighted that scrutiny should be given to the ecological risks of Cu (3.80 μg/L) in the Bohai Bay and Hg (0.23 μg/L) in the Laizhou Bay. Conversely, the Liaodong Bay exhibited negligible ecological risks related to trace metals. The risks of ΣPAHs in the Liaodong Bay, Bohai Bay, and Laizhou Bay were moderate, with mean concentrations of 368.16 ng/L, 731.93 ng/L, and 187.58 ng/L, respectively. The source allocation of trace metals and PAHs required consideration of spatial variability and anthropogenic factors, which greatly affected the distribution and composition of these pollutants. The combined ecological risks in the Bohai Bay (6.80 %) and Laizhou Bay (5.43 %) deserved more attention.

Polycyclic aromatic hydrocarbons in 55-120 μm phytoplankton

A baseline study was undertaken on polycyclic aromatic hydrocarbons (PAHs) in phytoplankton. Plankton samples from six stations (duplicates) in Kaohsiung Harbor (KH), Taiwan along with a phytoplankton control sample afar from the harbor, were collected. We applied size-fractionation to isolate phytoplankton (55-120 μm), followed by sedimentation and centrifugation to remove abiogenic particulates. The phytoplankton was freeze-dried, extracted with acetone: n-hexane (1:1, v/v), and analyzed using GC-MS. ΣPAHs in phytoplankton ranged between 5204 and 28,903 ng/g dry weight (mean: 12,150 ng/g). The ΣPAHs in KH were >7 times than the control site (C1: 3972 ng/g). Cluster analysis showed spatial gradients (northern < southern KH). Accumulated PAHs in phytoplankton were from petrogenic (fishing ports and ships) and pyrogenic (river outflows), dominated by lower-ring PAHs, likely due to their higher bioavailability in the dissolved phase. We present a practical phytoplankton isolation technique for more accurate phytoplankton PAH concentrations with insights into their distribution and sources.

Assessing the effects of a mixture of hydrophobic contaminants on the algae Rhodomonas salina using the chemical activity concept

The production and release of chemicals from human activities are on the rise. Understanding how the aquatic environment is affected by the presence of an unknown number of chemicals is lacking. We employed the chemical activity concept to assess the combined effects of hydrophobic organic contaminants on the phytoplankton species Rodomonas salina. Chemical activity is additive, and refers to the relative saturation of a chemical in the studied matrix. The growth of R. salina was affected by chemical activity, following a chemical activity-response curve, resulting in an Ea50 value of 0.078, which falls within the baseline toxicity range observed in earlier studies. The chlorophyll a content exhibited both increases and decreases with rising chemical activity, with the increase possibly linked to an antioxidant mechanism. Yet, growth inhibition provided more sensitive and robust responses compared to photosynthesis-related endpoints; all measured endpoints correlated with increased chemical activity. Growth inhibition is an ecologically relevant endpoint and integrates thermodynamic principles such as membrane disruption. Our study utilized passive dosing, enabling us to control exposure and determine activities in both the medium and the algae. The concept of chemical activity and our results can be extended to other neutral chemical groups as effects of chemical activity remain independent of the mixture composition.

The interference of marine accidental and persistent petroleum hydrocarbons pollution on primary biomass and trace elements sink

More than 80 % of the primary biomass in marine environments is provided by phytoplankton. The primary mechanism in the trace element sink is the absorption of trace elements by phytoplankton. Because of their difficult degradability and bioaccumulation, petroleum hydrocarbons are one of the most significant and priority organic contaminants in the marine environment. This study chose Chlorella pyrenoidosa as the model alga to be exposed to short and medium-term petroleum hydrocarbons. The ecological risk of accidental and persistent petroleum hydrocarbon contamination was thoroughly assessed. The interaction and intergenerational transmission of phytoplankton physiological markers and trace element absorption were explored to reflect the change in primary biomass and trace element sink. C. pyrenoidosa could produce a large number of reactive oxygen species stimulated by the concentration and exposure time of pollutants, which activated their antioxidant activity (superoxide dismutase (SOD) activity, β-carotene synthesis, antioxidant trace elements uptake) and peroxides production (hydroxyl radicals and malondialdehyde). The influence of the growth phase on SOD activity, copper absorption, and manganese adsorption in both persistent and accidental pollution was significant (p < 0.05, F > Fα). Adsorption of manganese and selenium positively connected with SOD, malondialdehyde, and Chlorophyl-a (p < 0.01). These findings convincingly indicate that petroleum hydrocarbon contamination can interfere with primary biomass and trace element sinks.

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.

Seasonal variation of temperature affects HMW-PAH accumulation in fishery species by bacterially mediated LMW-PAH degradation

Currently, the specific mechanism generating seasonal variation in polycyclic aromatic hydrocarbons (PAHs) via bacterial biodegradation remains unclear, and whether this alteration affects PAH bioaccumulation is unknown. Therefore, we performed a study between 2015 and 2020 to investigate the effects of seasonal variation on bacterial communities and PAH bioaccumulation in the Pearl River Estuary. Significantly high PAH concentrations in both aquatic and fishery species were determined in dry seasons (the mean ∑₁₆PAH concentration: water, 37.24 ng/L (2015), 30.83 ng/L (2020); fish, 51.01 ng/L (2015) and 72.60 ng/L (2020)) compared to wet seasons (the mean ∑₁₆PAH concentration: water, 22.38 ng/L (2015), 19.40 ng/L(2020); fish, 25.28 ng/L (2015) and 32.59 ng/L (2020)). Distinct differences in taxonomic and functional composition of bacterial communities related to biodegradation of low molecular weight PAHs (LMW-PAHs) were observed between seasons, and the concentrations of PAHs were negatively correlated with seasonal variation in temperature. Temperature-related specific bacterial taxa (e.g., Stenotrophomonas) directly or indirectly participated in LMW-PAH degradation via encoding PAH degradation enzymes (e.g., protocatechuate 4,5-dioxygenase) that subsequently led to bioaccumulation of high molecular weight PAHs (HMW-PAHs) in wild and fishery species due to LMW-PAHs in the water. Based on this alteration, the ecological risk posed by PAHs decreased in wet seasons, and an unbalanced spatio-temporal distribution of PAHs was observed in this estuary. These results suggest that seasonal variation of temperature affects HMW-PAH accumulation in fishery species via bacterially mediated LMW-PAH biodegradation.

Contrasting sensitivity among oligotrophic marine microbial communities to priority PAHs

Oligotrophic areas represent a large proportion of the oceans, wherein microbial food webs largely determine carbon flux dynamics and biogeochemical cycles. However, little is known regarding the sensitivity of microbial planktonic communities to pollutants in such areas. Organic pollutants such as polycyclic aromatic hydrocarbons (PAH/s) are toxic oil derivatives that occur as complex mixtures and reach marine environments through different sources. Therefore, our study analyzed the PAH tolerance of natural photosynthetic and heterotrophic bacteria and eukaryotes from the oligotrophic Red Sea, which is uniquely susceptible to high oil contamination. Natural communities sampled from the surface layer were exposed to a concentration gradient of a mixture of 16 priority PAHs at in situ conditions for 48 h. The populations of the dominant picocyanobacteria Synechococcus sp., picophytoeukaryotes, and low nucleic acid (LNA) bacteria decreased upon exposure to PAHs in a strong dose-dependent manner. Chlorophyll-a, which was measured as an indicator of the total autotrophic community response, also decreased substantially. High nucleic acid (HNA) bacteria, however, exhibited lower growth inhibition (<50%). The lethal concentration (LC₁₀) thresholds to the 16-PAH mixture demonstrated contrasting sensitivities among the microbial communities studied increasing from picoeukaryotes (5.98 ± 2.08 μg L^-1) < chlorophyll-a (19.51 ± 8.11 μg L^-1) < LNA bacteria (23.63 ± 10.64 μg L^-1) < Synechococcus sp. (26.77 ± 13.34 μg L^-1) < HNA bacteria (97.13 ± 17.28 μg L^-1). The sensitivity of Red Sea Synechococcus and picophytoeukaryotes to the 16-PAH mixture was between 2 and 6.5 times higher compared to single PAH compounds tested previously. However, some populations of HNA bacteria and Synechococcus sp., were highly tolerant, suggesting an adaptation to chronic pollution. Concerningly, the LC₁₀ toxicity thresholds approached the ambient PAH concentrations in the Red Sea, suggesting that environmental oil pollution actively shapes the microbial community structures in the region.

Chemical contamination alters the interactions between bacteria and phytoplankton

Bacteria and phytoplankton are key players in aquatic ecosystem functioning. Their interactions mediate carbon transfer through the trophic web. Chemical contamination can alter the function and diversity of phytoplankton and bacterioplankton, with important consequences for ecosystem functioning. The aim of the present study was to assess the impact of chemical contamination on the interactions between both biological compartments. Two contrasting marine coastal ecosystems, offshore waters and lagoon waters, were exposed to chemical contamination (artificial or produced from resuspension of contaminated sediment) in microcosms in four seasons characterized by distinct phytoplankton communities. Offshore waters were characterized by a complex phytoplankton-bacterioplankton network with a predominance of positive interactions between both compartments, especially with Haptophyta, Cryptophyta, and dinoflagellates. In contrast, for lagoon waters, the phytoplankton-bacterioplankton network was simpler with a prevalence of negative interactions with Ochrophyta, Cryptophyta, and flagellates. Contamination with an artificial mix of pesticides and trace metal elements resulted in a decrease in the number of interactions between phytoplankton and bacterioplankton, especially for offshore waters. Resuspension of contaminated sediment also altered the interactions between both compartments. The release of nutrients stored in the sediment allowed the growth of nutrient limited phytoplankton species with marked consequences for the interactions with bacterioplankton, with a predominance of positive interactions, whereas in lagoon waters, negative interactions were mostly observed. Overall, this study showed that chemical contamination and sediment resuspension resulted in significant effects on phytoplankton-bacterioplankton interactions that can alter the functioning of anthropogenic coastal ecosystems.

Combined effects of seawater acidification and benzo(a)pyrene on the physiological performance of the marine bloom-forming diatom Skeletonema costatum

The combined effects of polycyclic aromatic hydrocarbons and seawater acidification are poorly understood. Hence, we exposed the bloom-forming diatom Skeletonema costatum to four concentrations (0, 0.1, 1 and 10 μg L^-1) of benzo(a)pyrene and two pCO₂ levels (400 and 1000 μatm) to investigate its physiological performance. The growth and photosynthesis of S. costatum were tolerant to low and moderate benzo(a)pyrene concentrations regardless of the pCO₂ level. However, the highest benzo(a)pyrene concentration had remarkably adverse effects on most parameters, decreasing the growth rate by 69%. Seawater acidification increased the sensitivity to high light stress, as shown by the lower maximum relative electron transport rate and light saturation point at the highest benzo(a)pyrene concentration. Our results suggested that benzo(a)pyrene could be detrimental to diatoms at a habitat-relevant level, and seawater acidification might further decrease its light tolerance, which would have important ramifications for the community structure and primary production in coastal waters.

Bioremediation of polycyclic aromatic hydrocarbons contaminated soil under the superimposed electric field condition

An innovative superimposed electric field (SEF) was designed with the aim to achieve uniform removal of polycyclic aromatic hydrocarbons (PAHs) in soil. Also the influence of SEF on the bioremediation efficiency of PAHs was investigated in compared with the common electric field (CEF). Five experiments were conducted in this study, namely EK-CEF (applied CEF), EKB-CEF (CEF enhanced bioremediation), EK-SEF (applied SEF), EKB-SEF (SEF enhanced bioremediation), and Bio (bioremediation). The results indicated that electric field with periodically reversed polarity could effectively prevent the occurrence of large changes in soil pH, temperature, and electric current. The electric field intensity of SEF was concentrated in the range of 0.5-1.5 V/cm, and the difference between the maximum and minimum PAHs removal percentage in EK-SEF was just 5.4%, in comparison to 14.8% in EK-CEF. The bioremediation promoting effect did not show significant difference between SEF and CEF. Compared to Bio, the removal percentages of the 5-ring and 6-ring PAHs attributed to the degrading bacteria were much higher in EKB-SEF and EKB-CEF. Moreover, the microbial number increased with the distance away from electrodes, and the microbial community changed correspondingly. All these would be resulted in differences removal efficiencies among different PAHs components. Despite its intrinsic advantages, the influence of SEF on soil physicochemical and biological properties needs further study.

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.

Recent advances in biochar engineering for soil contaminated with complex chemical mixtures: Remediation strategies and future perspectives

Heavy metal/metalloids (HMs) and polycyclic aromatic hydrocarbons (PAHs) in soil have caused serious environmental problems, compromised agriculture quality, and have detrimental effects on all forms of life including humans. There is a need to develop appropriate and effective remediation methods to resolve combined contaminated problems. Although conventional technologies exist to tackle contaminated soils, application of biochar as an effective renewable adsorbent for enhanced bioremediation is considered by many scientific researchers as a promising strategy to mitigate HM/PAH co-contaminated soils. This review aims to: (i) provide an overview of biochar preparation and its application, and (ii) critically discuss and examine the prospects of (bio)engineered biochar for enhancing HMs/PAHs co-remediation efficacy by reducing their mobility and bioavailability. The adsorption effectiveness of a biochar largely depends on the type of biomass material, carbonisation method and pyrolysis conditions. Biochar induced soil immobilise and remove metal ions via various mechanisms including electrostatic attractions, ion exchange, complexation and precipitation. PAHs remediation mechanisms are achieved via pore filling, hydrophobic effect, electrostatic attraction, hydrogen bond and partitioning. During last decade, biochar engineering (modification) via biological and chemical approaches to enhance contaminant removal efficiency has garnered greater interests. Hence, the development and application of (bio)engineered biochars in risk management, contaminant management associated with HM/PAH co-contaminated soil. In terms of (bio)engineered biochar, we review the prospects of amalgamating biochar with hydrogel, digestate and bioaugmentation to produce biochar composites.

Exploration of nanocellulose washing agent for the green remediation of phenanthrene-contaminated soil

Polycyclic aromatic hydrocarbons are hazardous contaminants existing ubiquitously in polluted soil. In this study, using nanocellulose (CNC) fluid as an eco-friendly agent was proposed for the first time in the remediation of phenanthrene (PHE) contaminated soil. The effects of environmental factors on the mobilization of PHE in soil by CNC nanofluid was investigated using factorial analysis. The results showed that temperature and ionic strength had a significant influence on PHE removal, which were associated with the viscosity and zeta potential change in the nanofluid. The analysis based on two-dimensional correlation spectroscopy integrated with FTIR and synchrotron-based XRF imaging revealed that metals and minerals in soil played important roles in PHE detachment. The hydroxyl groups on CNC bonded with Fe-O, Si-O, and Mn-O in soil as time went on, and eventually achieved PHE mobilization through the interruption of PHE/SOM-metal/mineral linkages. The complexation and transport of PHE/SOM-metals/minerals from soil particles to the aqueous phase could be the primary PHE removal mechanism. Besides, the biotoxicity study displayed a detoxification effect of CNC nanofluid on PHE contaminants in soil. This study offers new insight into a cost-effective and biodegradable nanocellulose washing agent, which can be a good alternative to the available site remediation options.

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.

Phenanthrene and pyrene disturbed the growth of Microcystis aeruginosa as co-cultured with Chlorella pyrenoidosa

Significant levels of polycyclic aromatic hydrocarbons (PAHs) were detected in lakes. The competition between algae would be disturbed by PAHs resulted in variations of algal growth. For controlling the cyanobacterial blooms, it is important to understand this disturbed competition between Microcystis aeruginosa and other algae. A 6-day cultivation experiment was designed to investigate the responses of M. aeruginosa to PAHs in presence of green algae. A popular green alga Chlorella pyrenoidosa was used as a representative of green algae, and phenanthrene and pyrene were selected as representatives of PAHs. The results showed that M. aeruginosa outcompeted C. pyrenoidosa under PAH contamination, and PAHs and M. aeruginosa significantly inhibited the survival of C. pyrenoidosa. PAHs disturbed the growth of algae by influencing photosynthetic pigments and phycobiliproteins, and the different alteration of F_v/F_m ratios implied that shifted algal community composition would be induced by PAHs. The F_v/F_m of the two algal mixture and individual C. pyrenoidosa was significantly negatively correlated with phenanthrene levels. However, there were no significant correlations between the F_v/F_m of M. aeruginosa and the exposure levels of phenanthrene or pyrene. Remarkably, the F_v/F_m significantly increased in M. aeruginosa at 0.15 mg L^-1 pyrene, suggesting that PSII resistance to pyrene was enhanced in M. aeruginosa. Our results pointed out an increasing frequency and intensity of cyanobacterial blooms could be induced by PAHs in contaminated waters.

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.

Influence of eugenol on algal growth, cell physiology of cyanobacteria Microcystis aeruginosa and its interaction with signaling molecules

Essential oils (EOs) are naturally occurring substances that have shown great prospect in the field of antimicrobial, antioxidant and pest control by nontoxic mechanisms. In this regard, EOs are considered the promising and eco-friendly approach for controlling harmful algae. In this study, the anti-cyanobacterial activity of EOs eugenol against Microcystis aeruginosa are evaluated from the perspective of photosynthetic efficiency, the behavior of extracellular organic matter (EOM), endogenous plant hormone synthesis, and nitric oxide signaling pathway. Results showed that the photosynthetic activity of M. aeruginosa decreased significantly after eugenol treatments. Eugenol treatment resulted in cells rupture and the release of EOM. Levels of endogenous plant hormones salicylic acid (SA) and jasmonic acid (JA) were enhanced separately by 2.32 and 2.01 times after 4 d of exposure to eugenol. And the inhibition of SA and JA biosynthesis further promotes the inhibitory effects of eugenol on algae. Additionally, the signaling molecule nitric oxide (NO) increased significantly by 3.78-fold. Furthermore, the influence of NO on microalgae exposed to eugenol was also determined, suggesting that the inhibitory effect of eugenol stress might be associated with NO generation in M. aeruginosa. These findings will be helpful for the understanding of the fate and potential of eugenol in harmful algae control.

Bioaccumulation and oxidative damage of polycyclic aromatic hydrocarbon mixtures in Manila clam Ruditapes philippinarum

The aim of this study was to investigate the bioaccumulation and oxidative damage of Manila clam, Ruditapes philippinarum, exposed to four selected mixtures of polycyclic aromatic hydrocarbons (PAHs; benzo (a) pyrene (BaP), benzo (a) anthracene (BaA), benzo (b) fluoranthene (BbF), and chrysene (Chr) in equal proportion. For this purpose, clams were exposed to PAHs (BaP:BbF:BaA:Chr = 1:1:1:1) at different concentrations (0.05, 0.5, and 5 μg/L) for 21 days, followed by a 15-day depuration period. All four PAHs accumulated in the gill, digestive gland, adductor muscle, and soft tissue of Manila clams, and all PAH treatment groups showed clear time and dose dependence. The decreasing order of bioaccumulation for the four PAHs in the exposure experiment was Chr > BaA > BaP > BbF. Moreover, the order of PAH bioaccumulation for the four tissues during the whole experiment was digestive gland > gill > soft tissues > adductor muscles. Although the initial concentrations of the four PAHs were the same, the final accumulated contents were different. Therefore, we also determined the detoxification processes of the four PAH mixtures in gills and digestive glands. The bioaccumulation of Chr was higher than the other three PAHs, probably because clams have a lower metabolic capacity for Chr than for BaP, BbF, and BaA. Exposure to PAH mixtures can result in oxidative damage, as indicated by the fact that DNA strand breaks, lipid peroxidation (LPO), and protein carbonyl (PC) were induced significantly (P < 0.05), except in the low-dose groups of PAHs, and different trends were detected with time of exposure. According to the correlation analysis, aryl hydrocarbon hydroxylase, glutathione s-transferase, superoxide dismutase, DNA strand break, PC, and LPO in both the gill and digestive gland are potential early indicators of PAH mixtures. We investigated the accumulation rules of R. philippinarum exposed to the selected PAHs and screened the potential biomarkers. The results of our study provide important scientific information for the purpose of monitoring marine pollution.

How will marine plastic pollution affect bacterial primary producers?

We demonstrated in our recent Communications Biology paper how marine photosynthetic bacteria, Prochlorococcus, are adversely affected by leachates from commonly used plastics. This study was one of the first to consider how substances leaching from plastics may affect marine primary producers and demonstrated that plastic pollution has the potential to negatively impact a wider range of organisms than previously appreciated. We outline here key outstanding questions regarding how ocean plastic pollution may impact small, but essential, marine microbes and discuss how these can be addressed.

Following up on their recent study in Communications Biology Sasha Tetu et al discuss how plastic pollution of the oceans may affect marine microbes as well as strategies to identify the substances responsible for leachate toxicity and to further understand their impact.

Pyrene metabolites by bacterium enhancing cell division of green alga Selenastrum capricornutum

Collaborations between multiple microbial species are important for understanding natural clearance and ecological effects of toxic organic contaminants in the environment. However, the interactions between different species in the transformation and degradation of contaminants remain to address. In this study, the effects of pyrene and its bacterial metabolites on the algal growth (Selenastrum capricornutum) were examined. The specific growth rate of algal cells incubated with bacterial pyrene metabolites (1.18 d^-1) was highest among all treatment, followed by the controls (1.07 d^-1), treated with pyrene-free bacterial metabolites (1.04 d^-1) and those treated with pyrene (0.55 d^-1). G₁ phase is the key growth phase for the cells to synthesize biomolecules for subsequent cell division in the cell cycle. Approximately 76.9% of the cells treated with bacterial pyrene metabolites were at the G₁ phase and significantly lower than those with the controls (85.3%), pyrene-free bacterial metabolites (85.5%) and pyrene treatment (92.5%). Transcriptomic analysis of algae showed that the expression of 47 ribosomal unigenes was down-regulated by 5 mg L^-1 of pyrene, while 308 unigenes related to the preparation of cell division (DNA replication and protein synthesis) were up-regulated by bacterial pyrene metabolites. It indicated that basal metabolism associated with the growth and proliferation of algal cells could be significantly promoted by bacterial pyrene metabolites. Overall, this study suggests a close relationship between algae and bacteria in the transformation and ecological effects of toxic contaminants.