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.

Monoculture and co-culture tests of the toxicity of four typical herbicides on growth, photosynthesis and oxidative stress responses of the marine diatoms Pseudo-nitzschia mannii and Chaetoceros decipiens.

The toxicity of four herbicides in mixture (alachlor, diuron, des-isopropyl-atrazine and simazine) on the growth and the photosynthesis parameters of two marine diatoms Pseudo-niszchia mannii and Chaetoceros decipiens have been investigated for 9 days in monoculture and co-culture tests. The catalase (CAT) and guaiacol peroxidase (GPX) were also monitored to assess the oxidative stress response. In single-species assays, while both species displayed no affected instantaneous growth rate by herbicides, their physiological responses were different. Chl a content of P. mannii significantly decreased upon herbicide exposure, due probably to pigment destruction or inhibition of their synthesis. This decrease was associated with a reduction in the chlorophyll fluorescence parameters (ABS0/RC, TR0/RC, ET0/RC and DI0/RC). In contrast, C. decipiens maintained an effective photosynthetic performance under herbicide exposure, as Chl a per cell content and the specific energy fluxes per reaction center remained unchanged relative to control values. GPX activity was significantly higher in contaminated P. mannii and C. decipiens monocultures than in controls at early herbicide exposure (1 day), whereas a significant induction of CAT activity occurred later (from day 3 for C. decipiens and at day 9 for P. mannii) in response to herbicides. In control co-culture, P. mannii was eliminated by C. decipiens. As observed in the monoculture, the herbicides did not affect the photosynthetic performance of C. decipiens in co-culture, but significantly reduced its instantaneous growth rate. The oxidative stress response in co-culture has similar trends to that of C. decipiens in monoculture, but the interspecies competition likely resulted in higher CAT activity under herbicide exposure. Results of this study suggest that herbicide toxicity for marine diatoms might be amplified by interspecies interactions in natural communities, which might lead to different physiological and growth responses.

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.

Impacts of chemical contamination on bacterio-phytoplankton coupling.

Phytoplankton and bacterioplankton are the key components of the organic matter cycle in aquatic ecosystems, and their interactions can impact the transfer of carbon and ecosystem functioning. The aim of this work was to assess the consequences of chemical contamination on the coupling between phytoplankton and bacterioplankton in two contrasting marine coastal ecosystems: lagoon waters and offshore waters. Bacterial carbon demand was sustained by primary carbon production in the offshore situation, suggesting a tight coupling between both compartments. In contrast, in lagoon waters, due to a higher nutrient and organic matter availability, bacteria could rely on allochthonous carbon sources to sustain their carbon requirements, decreasing so the coupling between both compartments. Exposure to chemical contaminants, pesticides and metal trace elements, resulted in a significant inhibition of the metabolic activities (primary production and bacterial carbon demand) involved in the carbon cycle, especially in offshore waters during spring and fall, inducing a significant decrease of the coupling between primary producers and heterotrophs. This coupling loss was even more evident upon sediment resuspension for both ecosystems due to the important release of nutrients and organic matter. Resulting enrichment alleviated the toxic effects of contaminants as indicated by the stimulation of phytoplankton biomass and carbon production, and modified the composition of the phytoplankton community, impacting so the interactions between phytoplankton and bacterioplankton.

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.

Effects of nitrogen supply on Pseudo-nitzschia calliantha and Pseudo-nitzschia cf. seriata: field and laboratory experiments.

The effects of inorganic and organic nitrogen supply on the growth and domoic acid (DA) production of Pseudo-nitzschia cf. seriata and Pseudo-nitzschia calliantha from Bizerte Lagoon (SW Mediterranean Sea) were studied during field and laboratory experiments. Nitrogen enrichments (40 µM NO3 (-); 10 µM NH4 (+); 20 µM CH4N2O) and a control, with no added N, were carried out in separate carboys with seawater collected from Bizerte Lagoon. In the field experiments, all N-enrichments resulted in significant increases in chlorophyll a concentration, and maintained exponential growth until the end of the experiment. The initial diatom community was dominated by a bloom of P. cf. seriata (9.3 × 10(5) cells l(-1)). After 6 days of incubation, the abundance of P. cf. seriata was greatest in the urea addition (1.52 × 10(6) cells l(-1)), compared to the ammonium treatment (0.47 × 10(6) cells l(-1)), nitrate treatment (0.70 × 10(6) cells l(-1)) and control (0.36 × 10(6) cells l(-1)). The specific growth rates, calculated from increases in chlorophyll a and cell abundance, were statistically different across all treatments, with the highest in the urea and nitrate additions. Similar results were obtained from the laboratory experiments. These were carried out with P. calliantha isolated from Bizerte Lagoon and grown in f/2 medium enriched with 40 µM nitrate, 10 µM ammonium and 20 µM urea. The exponential growth rate was significantly faster for the cells cultured with urea (1.50 d(-1)) compared to the nitrate (0.90 d(-1)) and ammonium (0.80 d(-1)) treatments and the control (0.40 d(-1)). Analysis of DA, performed at the beginning and the end of the both experiments in all treatments, revealed very low concentrations (below the limit of quantification, 0.02- 1.310(-7) pg cell(-1), respectively).The field and laboratory experiments demonstrate that P.cf. seriata and P. calliantha are able to grow efficiently on the three forms of N, but with a preference for urea.

Three-dimensional (3-D) fluorescence spectroscopy analysis of the fluorescent dissolved organic matter released by the marine toxic dinoflagellate Alexandrium catenella exposed to metal stress by zinc or lead.

We investigated the effects of zinc or lead on growth and on exudation of fluorescent dissolved organic matter (FDOM) by the marine toxic dinoflagellate Alexandrium catenella (Whedon & Kofoid) Balech. The species was exposed to increasing free zinc (1.34 × 10(-7) M-3.98 × 10(-6) M) or lead (5.13 × 10(-9) M-1.82 × 10(-7) M) concentra-tions. Low metal levels ([Zn(2+) ] = 1.34 × 10(-7) M; [Pb(2+) ] = 5.13 × 10(-9) M) had no effect on cell growth. Toxic effects were observed from higher metal contamination ([Zn(2+) ] = 3.98 × 10(-6) M; [Pb(2+) ] = 6.54 × 10(-8) M), as a conversion of vegetative cells into cysts. Analysis of the released FDOM by three-dimensional (3-D) fluorescence spectroscopy was achieved, using the parallel factor analysis (PARAFAC). The PARAFAC modeling revealed four components associated with two contributions: one related to the biological activity; the other linked to the organic matter decomposition in the culture medium. The C1 component combined a tryptophan peak and characteristics of humic substances, whereas the C2 component was considered as a tryptophan protein fluorophore. The two others C3 and C4 components were associated with marine organic matter production. Relea-sed fluorescent substances were induced by low ([Zn(2+) ]= 1.34 × 10(-7) M; [Pb(2+) ] = 5.13 × 10(-9) M) and moderate ([Zn(2+) ] = 6.21 × 10(-7) M; [Pb(2+) ] = 2.64× 10(-9) M) metal concentrations, suggesting the activation of cellular mechanisms in response to metal stress, to exudate FDOM that could complex metal cations and reduce their toxicity toward A. catenella cells.

[Seasonal dynamics of genus Alexandrium (potentially toxic dinoflagellate) in the lagoon of Bizerte (North of Tunisia) and controls by the abiotic factors]. / Dynamique saisonnière du genre Alexandrium (dinoflagellé potentiellement toxique) dans la lagune de Bizerte (Nord de la Tunisie) et contrôle par les facteurs abiotiques environnants.

Some species of the genus Alexandrium are known as potential producers of saxitoxin, a neurotoxin that causes the paralytic shellfish poisoning (PSP) syndrome. Blooming of these species, especially in shellfish farms can affect the aquaculture production and harm human health. Seasonal dynamics of Alexandrium spp. abundance in relationship to environmental factors was investigated from November 2007 to February 2009 at six stations in the Bizerte lagoon, an important shellfish farming area situated in SW Mediterranean. The sampling stations represented different hydrological and trophic conditions: one station TJ (Tinja) is affected by the river plume; two stations (Chaara [Ch] and Canal [Ca]) are influenced by marine inflow (particularly in summer), industrial and urban effluents; and the three other stations (Menzel Abdelrahmen [MA], Menzel Jemil [MJ] and Douaouda [Do]) are located close to shellfish farms. Cell abundance of Alexandrium spp. varied among stations and months. Species of this genus showed a sporadic appearance, but they reached high concentration (0.67-7 × 10(5)cells L(-1)). Maximal cell density was detected in autumn (November 2007; station MA), at salinity of 37.5, temperature of 16 °C and NH(4)(+) level of 55.45 µM. During this month, Alexandrium spp. abundance accounted for a large fraction (61%) of the harmful phytoplankton. The statistical analysis revealed that Alexandrium concentrations were positively correlated with N:P ratio and NH4+ levels. Thus, the eutrophic waters of the lagoon favour the growth of Alexandrium, which seemed to have preference for N-nutrient loading from antrophogenic activities, as ammonium. Blooms of these potential harmful algae may constitute a potential threat in this coastal lagoon of the southern Mediterranean. Consequently, it is necessary to be well vigilant and to do regular monitoring of Alexandrium species.