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.

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.

Seasonal changes of chemical contamination in coastal waters during sediment resuspension.

The potential of remobilization of pollutants is a major problem for anthropogenic ecosystems, because even when the anthropogenic source of pollution is identified and removed, pollutants stored in sediments can be released into the water column and impact pelagic communities during sediment resuspension provoked by dredging, storms or bottom trawling. The objectives of the present study were to assess the changes observed in the chemical composition of the water column following resuspension of a polluted marine sediment and the consequences for the chemical composition of adjacent marine waters according to season. For that purpose, an experimental sediment resuspension protocol was performed on four distinct occasions, spring, summer, fall and winter, and the changes in nutrients, organic contaminants and inorganic contaminants were measured after mixing sediment elutriate with lagoon waters and offshore waters sampled nearby. Significant seasonal variations in the chemical composition of the contaminated sediments were observed, with a strong accumulation of PAHs in fall, whereas minimum PAH concentrations were observed during winter. In all seasons, sediment resuspension provoked a significant enrichment in nutrients, dissolved organic carbon, and trace metal elements like Ni, Cu, and Zn in offshore waters and lagoon waters, with enrichment factors that were season and site dependent. The most pronounced changes were observed for offshore waters, especially in spring and winter, whereas the chemical composition of lagoon waters was weakly impacted by the compounds supplied by sediment resuspension.

Consequences of contaminant mixture on the dynamics and functional diversity of bacterioplankton in a southwestern Mediterranean coastal ecosystem.

Contamination of coastal environments is often due to a complex mixture of pollutants, sometimes in trace levels, that may have significant effects on diversity and function of organisms. The aim of this study was to evaluate the short-term dynamics of bacterioplankton exposed to natural and artificial mixtures of contaminants. Bacterial communities from a southwestern Mediterranean ecosystem, lagoon and the bay (offshore) of Bizerte were exposed to i) elutriate from resuspension of contaminated sediment, and ii) an artificial mixture of metals and herbicides mimicking the contamination observed during sediment resuspension. Elutriate incubation as well as artificial spiking induced strong enrichments in nutrients (up to 18 times), metals (up to six times) and herbicides (up to 20 times) relative to the in situ concentrations in the offshore station, whereas the increases in contaminants were less marked in the lagoon station. In the offshore waters, the artificial mixture of pollutants provoked a strong inhibition of bacterial abundance, production and respiration and significant modifications of the potential functional diversity of bacterioplankton with a strong decrease of the carbohydrate utilization. In contrast, incubation with elutriate resulted in a stimulation of bacterial activities and abundances, suggesting that the toxic effects of pollutants were modified by the increase in nutrient and DOM concentrations due to the sediment resuspension. The effects of elutriate and the artificial mixture of pollutants on bacterial dynamics and the functional diversity were less marked in the lagoon waters, than in offshore waters, suggesting a relative tolerance of lagoon bacteria against contaminants.

Copper and cadmium effects on growth and extracellular exudation of the marine toxic dinoflagellate Alexandrium catenella: 3D-fluorescence spectroscopy approach.

In this study, metal contamination experiments were conducted to investigate the effects of copper and cadmium on the growth of the marine toxic dinoflagellate Alexandrium catenella and on the production of dissolved organic matter (Dissolved Organic Carbon: DOC; Fluorescent Dissolved Organic Matter: FDOM). This species was exposed to increasing concentrations of Cu(2+) (9.93 × 10(-10)-1.00 × 10(-7)M) or Cd(2+) (1.30 × 10(-8)-4.38 × 10(-7)M), to simulate polluted environments. The drastic effects were observed at pCu(2+)=7.96 (Cu(2+): 1.08 × 10(-8)M) and pCd(2+)=7.28 (Cd(2+): 5.19 × 10(-8)M), where cyst formation occurred. Lower levels of Cu(2+) (pCu(2+)>9.00) and Cd(2+) (pCd(2+)>7.28) had no effect on growth. However, when levels of Cu(2+) and Cd(2+) were beyond 10(-7)M, the growth was totally inhibited. The DOC released per cell (DOC/Cell) was different depending on the exposure time and the metal contamination, with higher DOC/Cell values in response to Cu(2+) and Cd(2+), comparatively to the control. Samples were also analyzed by 3D-fluorescence spectroscopy, using the Parallel Factor Analysis (PARAFAC) algorithm to characterize the FDOM. The PARAFAC analytical treatment revealed four components (C1, C2, C3 and C4) that could be associated with two contributions: one, related to the biological activity; the other, linked to the decomposition of organic matter. The C1 component combined a tryptophan peak and a characteristic humic substances response, and the C2 component was considered as a tryptophan protein fluorophore. The C3 and C4 components were associated to marine organic matter production.

Characterization of exudates released by the marine diatom Skeletonema costatum exposed to copper stress: a 3D-fluorescence spectroscopy approach.

In a laboratory study, metal contamination experiments were conducted to investigate the effects of two free copper concentrations (10(-9) and 10(-8) M) on cell growth and on dissolved organic matter exudation by a marine diatom Skeletonema costatum. Throughout incubation, the growth kinetics and exudation of extracellular molecules (i.e. dissolved organic carbon (DOC) and the fluorescent organic matter) were determined. Results revealed an inhibition of S. costatum growth when the free copper level increased (from 10(-9) to 10(-8)). Furthermore, DOC release was more significant in cultures contaminated by 10(-9) M Cu(2+) than in control, suggesting a coping mechanism developed by this species. In this study, samples were daily analysed by 3D-fluorescence and PARAFAC algorithm, in order to compare the fluorescent material produced during growth under different contaminations. PARAFAC treatment revealed two main contributions: one related to the biological activity (C1), the other linked to the marine organic matter (C2). The third component C3 was typically protein-like. This fluorophore was considered as a tryptophan-like fluorophore, whereas the C1 and the C2 components were associated to marine production such as humic matter.

Toxicity of benz(a)anthracene and fluoranthene to marine phytoplankton in culture: does cell size really matter?

The toxicity of benz(a)anthracene and fluoranthene (polycyclic aromatic hydrocarbons, PAHs) was evaluated on seven species of marine algae in culture belonging to pico-, nano-, and microphytoplankton, exposed to increasing concentrations of up to 2 mg L(-1). The short-term (24h) toxicity was assessed using chlorophyll a fluorescence transients, linked to photosynthetic parameters. The maximum quantum yield Fv/Fm was lower at the highest concentrations tested and the toxicity thresholds were species-dependent. For acute effects, fluoranthene was more toxic than benz(a)anthracene, with LOECs of 50.6 and 186 µg L(-1), respectively. After 72 h exposure, there was a dose-dependent decrease in cell density, fluoranthene being more toxic than benz(a)anthracene. The population endpoint at 72 h was affected to a greater extent than the photosynthetic endpoint at 24h. EC50 was evaluated using the Hill model, and species sensitivity was negatively correlated to cell biovolume. The largest species tested, the dinoflagellate Alexandrium catenella, was almost insensitive to either PAH. The population endpoint EC50s for fluoranthene varied from 54 µg L(-1) for the picophytoplankton Picochlorum sp. to 418 µg L(-1) for the larger diatom Chaetoceros muelleri. The size/sensitivity relationship is proposed as a useful model when there is a lack of ecotoxicological data on hazardous chemicals, especially in marine microorganisms.

[Seasonal phytoplankton responses to N:Si:P enrichment ratio in the Bizerte Lagoon (southwestern Mediterranean)]. / Réponses saisonnières du phytoplancton aux rapports d'enrichissements N:Si:P dans la lagune de Bizerte (Sud-Ouest de la Méditerranée).

This study investigated the role of N, P and Si enrichments on phytoplankton in the Bizerte Lagoon (southwestern Mediterranean Sea, Tunisia) during March, June, August, October and December 2004. Polycarbonate bottles were enriched with different nutrients according to four treatments N:Si:P ratios [+NSi/-P (40:40:1), +P/-NSi (20:20:2,5), +NP/-Si (16:0:1) and +Si/-NP (16:32:1)] and incubated in situ during six days. Chl a and carbon biomass of phytoplankton varied significantly during the course of months, with the highest levels recorded in summer (4-4.4 microg Chl a L(-1) or 1126-1721 microg C L(-1)). Dinoflagellates dominated the initial phytoplankton communities, except in August, when diatoms represented a high fraction of microalgae (48%). Enrichment experiments induced significant increases in Chl a and in the final phytoplankton carbon biomasses. In summer (June/August), Si was the main limiting element for phytoplankton. Diatoms strongly responded to +Si/-NP and +NSi/-P enrichments and dominated the final phytoplankton communities (52-61%) in both treatments. Si played the most important role in the growth and development of diatoms. The biomasses and growth rates of dinoflagellates were significantly stimulated by +P/-NSi and +NP/-Si enrichments. After 6 days, dinoflagellates represented more than 70% of the total phytoplankton biomass in samples subjected to these treatments. Moreover, the addition of +P/-NSi increased the biomasses of several dinoflagellates. This suggests that dinoflagellates were mostly controlled by P availability. Unlike diatoms and dinoflagellates, flagellates showed weak responses to nutrient treatments during only some months of the year. The results showed that phytoplankton dynamics in the lagoon were influenced by nutrients in different manners.