Viabilité des bactéries hétérotrophes dans la baie de Marseille

Marine microorganism activities are commonly assessed by bulk methods and assigned to the total cell count. The presence in significant amounts of ghost, dead, and damaged cells makes such as assignment a non-correct one. A Nucleic Acid Double Staining protocol (NADS) of fresh water bacteria (Barbesti et al., Cytometry 40 (2000) 214-218) has been adapted to resolve viable, damaged and dead cells in marine environments (Grégori et al., Appl. Environ. Microbiol. 67 (2001) 4662-4670). The present reports the first in situ application of this approach, conducted in the Bay of Marseilles in winter and spring periods at two sites with contrasted features.

Effects of dispersed and adsorbed crude oil on microalgal and bacterial communities of cold seawater

: Mesocosm facilities consisting of five 3.5 m(3) stainless steel tanks filled with seawater from the St Lawrence Estuary (Québec, Canada) were used to conduct a 2 month experiment under the natural conditions prevailing at the end of the winter in subarctic environments, with seawater temperatures ranging from-1.5°C (surface ice cover) to 3°C. Various oil treatments were simulated in mesocosms: Forties crude oil was chemically dispersed, adsorbed onto an immersed substrate and spilled without any treatment. Total oil concentrations ranged from <1 mgl(-1) (untreated oil) to 44.6 mgl(-1) (dispersed oil). Contrasting with the parent crude oil and dispersed oil, the dissolved phase was enriched with low molecular weight polycyclic aromatic hydrocarbons (PAH). As revealed by pH variations, chlorophyll a contents and degraded pigments, the phytoplankton growth was inhibited early in tanks contaminated with dispersed and adsorbed oil. Although global measurements showed a recovery of the microalgal activity while the dispersed oil was diluted in a flow-through cascade system, the specific composition was quite different from the control, with an increase in small microflagellate species and a marked decline in the diversity of centric diatoms. Small microflagellates also dominated in the heavily oil-contaminated surface microlayer. The growth of viable heterotrophic bacteria (VHB) was immediately stimulated by both dispersed and adsorbed oil (10(4)-10(5) colony forming units per ml) and oil-degrading bacteria (ODB) reached maximum densities (10(2) CFU ml(-1)) later in the experiment. The adaptation of the indigenous community was assessed using the ODB/VHB ratio, which increased by ten times in the seawater contaminated with dispersed oil. No significant bacterial enhancement was observed in the tank that received untreated oil. No bacterial enrichment was found in the surface microlayer. In sediment traps, the bacterial density increased with the amount of total settling matter and oil residues.

Changes in bacterial populations during red tides caused by Mesodinium rubrum and Gymnodinium catenatum in North West Coast of Spain

Heterotrophic bacterial communities associated with four red tides caused by Mesodinium rubrum and Gymnodinium catenatum in two Galician Rias (North West Spain) were examined. Three of these were produced by the Mesodinium rubrum and the causative organism of a toxic bloom was Gymnodinium catenatum. In early stages of all the blooms, the diversity decreased but the total marine bacterial counts increased by one or two logs. Vibrio numbers were also incremented by two logs in two blooms of M. rubrum, while in the other bloom of this organism and in the red tide caused by G. catenatum a decrease in number of these bacteria was observed. A total of 116 bacterial strains were identified at the genus level and grouped into 12 phena. During the decomposition processes of two blooms of M. rubrum a zooplanktonic-type bacterial succession was observed (Vibrio, pseudomonads and Moraxella-Acinetobacter). On the other hand, during decomposition of the other red tide of M. rubrum and the bloom of G. catenatum, a typical phytoplanktonic-type succession occurred, as Pseudomonas and Moraxella groups became dominant for all the process. These results support the conflicting taxonomical position of M. rubrum. After the blooms, the changes in the community point towards the restablishment of the normal bacterial flora of the estuary (increase in diversity and decreases of bacterial numbers). Only the Vibrio strains, isolated from the non-toxic first and second red tides, displayed cytotoxic activities. A relationship among bacterial cytotoxicity and toxic effects of blooms cannot therefore be established.