In order to evaluate the impact of an urban effluent on antibiotic resistance of freshwater bacterial populations, water samples were collected from the Arga river (Spain), upstream and downstream from the wastewater discharge of the city of Pamplona. Strains of Enterobacteriaceae (representative of the human and animal commensal flora) (110 isolates) and Aeromonas (typically waterborne bacteria) (118 isolates) were selected for antibiotic susceptibility testing. Most of the Aeromonas strains (72%) and many of the Enterobacteriaceae (20%) were resistant to nalidixic acid. Singly nalidixic acid-resistant strains were frequent regardless of the sampling site for Aeromonas, whereas they were more common upstream from the discharge for enterobacteria. The most common resistances to antibiotics other than quinolones were to tetracycline (24.3%) and beta-lactams (20.5%) for Enterobacteriaceae and to tetracycline (27.5%) and co-trimoxazole (26.6%) for Aeromonas. The rates of these antibiotic resistances increased downstream from the discharge at similar degrees for the two bacterial groups; it remained at high levels for enterobacteria but decreased along the 30-km study zone for Aeromonas. Genetic analysis of representative strains demonstrated that these resistances were mostly (enterobacteria) or exclusively (Aeromonas) chromosomally mediated. Moreover, a reference strain of Aeromonas caviae (CIP 7616) could not be transformed with conjugative R plasmids of enterobacteria. Thus, the urban effluent resulted in an increase of the rates of resistance to antibiotics other than quinolones in the riverine bacterial populations, despite limited genetic exchanges between enterobacteria and Aeromonas. Quinolone resistance probably was selected by heavy antibiotic discharges of unknown origin upstream from the urban effluent.

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Microbial mats that develop in the gypsum crust of the hypersaline ponds of Salins‐de‐Giraud (Camargue, France) were carefully investigated between 1989 and 1991. During the warm seasons, when these mats were fully developed, analyses of microbial activities and microprofiles of oxygen and sulfide have shown a great activity of the different kinds of bacteria found in the mat below the gypsum crust. Oxygen production could amount to 2 μmol cm−3 h−1 during the maximum daylight whereas the oxidation of sulfide in the light was calculated to be 12.7 μmol cm−3 h−1, i.e. 300 to 180 mmol m−2 day−1 assuming 8–10 hours of constant daylight and a sulfide oxidation zone of 3 mm in thickness. This sulfide oxidation consumes about 65–95% of the diel sulfide production which has been estimated to be 400 to 450 mmol m−2 day−1 originating from sulfate reduction which takes place in the 6 cm depth horizon of sediment plus mat. According to the amounts of sulfate precipitated at the sediment surface in the form of gypsum, sulfate reduction is never limited and was found to be among the highest values reported in the literature (average value of 8200 nmols cm−3 day−1). Completely covered by the gypsum crust, this ecosystem has been found to react as a closed system. Consequently, the sulfide does not escape and accumulate below the crust. It was detected up to the top of the mat after a few hours of darkness. It is reoxidized during the day by the photosynthetic organisms that from the mats. These latter mats were composed of 2 to 3 laminated layers of phototrophic organisms: an upper brown layer of the cyanobacterium Aphanothece, an intermediate green layer of the cyanobacterium Phormidium and an underlying red layer of purple sulfur‐oxidizing bacteria from which two new halophilic species were isolated (Chromatium salexigens and Thiocapsa halophila). It has been found that the accumulated sulfide is oxidized not only by the phototrophic bacteria in the sulfide oxidation zone but also by the oxygen produced by the cyanobacteria which are able to photosynthesize in the presence of sulfide.

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