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Bacillus sp. ORAs2 and Pseudomonas sp. ORAs5, two arsenic-resistant bacterial strains previously isolated from sediments of the Orbetello Lagoon, Italy, were tested for their adaptation to mixed contaminants on the level of membrane fatty acid composition. The two bacterial strains were characterized by high levels of arsenic resistance, and Pseudomonas sp. ORAs5 was also shown to be solvent-tolerant. The bacterial strains were exposed to mixtures of two toxic compounds: arsenic at fixed concentrations and toluene in variable amounts or, alternatively, toluene at constant values along with arsenic added at variable concentrations. Both strains react to the contaminants by changing the composition of their membrane fatty acids. Bacillus sp. strain ORAs2 showed a correlation between growth rate decreases and fatty acids degree of saturation increases in both cases, although pointedly in the presence of 1, 2, and 3 mM of toluene and different additions of arsenic, counteracting membranes fluidity induced by toxic compounds. In Pseudomonas sp. ORAs5, adaptive changes in membrane composition was observed both in terms of increases in the degree of saturation and in the trans/cis ratio of unsaturated fatty acids in the presence of varying toluene and constant arsenic concentrations, whereas only minor changes occurred with increasing arsenic and constant toluene concentrations. Thus, on the level of membrane composition, Bacillus sp. ORAs2 showed a higher potential for adaptation to the presence of mixed pollutants, suggesting its probable suitability for bioremediation purposes.

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Aims:  To isolate and characterize an anaerobic bacterial strain from the deeper polluted lagoon sediment able to use as electron acceptors [As(V)] and sulfate (), using lactate as an electron donor. Methods and Results:  Methods for isolation from polluted lagoon sediments included anaerobic enrichment cultures in the presence of As(V) and . Reduction of As(V) to As(III) was observed during the growth of the bacterial strain, and the final concentration of As(III) was lower than the initial As(V) one, suggesting the immobilization of As(III) in the yellow precipitate. The precipitate was identified by energy dispersive spectroscopy X‐ray as arsenic sulfide. Scanning electron microscopy (SEM) revealed rod‐shaped bacterial cells embedded in the precipitate, where net‐like formations strictly related to the bacterial cells were visible. The surface of the precipitate showed the adhesion of bacterial cells, forming clusters. Transmission electron microscopy (TEM) also highlighted precipitates inside the bacterial cells and on their surface. Following 16S rRNA sequencing, the bacterial strain 063 was assigned to the genus Desulfosporosinus. Conclusions:  This study reports, for the first time, the isolation from the polluted lagoon sediments of a strain capable of respiring and using As(V) and as electron acceptors with lactate as the sole carbon and energy source with the formation of an arsenic sulfide precipitate. Significance and Impact of the Study:  The identification of these properties provides novel insight into the possible use of the anaerobic strain in bioremediation processes and also adds to the knowledge on the biogeochemical cycling of arsenic.