Photosynthetic sulfur bacteria from Zimapan Reservoir (Mexico): Seasonal variations and toxic effects of zinc in a microcosm system

Photosynthetic sulfur bacteria oxidize the sulfide produced by dissimilatory sulfate-reducing bacteria, thus preventing the occurrence of pollution by these compounds in the oxygenic zone of aquatic systems. Zinc is one of the most abundant heavy metals. Its toxic effects have been documented in various organisms that can photosynthesize, but a lack of information prevails in this respect, about photosynthetic sulfur bacteria. The aim of the present study was to evaluate the toxic effects of zinc on green sulfur bacteria (Chlorobiaceae) and purple sulfur bacteria (Chromatiaceae) in a microcosm as well to explore the possible relationships between the bacterial population growths with the physicochemical properties of the metalimnion of Zimapan Reservoir (Mexico) during a 1-year period. Hydrosoluble organic carbon (HOC) and light availability in the metalimnion were the limiting factors for growth in these bacteria. Higher HOC levels occurred in Spring, at the end of the wet season and in Winter. Current zinc levels in the reservoir seem to elicit toxic effects. At lethal zinc concentrations, the best concentration-response relationship was shown. The biomarker for purple sulfur bacteria was the bacteriochlorophyll content, while total reducing sugar was the best biomarker for green sulfur bacteria. At sublethal concentrations, zinc alters adenosine triphosphate levels, total reducing sugar, total protein content, and bacteriochlorophyll content. The observed responses indicate that green sulfur bacteria are more sensitive than purple sulfur bacteria and are suitable target organisms for use in establishing the accepted threshold concentration of zinc (NOEC 0.056–0.097 mg L^?1 of Zn) in anoxic deep water.     

A novel low-cost detection method for screening of arsenic in groundwater

In the present work, a novel and simple detection system for As inorganic species contained in groundwater is presented. To reach the required detection limit, the proposed methodology is based on two steps: first is the transport and preconcentration of the inorganic arsenic species using a polymer inclusion membrane (PIM) system and second is the formation of a coloured complex, the  absorbance of which is measured. Different parameters related to the membrane composition and the transport kinetics have been studied, and it was found that membranes made of polyvinyl chloride (PVC) as a polymer, and Aliquat 336 as a carrier, ensured efficient arsenic transport when the carrier content was at least 31 % (w/w). The implementation of the designed PIM in a special device that contained only 5 mL of the stripping solution (0.1 M NaCl) allowed As preconcentration from a 100-mL water sample, thus facilitating its detection with the colorimetric method. The new method developed here was validated, and its analytical figures of merit were determined, i.e. limit of detection of 4.5 μg L^?1 at 820 nm and a relative standard deviation within the range 8–10 %. Finally, the method was successfully applied to the analysis of different water samples from Catalonia region with naturally occurring As.