Neslihan Semerci

Inhibition of respiration and distribution of Cd, Pb, Hg, Ag and Cr species in a nitrifying sludge.

The study investigated the inhibitory effects of the heavy metals Cd, Pb, Hg, Ag and Cr (as Cr(3+) and Cr(6+)) on a nitrifying sludge. The aim was to assess the IC(50) concentrations leading to 50% inhibition. The method is based on respiration of nitrifying sludge in the presence of these metals. Both O(2) consumption and CO(2) production were taken into account. The order of the inhibitory effect was Ag>Hg>Cd>Cr(3+)=Cr(6+). Metal speciation was calculated in terms of free metal, inorganic metal complexes and bound metal. Pb largely precipitated and 50% inhibition was never reached. Ag was always in the form of the free ion or labile complexes. Hg had apparently a lower toxicity than Ag, since most of it was initially highly complexed with ammonia. Cd was present in the form of free ion and complexes which caused inhibition although a large part of them were precipitated. The inhibitory effects of trivalent chromium (Cr(3+)) and hexavalent chromium (Cr(6+)) were similar. The latter was present in the form of the anion CrO(4)(2-) and was not taken up by biomass. The study highlighted that IC(50) values alone do not have an explanatory power of inhibition unless speciation is also considered.

Monitoring of population shifts in an enriched nitrifying system under gradually increased cadmium loading

The changes in nitrifying bacterial population under cadmium loading were monitored and evaluated in a laboratory scale continuous-flow enriched nitrification system. For this purpose, the following molecular microbiological methods were used: slot-blot hybridization, denaturing gradient gel electrophoresis (DGGE), real-time PCR followed by melting curve analysis, cloning and sequence analysis. The initial cadmium concentration was incrementally increased from 1 to 10mg/l which led to a drop in ammonia removal efficiency from 99 to 10%. Inhibition was recovered when cadmium loading was stopped. During the second application of cadmium, nitrifying population became more tolerant. Even at 15mg/l Cd, only a minor inhibition was observed. To investigate the variations in ammonia and nitrite oxidizing bacteria populations in a period of 483 days, ammonia monooxygenase (amoA) and 16S rRNA genes-based molecular techniques were used. An obvious shift was experienced in the diversity of ammonia oxidizers after the first application of 10mg/l Cd. Metal-tolerant ammonia oxidizing species became dominant and the microbial diversity sharply shifted from Nitrosomonas and Nitrosococcus sp. to Nitrosospira sp. which were observed to tolerate higher cadmium loadings. This result indicated that the extent of nitrification inhibition was not only related to the metal concentration and quantity of microorganisms but also depended on the type of species.