The effect of cations (Na+, Mg2+, and Ca2+) on the activity and structure of nitrifying and denitrifying bacterial communities

Abstract

Wastewaters generated in regions with water scarcity usually have high alkalinity, hardness, and elevated osmotic pressure (OP). Those characteristics should be considered when using biological systems for wastewater treatment along with the salinity heterogeneity. The interaction of different salts in mixed electrolyte solutions may cause inhibition, antagonism, synergism, and stimulation effects on microbial communities. Little is known about those effects on microbial activity and community structure of nitrifying and denitrifying bacteria. In this work, factorial design was used to evaluate the effects of NaCl, MgCl₂ and CaCl₂ on nitrifying and denitrifying communities. Antagonistic relationships between all salts were observed and they had greater magnitude on the nitrifying community. Stimulus and synernism were more evident on the nitrifying and denitrifying experiments, respectively. For this reason, the highest nitrification and denitrification specific rates were 1.1 × 10^-1 mgN-NH₄⁺ gSSV^-1 min^-1 for condition 01 and 6.5 × 10^-2 mgN-NO₃^- gSSV^-1 min^-1 for control condition, respectively. The toxicity of the salts followed the order of NaCl > MgCl₂ > CaCl₂ and the antagonism between MgCl₂ and NaCl was the most significant. PCR/DGGE analyses showed that Mg²⁺ may be the element that expresses the least influence in the differentiation of microbial structure even though it significantly affects the activity of the autotrophic microorganisms. The same behavior was observed for Ca²⁺ on denitrifying microorganism. In addition, microbial diversity and richness was not negatively affected by different salinities. Genetic sequencing suggested that the genus Aeromonas, Alishewanella, Azospirillum, Pseudoalteromonas, and Thioalkalivibrio were outstanding on ammonium and nitrate removal under saline conditions. The specific toxicity of each salt and the interactions among them are the major effects on microbial activity in biological wastewater treatments rather than the osmotic pressure caused by the final salinity.

Keywords: Antagonism; Microbial activity; Saline wastewater; Salt interaction.