Urban stream denitrifier communities are linked to lower functional resistance to multiple stressors associated with urbanization
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The microbial communities in urban stream ecosystems are subject to complex combinations of stressors. These same microbial communities perform the critical ecosystem service of removing excess reactive nitrogen. We asked whether the denitrifying microbial communities in urban streams differ in their functional resistance to common urban stressors from communities from nonurban streams. We exposed inocula from a highly polluted urban stream and a nearby nonurban stream to three different stressors, added alone and in combination. Stressors represent the common urban impacts of thermal pollution (10°C), trace metal exposure (ionic silver (Ag+)), and salinization (addition of NaCl). We used reduction in nitrite (NO2−) concentrations under anaerobic conditions as a proxy for denitrification potential. Nonurban stream denitrifying microbial communities were more diverse than their urban counterparts. Denitrification potential for both communities was unaffected by exposure to any individual stressor. However, denitrification rates by the less diverse urban microbial inoculum decreased in response to combined heavy metal and salt stress, while nonurban communities were unaffected. These findings support the hypothesis that higher diversity may confer greater functional resistance in response to multiple stressors and do not support the idea that stressful conditions select for communities that are functionally resilient to multiple stressors.