Microbial nitrification in urban streams: from single cell activity to ecosystem processes

We monitored the effects of wastewater treatment plant (WWTP) inputs on the recovery of stream biofilmsafter a large flood event that eroded most of the former biofilm communities. We monitored biomass recovery,chlorophyll a, nitrogen content, and stable isotope natural abundance(15N) over 8 weeks in lightand dark-exposed biofilms upstream and downstream from WWTP inputs, respectively, as well as the abundance ofammonia oxidizers by quantitative polymerase chain reaction. Biomass and chlorophyll a recovered quickly (,2 weeks), and were significantly higher for lightthan for dark-exposed biofilms. There was no consistent effect ofWWTP inputs on these parameters, except for the biomass on dark-exposed biofilm that was higher at theWWTP-influenced sites. The influence of the WWTP inputs on stream-water ammonium concentration and itsisotopic15N signature increased as the flood receded. Biofilm 15N downstream of WWTP increased over time,tracking the increase in15N-ammonium from the WWTP waters. Bacterial and archaeal ammonia oxidizers werepresent within the biofilm assemblages from early stages of postflood recovery. However, spatial distribution ofthese two clades was clearly segregated among sites and between lightand dark-exposed biofilms, probablyrelated to ammonium availability and the development of photoautotrophic organisms. Streams transport dissolved and particulate materialsfrom adjacent terrestrial ecosystems to larger rivers andcoastal zones. Human activity alters stream nutrientconcentrations through nutrient-rich sources through point(e.g., effluents from wastewater treatment plants [WWTP])or diffuse (e.g., from agricultural activities) inputs. Inurban areas, nutrient point sources can be a significantcause of the urban stream syndrome (Walsh et al. 2005).High nutrient concentrations in WWTP-influenced streamslead to decreasing nutrient retention efficiency and loss ofspecies diversity, which ultimately results in eutrophicationof downstream ecosystems (Martı́ et al. 2004; Camargo andAlonso 2006; Sánchez-Pérez et al. 2009). However, thesestreams have also been reported as hot spots for microbialnitrification when they are subjected to large inputs ofammonium(NHz4 ) from the WWTPs (Merseburger et al.2005). In the Mediterranean region, both water scarcity, acommon feature that drives the hydrological regime ofthese streams, and relatively constant anthropogenic inputsfrom WWTPs have a very pronounced effect on streamecology and biogeochemistry because of the reduceddiluting capacity (Martı́ et al. 2010). Moreover, theIntergovernmental Panel to Climate Change has predictedfor the Mediterranean region consistent decreases inprecipitation and annual runoff (Bates et al. 2008), whichwill further exacerbate the local effects of anthropogenicinputs.Increases in nitrogen concentration (mainlyNHz4 ) arecommonly observed in streams loaded with inputs fromurban WWTP effluents (Martı́ et al. 2010).NHz4 is thepreferential N source for primary uptake and a potentiallimiting nutrient for stream communities (Borchardt 1996;Hall and Tank 2003). However, even at relatively lowconcentrations,NHz4 can be highly toxic to aquaticorganisms, whereas at high concentrations it may promoteeutrophication (Camargo and Alonso 2006). Streammicrobial communities (biofilms) can play a key rolecontrolling bioreactive N loads since microbes mostlymediate the processes of N transformation and retention(Peterson et al. 2001; Falkowski et al. 2008; Mulholland etal. 2008). In benthic ecosystems, biofilms are a substrata-attached, matrix-embedded, complex mixture of algae,bacteria, fungi, and microzoans (Lock et al. 1984; Battinet al. 2003). Their three-dimensional layer structure,compositional heterogeneity, and biomass accrual dependon flow velocity, light, and nutrient availability (Besemer etal. 2007; von Schiller et al. 2007; Singer et al. 2010).Microbial diversity and identity in biofilms determine theefficiency at which N is uptaken and transformed; and thus,it may influence N biogeochemistry at the whole-reachscale (Loreau et al. 2001; Prosser et al. 2007).Understanding both the structure of the biofilm and howit processes N inputs can provide insights on themechanisms driving global stream N cycling. In particular,excess ofNHz4 inputs can be biologically modulated byboth assimilation and microbial nitrification associatedwith biofilms (Merseburger et al. 2005). Nitrification is akey process in highly N-loaded streams since the endproduct (i.e., nitrate;NO3 ) can be further transformedunder anaerobic conditions into N2 gas through denitrifi-cation, which finally results in a net loss of N to theatmosphere. Microbial nitrification is a two-step oxidationprocess ofNHz4 to NO {3 via nitrite (NO {2 ). Ammoniaoxidation is the rate-limiting step of nitrification. This stepis carried out by two phylogenetically distant groups, whichinclude three genera of the Bacteria domain (Nitrosomonas,Nitrosococcus, and Nitrosospira; Koops and Pommerening-Röser 2001) and a few recently described members of thedomain Archaea, apparently restricted to the highly diverseThaumarchaeota phylum (Spang et al. 2010). Both bacterial*Corresponding author: [email protected]. Oceanogr., 56(3), 2011, 1054–1064E 2011, by the American Society of Limnology and Oceanography, Inc.doi:10.4319/lo.2011.56.3.1054