Microbial communities involved in nitrogen recycling

Ammonium transformations are crucial for aquatic ecosystem functioning since nitrogen could be lost or recycled when oxidized or assimilated; processes also sensitive to natural and induced salinization in the wetlands from the Southern Altiplano. Here we explore ammonium recycling processes and the active microbial communities involved by means of tracer and inhibitor experiments combined with 16S rRNA tag pyrosequencing (16S-pirolybraries) and qPCR of specific bacterial (AOB) and archaeal (AOA)-amoA, in low and high conductivity sites of Salar de Huasco. In high conductivity sites, nitrification rates were reduced >50%, whereas ammonium uptake increased (reaching up to 40 um l-1 d-1) and nitrogen fixation rates were similar to low conductivity sites (c.a. 0.02 um l-1 d-1). Nitrifying assemblages were mainly detected in low conductivity sites and were rare (<1% of total classified sequences) but diverse; AOB (Nitrosospira and Nitrosomonas), AOA (Nitrosopumilus) and NOB (Nitrospira). AOB presented higher amoA-qPCR counts compared with AOA, and was negatively and positively correlated with conductivity and ammonium, respectively. Also, the potentially active microbial community was richer and diverse in low conductivity compared with high conductivity sites, including the presence of other aerobic and anaerobic nitrogen cycling players like anammox bacteria. Our findings suggest that ammonium utilization was more sensitive to salinity stress compared with planktonic nitrogen fixation and that rare but very active AOB play a significant role in ammonium oxidation in altiplanic wetlands.