Relative importance of advective flow versus environmental gradient in shaping aquatic ammonium oxidizers near the Three Gorges Dam of the Yangtze River, China.

Construction of a dam in a large river alters its hydrodynamic condition and geochemical gradient, but the effect of such anthropogenic activity on microbial ecology remains poorly understood. To assess this effect, we investigated the relative importance of advective flow versus environmental condition in shaping ammonia oxidizing bacteria (AOB) and archaea (AOA) community from 110 km upstream to the Three Gorges Dam (TGD) of the Yangtze River, China. Water physicochemical conditions, including turbidity, conductivity, redox state and nutrient level, were fairly constant from 110 to 45 km upstream of the TGD, but significantly oscillated near the dam. AOB and AOA in the Yangtze River were dominated by Nitrosospira- and Nitrosopumilus-affiliated clusters, respectively, and these compositions were invariant throughout the sampled 110 km flow path, suggesting that AOB and AOA communities in the river were largely transported from upstream by advection with minor local and transient inputs from surrounding soils and tributaries. However, the abundance of AOB and AOA was influenced by local geochemical conditions, possibly via the growth/decay mechanisms. The source of AOB in the Yangtze River appeared to be derived from soil near the headwater, but its abundance was enhanced during downstream transport, likely due to survival and growth.