Epilithic biofilms as hotspots of in-stream nitrification in a high N loaded urban stream

Abstract:

Nitrification, the oxidation of ammonia to nitrate, is one of the most important biogeochemical processes in high nitrogen loaded urban streams. The first rate-limiting step of the nitrification process is carried out by ammonia-oxidizing (AO) archaea (AOB) and bacteria (AOB) that live in stream sediments and epilithic biofilms. Yet, the relative contribution of these two stream habitats to whole-reach nitrification is largely unknown.

We tested the well-established idea that whole-reach nitrification is mainly driven by AO present in hyporheic sediments because of their relative high active surface area compared to the thin epilithic biofilm interface. To do so, we examined substrata-specific nitrification rates and AO transcripts abundance (amoA gene) in mesocosms and scaled data to whole reach. Further, we compared the scaled data to in situ whole-reach nitrification rates and amoA transcript and gene abundances in a high N loaded urban stream downstream of a waste water treatment plant effluent. Against expectations, whole-reach in-stream nitrification was mainly driven by AOB embedded in biofilms growing on the sediment-facing side (> 60%) and light-exposed side (20%) of stream cobbles. Hyporheic sediments, which were mainly colonized by AOA, accounted for 11% of in situ whole-reach nitrification. Our study points epilithic biofilms as hot spots of nitrification within urban stream ecosystems.