Nitrate removal microbiology in woodchip bioreactors: A case-study with full-scale bioreactors treating aquaculture effluents

Sanni L Aalto; Suvi Suurnäkki; Mathis von Ahnen; Henri M P Siljanen; Per Bovbjerg Pedersen; Marja Tiirola

doi:10.1016/j.scitotenv.2020.138093

Nitrate removal microbiology in woodchip bioreactors: A case-study with full-scale bioreactors treating aquaculture effluents

Sci Total Environ. 2020 Jun 25:723:138093. doi: 10.1016/j.scitotenv.2020.138093. Epub 2020 Mar 21.

Authors

Sanni L Aalto¹, Suvi Suurnäkki², Mathis von Ahnen³, Henri M P Siljanen⁴, Per Bovbjerg Pedersen³, Marja Tiirola²

Affiliations

¹ Technical University of Denmark, DTU Aqua, Section for Aquaculture, The North Sea Research Centre, P.O. Box 101, DK-9850 Hirtshals, Denmark; Department of Biological and Environmental Science, Nanoscience Center, University of Jyväskylä, P.O. Box 35, 40014 Jyväskylä, Finland. Electronic address: sheaa@aqua.dtu.dk.
² Department of Biological and Environmental Science, Nanoscience Center, University of Jyväskylä, P.O. Box 35, 40014 Jyväskylä, Finland.
³ Technical University of Denmark, DTU Aqua, Section for Aquaculture, The North Sea Research Centre, P.O. Box 101, DK-9850 Hirtshals, Denmark.
⁴ Department of Environmental and Biological Sciences, University of Eastern Finland, P.O. Box 1627, 70211 Kuopio, Finland.

PMID: 32222508
DOI: 10.1016/j.scitotenv.2020.138093

Abstract

Woodchip bioreactors are viable low-cost nitrate (NO₃^-) removal applications for treating agricultural and aquaculture discharges. The active microbial biofilms growing on woodchips are conducting nitrogen (N) removal, reducing NO₃^- while oxidizing the carbon (C) from woodchips. However, bioreactor age, and changes in the operating conditions or in the microbial community might affect the NO₃^- removal as well as potentially promote nitrous oxide (N₂O) production through either incomplete denitrification or dissimilatory NO₃^- reduction to ammonium (DNRA). Here, we combined stable isotope approach, amplicon sequencing, and captured metagenomics for studying the potential NO₃^- removal rates, and the abundance and community composition of microbes involved in N transformation processes in the three different full-scale woodchip bioreactors treating recirculating aquaculture system (RAS) effluents. We confirmed denitrification producing di‑nitrogen gas (N₂) to be the primary NO₃^- removal pathway, but found that 6% of NO₃^- could be released as N₂O under high NO₃^- concentrations and low amounts of bioavailable C, whereas DNRA rates tend to increase with the C amount. The abundance of denitrifiers was equally high between the studied bioreactors, yet the potential NO₃^- removal rates were linked to the denitrifying community diversity. The same core proteobacterial groups were driving the denitrification, while Bacteroidetes dominated the DNRA carrying microbes in all the three bioreactors studied. Altogether, our results suggest that woodchip bioreactors have a high genetic potential for NO₃^- removal through a highly abundant and diverse denitrifying community, but that the rates and dynamics between the NO₃^- removal pathways depend on the other factors (e.g., bioreactor design, operating conditions, and the amount of bioavailable C in relation to the incoming NO₃^- concentrations).

Keywords: DNRA; Denitrification; Nitrogen removal; Nitrous oxide; Recirculation aquaculture systems.

MeSH terms

Aquaculture
Bioreactors*
Denitrification*
Nitrates
Nitrogen

Substances

Nitrates
Nitrogen