Articles | Volume 15, issue 6
Biogeosciences, 15, 1863–1878, 2018
Biogeosciences, 15, 1863–1878, 2018

Research article 29 Mar 2018

Research article | 29 Mar 2018

Carbon amendment stimulates benthic nitrogen cycling during the bioremediation of particulate aquaculture waste

Georgina Robinson1,2,a, Thomas MacTavish3, Candida Savage3,4, Gary S. Caldwell1, Clifford L. W. Jones2, Trevor Probyn5, Bradley D. Eyre6, and Selina M. Stead1 Georgina Robinson et al.
  • 1School of Natural and Environmental Sciences, Newcastle University, Newcastle, NE1 7RU, UK
  • 2Department of Ichthyology and Fisheries Science, Rhodes University, Grahamstown 6140, South Africa
  • 3Department of Marine Science, University of Otago, Dunedin 9016, New Zealand
  • 4Department of Biological Sciences and Marine Research Institute, University of Cape Town, Rondebosch 7700, Cape Town, South Africa
  • 5Marine and Coastal Management, Private Bag X2, Rogge Bay 8012, Cape Town, South Africa
  • 6Centre for Coastal Biogeochemistry, School of Environment, Science and Engineering, Southern Cross University, Lismore, NSW 2480, Australia
  • acurrent address: The Scottish Association for Marine Science, Scottish Marine Institute, Oban, PA37 1QA, UK

Abstract. The treatment of organic wastes remains one of the key sustainability challenges facing the growing global aquaculture industry. Bioremediation systems based on coupled bioturbation–microbial processing offer a promising route for waste management. We present, for the first time, a combined biogeochemical–molecular analysis of the short-term performance of one such system that is designed to receive nitrogen-rich particulate aquaculture wastes. Using sea cucumbers (Holothuria scabra) as a model bioturbator we provide evidence that adjusting the waste C : N from 5 : 1 to 20 : 1 promoted a shift in nitrogen cycling pathways towards the dissimilatory nitrate reduction to ammonium (DNRA), resulting in net NH4+ efflux from the sediment. The carbon amended treatment exhibited an overall net N2 uptake, whereas the control receiving only aquaculture waste exhibited net N2 production, suggesting that carbon supplementation enhanced nitrogen fixation. The higher NH4+ efflux and N2 uptake was further supported by meta-genome predictions that indicate that organic-carbon addition stimulated DNRA over denitrification. These findings indicate that carbon addition may potentially result in greater retention of nitrogen within the system; however, longer-term trials are necessary to determine whether this nitrogen retention is translated into improved sea cucumber biomass yields. Whether this truly constitutes a remediation process is open for debate as there remains the risk that any increased nitrogen retention may be temporary, with any subsequent release potentially raising the eutrophication risk. Longer and larger-scale trials are required before this approach may be validated with the complexities of the in-system nitrogen cycle being fully understood.

Short summary
This study examined the effect of adding carbon to a sediment-based effluent treatment system to treat nitrogen-rich aquaculture waste. The research was conducted in incubation chambers to measure the exchange of gases and nutrients across the sediment–water interface and examine changes in the sediment microbial community. Adding carbon increased the amount of nitrogen retained in the treatment system, thereby reducing the levels of nitrogen needing to be discharged to the environment.
Final-revised paper