Abstract
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.
Highlights
Intensive land-based aquaculture produces nitrogen-rich effluent that may detrimentally impact water quality and other environmental parameters
The biomass density decreased from 1,034.00 ± 12.73 to 874.97 ± 18.31 g m−2, the initial stocking density was comparable to the final densities (1011.46 ± 75.58 g m−2) achieved in previous carbon amended cultures standardized at 200 mmol C m−2 day−1 (Robinson et al, 2018)
Benthic fluxes of dissolved oxygen and dissolved inorganic carbon (DIC) can provide an indication of overall benthic metabolism in response to organic enrichment (Eyre et al, 2011)
Summary
Intensive land-based aquaculture produces nitrogen-rich effluent that may detrimentally impact water quality and other environmental parameters. G. Robinson et al.: Carbon amendment stimulates benthic nitrogen cycling to dinitrogen gas (Roy et al, 2010). In zero exchange biofloc systems, carbon-to-nitrogen ratios (C : N) are increased through the addition of labile carbon sources to promote ammonia assimilation from the water column by heterotrophic bacteria (Avnimelech, 1999; Crab et al, 2012). Technological advances are focused on the development of dissimilatory processes to permanently remove nitrogen from the system as N2 gas, while ecologically based systems, such as biofloc, aim to recycle and re-use nitrogen within the culture system. This study aims to advance ecologically based aquaculture bioremediation systems that may provide an alternative to closing the nitrogen cycle through the promotion of assimilatory processes (Robinson, 2018)
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