Redox stratification drives enhanced growth in a deposit-feeding invertebrate: implications for aquaculture bioremediation

Abstract

Effective and affordable treatment of waste solids is a key sustainability challenge for the aquaculture industry. Here, we investigated the potential for a deposit-feeding sea cucumber, Holothuria scabra, to provide a remediation service whilst concurrently yielding a high-value secondary product in a land-based recirculating aquaculture system (RAS). The effect of sediment depth, particle size and redox regime were examined in relation to changes in the behaviour, growth and biochemical composition of juvenile sea cucumbers cultured for 81 d in manipulated sediment systems, describing either fully oxic or stratified (oxic−anoxic) redox regimes. The redox regime was the principal factor affecting growth, biochemical composition and behaviour, while substrate depth and particle size did not significantly affect growth rate or biomass production. Animals cultured under fully oxic conditions exhibited negative growth and had higher lipid and carbohydrate contents, potentially due to compensatory feeding in response to higher microphyto benthic production. In contrast, animals in the stratified treatments spent more time feeding, generated faster growth and produced significantly higher biomass yields (626.89 ± 35.44 g m ; mean ± SE). Further, unlike in oxic treatments, growth in the stratified treatments did not reach maximum biomass carrying capacity, indicating that stratified sediment is more suitable for culturing sea cucumbers. However, the stratified sediments may exhibit reduced bioremediation ability relative to the oxic sediment, signifying a trade-off between remediation efficiency and exploitable biomass yield.

Profiling bacterial communities associated with sediment-based aquaculture bioremediation systems under contrasting redox regimes

“…In the present study, the final sea cucumber biomass densities were significantly higher than those achieved in Robinson et al 10. (1028.50 ± 117.46 versus 626.89 ± 35.44 g m −2 in the oxic-anoxic treatment, and 837.96 ± 99.70 versus 454.84 ± 14.30 g m −2 in the oxic treatment).…”

“…Redox potential was the principal driver of shifts in bacterial community composition and predicted functional capacity. However, these distinctions did not translate into significant differences in terms of sea cucumber growth as previously observed10.…”

“…This study presents novel findings that advance observations detailed in Robinson et al 10. by characterising the diversity, structure and predicted metabolic functions of the microbial communities present in the sediment of H. scabra culture tanks subjected to contrasting redox regimes (oxic and oxic-anoxic).…”

“…The growth rate and final biomass density differences between studies can be attributed to seasonal differences between the respective trials. The Robinson et al 10. study ran from the end of the austral summer into mid-winter, whereas the current trial ran from the start of the summer and benefitted from longer day lengths (11.45 to 14.20 daylight hours compared to 10 hours), stronger irradiances and significantly higher ambient temperatures in the culture facility (mean temperatures of 29.49 ± 0.09 versus 25.71 ± 0.05 °C).…”

“…sea cucumbers) grown on aquaculture effluents remains a largely novel and unexplored concept. Robinson et al 10. investigated the effects of manipulated sediment culture systems, describing either fully oxic or redox-stratified (oxic-anoxic) sediments, on the growth and biomass carrying capacity of the sea cucumber Holothuria scabra .…”