Nutrients and organic matter reduction by sea cucumbers in marine multitrophic aquaculture system

Resumen

Wastewater from traditional aquaculture contains high concentrations of mineral nutrients and organic compounds that generate environmental problems as eutrophication, oxygen depletion and water transparency reduction in the recipient ecosystems. Therefore, a sustainable aquaculture is the global challenge for scientists, policy makers and food producers. The integrated multitrophic aquaculture (IMTA) emerges as a new procedure that can alleviate these handicaps of the traditional aquaculture (Diana et al. 2013). This procedure includes the use of species trophically complementary, where the excretion and fecal wastes derived from the primary species are nutritional resources of the secondary species named as “extractive species” (Chopin et al. 2012). Sea cucumbers are frequently selected as extractive species due to capacity to uptake particulate matter and detritus (Slater & Carton 2009; Yokoyama 2013). However, their effects on mineral nutrients and dissolved organic components have been scantly explored. In this PhD thesis, we determined the effects of the presence of two species of holothurians (Holothuria tubulosa and Holothuria forskali) co-cultured with Anemonia sulcata as primary species on water transparency and quality. We determined the effects of these sea cucumbers on mineral nutrients, particulate organic components and optical properties of dissolved organic matter (DOM) including chromophoric and fluorescence components. Finally, we used fluorescence spectroscopy (excitation-emission matrixes (EEMs) and PARAFAC analysis) to assess the actual influence of holothurians on humic-like compounds (biologically more refractory) and amino acid-like compounds (biologically more labile). To reach these objectives we used two approaches: time-series monitoring in two big tanks with and without holothurians and short-term experiments in smaller tanks with all the other sources of variability controlled to corroborate the observations obtained in the time series. In the time-series of two big-volume tanks, we monitored during more than one year the concentration of mineral nutrients (ammonium, nitrite, nitrate, and total phosphorus), total organic carbon (TOC), particulate organic matter (POM), transparent exopolymer particles (TEP), chlorophyll-a, bacterial abundance, chromophoric dissolved organic matter (CDOM) including absorption coefficients and spectral slopes and fluorescence dissolved organic matter (FDOM). The short-term experiments consisted of manipulate the presence of holothurians. Each experiment was carried out in seven tanks that contained individuals of A. sulcata. At the initial time, in three of the tanks we also included individuals of H. tubulosa. These three tanks are the replicates of the +holothurians treatment. The other four tanks only contained A. sulcata and represent the replicates of the –holothurians treatment. We found that the concentration of ammonium, nitrate, TOC, POM, TEP, and bacterial abundance in the effluent waters from the tank with holothurians was significantly lower than the effluent waters of the tank without holothurians. The consumption of transparent exopolymeric particles by holothurians was a result particularly remarkable and novel. The experiments confirmed the time-series results with reductions statistically significant in nitrates, bacteria and TEP concentration in the treatments with holothurians. This TEP and bacteria consumption by holothurians is of great relevance in the maintenance of the tank hygiene and the control of potentially pathogenic bacterial outbreaks that can be associated to biofilm generation by exopolymeric particles. In relation to DOM optical characterization, in the time-series we observed that absorption coefficients (a325) and spectral slopes (S275-295) were significantly lower in the effluent of the +holothurian tank than in the effluent of the -holothurian tank. This reduction in the absorption of the dissolved organic matter appears to be mediated by the POM consumption by holothurians. The experiments confirmed the results observed in the time-series. The a325 and S275-295 values were significantly lower in the treatment with holothurians than in the treatment without holothurians indicating a reduction in the concentration of chromophoric organic compounds, particularly of compounds with lower molecular weight. This reduction in the concentration of particulate and chromophoric organic matter can affect positively to water transparency by reducing light scattering and absorption. EEMs and PARAFAC analysis provided a six components model with four humic-like components (C1, C2, C3 and C4) and two amino acid-like components (C5 and C6). We observed than holothurians were able to reduce significantly both humic-like and amino acid-like components, likely due to their great efficiency removing particulate organic matter. In the short-term experiments we confirmed that holothurian were able to reduce two of the humic-like components (C2 and C4) and the two amino acid-like components (C5, tryptophan-like and C6, tyrosine-like). We suggest that symbiotic bacteria in the tentacle epidermis and below the cuticle in holothurians could directly assimilate these amino acids and explain the remarkable significant reduction of the two components that we observed in both the time-series and particularly the short-term experiments. Roberts et al. (1991); Lawrence et al. (2010) found symbiotic bacteria in the tentacle epidermis and below the cuticle and Brothers et al. 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