Sadeghi-Nassaj SM et al. (2018), Sea cucumbers reduce chromophoric dissolved org...

ECB-ART-46126

PeerJ 2018 Jan 01;6:e4344. doi: 10.7717/peerj.4344.

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Sea cucumbers reduce chromophoric dissolved organic matter in aquaculture tanks.

Sadeghi-Nassaj SM , Catalá TS , Álvarez PA , Reche I .

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Background: Mono-specific aquaculture effluents contain high concentrations of nutrients and organic matter, which affect negatively the water quality of the recipient ecosystems. A fundamental feature of water quality is its transparency. The fraction of dissolved organic matter that absorbs light is named chromophoric dissolved organic matter (CDOM). A sustainable alternative to mono-specific aquaculture is the multitrophic aquaculture that includes species trophically complementary named "extractive" species that uptake the waste byproducts. Sea cucumbers are recognized as efficient extractive species due to the consumption of particulate organic matter (POM). However, the effects of sea cucumbers on CDOM are still unknown. Methods: During more than one year, we monitored CDOM in two big-volume tanks with different trophic structure. One of the tanks (-holothurian) only contained around 810 individuals of Anemonia sulcata, whereas the other tank (+holothurian) also included 90 individuals of Holothuria tubulosa and Holothuria forskali. We routinely analyzed CDOM absorption spectra and determined quantitative (absorption coefficients at 325 nm) and qualitative (spectral slopes) optical parameters in the inlet waters, within the tanks, and in their corresponding effluents. To confirm the time-series results, we also performed three experiments. Each experiment consisted of two treatments: +holothurians (+H) and -holothurians (-H). We set up three +H tanks with 80 individuals of A. sulcata and 10 individuals of H. tubulosa in each tank and four -H tanks that contained only 80 individuals of A. sulcata. Results: In the time-series, absorption coefficients at 325 nm (a325) and spectral slopes from 275 to 295 nm (S275-295) were significantly lower in the effluent of the +holothurian tank (average: 0.33 m-1 and 16 µm-1, respectively) than in the effluent of the -holothurian tank (average: 0.69 m-1 and 34 µm-1, respectively), the former being similar to those found in the inlet waters (average: 0.32 m-1 and 22 µm-1, respectively). 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 low molecular weight. Discussion: Consequently, sea cucumbers appear to improve water transparency in aquaculture tanks. The underlying mechanism of this improvement might be related to the POM consumption by holothurians, which reduces the concentration of CDOM derived from POM disaggregation or to the direct assimilation of dissolved compounds of low molecular weight as chromophoric amino acids.

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Genes referenced: LOC100887844 LOC100893907 LOC583082

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	Figure 1. Exponential decay of absorption coefficients as wavelengths increase from the ultraviolet to the visible band.Example of a decay curve of the chromophoric dissolved organic matter in a sample taken on January 30th of 2014 from the big-volume tank that contains holothurians. The wavelengths selected for the calculation of the optical parameters used in this study are marked on the decay curve.
	Figure 2. Time-series of the optical parameters of chromophoric dissolved organic matter in the big-volume tanks.Values of (A) absorption coefficients at 325 nm (a325), (B) spectral slopes from 275 to 295 nm (S275−295), and (C) spectral slope ratios (SR) in the inlet water (white circles), in the effluent water of the +holothurian tank (red squares) and in the effluent water of the –holothurian tank (grey triangles).
	Figure 3. Median values (line), the 25–75% percentiles (box), and the non-outlier ranges (whisker) of the optical parameters of chromophoric dissolved organic matter pooling all time-series data.Values of (A) absorption coefficients at 325 nm (a325), (B) spectral slopes from 275 to 295 nm (S275−295) and (C) spectral slope ratios (SR) in the inlet water (white box), in the +holothu rian effluent and tank (red boxes) and in the –holothurian effluent and tank (grey boxes).

	Figure 5. Changes in optical parameters of chromophoric dissolved organic matter in the three experiments.Mean (bars) and the standard deviations (whiskers) of the replicates of (A) absorption coefficients at 325 nm (a325), (B) absorption coefficients at 443 nm (a443), and (C) spectral slopes from 275 to 295 nm (S275−295) in the treatments with holothurians (+H) and without holothurians (−H) at the initial and final times. Red bars represent the treatment with holothurians and grey bars represent the treatment without holothurians. Asterisks show the statistically significant results at the final time of the experiments (more details in Table 2).

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