NOM quality differences in sediments of land based aquacultures – insights by liquid chromatography-high resolution mass spectrometry

Natural organic matter (NOM) was widely investigated in natural waters, sea water, river water, lake water, groundwater. The highest molecular resolution of NOM can be achieved by Fourier-transform ion cyclotron resonance mass spectroscopy (FT-ICR-MS). This analytical tool generates molecular formulas (MF) for thousands of NOM components. By coupling liquid chromatography to FT-ICR-MS insights into the polarity (hydrophilic versus hydrophobic) of NOM compounds can be achieved.

Anthropogenic inputs have an imprint on NOM and it changes its overall composition. Aquaculture is one of the fastest growing sectors of food production by covering over 8 Mio ha. The consequences of fish farming for the organic matter quality in fish pond waters and sediments are poorly understood. Here we investigated the stages of pollution by comparison the water extractable organic matter (WEOM) in the sediment at the main site of fish feed application and open water sediments at different distances (transect) to the feeding site.

Full profile HPLC-FTICR-MS chromatograms were segmented into approx. one-minute wide segments between 10 and 15 Min (five segments), the main eluting region of WEOM. MF were calculated for the mass range 150 - 1000 Da with an error threshold of 1 ppm using in-house software considering the following carbon (C), hydrogen (H), oxygen (O), nitrogen (N) and sulphur (S) elements: ¹²C_0–60, ¹³C_0–1, ¹H_0–122, ¹⁶O_0–40, ¹⁴N_0–8, ³²S_0–3, and ³⁴S_0–1.

For all retention times, N3, N4, N5, N6, N7 (CHNO) MF with 0.2 < O/C < 0.5 and 1.5 < H/C < 2.0 showed relative higher intensities in the feeding center compared to the distant sites. For some MF like C₂₀H₃₇N₅O₇, C₂₂H₃₉N₅O₇, C₂₄H₄₂N₆O₁₀ the intensity was more than five times higher in the center compared to open water site. Such components can be suggested to be oligopeptides (Leu-Asn-Thr-Ile, Glu-Pro-Lys-Ile, Leu-Leu-Asp-Ser-Gln as possible isomeric solutions). The sediment in the feeding center exhibited a prevalence of CHNOS and CHOS2 MF, whereas N1, N2, CHOS1, and CHO displayed relatively uniform intensities along the transect, with some instances of slightly higher intensities observed away from the feeding site. The number and abundance of CHNOS MF decreased with increasing retention time (decreasing polarity). Notably, these compounds appear to be inherently hydrophilic, characterized by predominantly low molecular weights (< 400 Da).

The results obtained suggest the following biogeochemical processes: Initially, the protein-rich fish feed undergoes hydrolysis, leading to the formation of oligopeptides. Subsequent partial desamination of these molecules facilitates their interaction with inorganic sulphides, resulting in the formation of CHNOS MF. Notably, the component C₈H₁₃N₁O₃S₁, exhibiting a five-fold intensity at the feeding site, appears to correspond to one of the possible metabolites (Sulfanylpropanoyl-proline).

The results indicate a potential overfertilization with easily biodegradable protein-rich substances. The WEOM quality seems highly affected by additional input of CHNO, CHNOS and CHOS.