Deciphering the Composition of Fulvic Acids from Recirculating Aquaculture Systems by Quadrupole Time-of-Flight Mass Spectrometry

The constant reuse of waters in recirculating aquaculture systems (RAS) together with the continuous addition of organic matter from fish degradation components, faeces and non-consumed feed promote the accumulation of dissolved organic matter (DOM), particularly of fulvic acids (FA), leading to the yellow discoloration of their waters. The accumulation of these acids in RAS can have potential effects on its water quality, and consequently fish health and welfare. Thus, the characterization of FA is paramount for improving water quality, and subsequently fish productivity in RAS. In this study, a non-targeted analysis by quadrupole time-of-flight mass spectrometry (QTOF-MS) was used to characterize the recirculating aquaculture systems fulvic acids (RASFA) and then compare their molecular fingerprints with actual reference standards of fulvic acids (FA) (Suwannee River; SRFA, Elliott soil; ESFA and Pahokee Peat; PPFA) purchased from the International Humic Substance Society (IHSS). The results of this study demonstrated the applicability of QTOF-MS as a rapid and comprehensive screening technique to characterize the FA fraction of DOM from RAS and to monitor differences in their molecular fingerprints when compared with other FA samples (SRFA, ESFA and PPFA). The QTOF-MS data from SRFA and ESFA standards matched the list of formulas obtained by 17 high-resolution mass spectrometry (HRMS) instruments with 90 and 76% accuracy, respectively, which guaranteed the power of QTOF-MS without the need for further coupling to liquid chromatography (LC). RASFA was found to be rich in low- and high-oxygen unsaturated classes of compounds (lowOC: 61.73% and highOC: 19.28%) and was similar in composition to SRFA. On the contrary, the ESFA and PPFA soil standards consisted mainly of aliphatic compounds (36.77 and 55.74%, respectively) and differed significantly in composition from the RASFA and SRFA water samples. RASFA matched with 66% of the elemental compositions obtained from the DOM of makeup waters analyzed in a previous experiment, indicating freshwater and seawater origins with a high fraction of terrestrial-derived organic matter (Tanimoto score: 0.53 between RASFA and SRFA). The unique information obtained from the molecular-level analysis of FA samples by QTOF-MS highlights the importance of this technique to characterize and compare FA from different origins rendering it possible to track the FA compositional changes along the water treatment processes of RAS. To our knowledge, this is the first study that characterizes the molecular fingerprints of the RASFA by QTOF-MS and compares them with the available FA reference standards from IHSS