The characterisation of Antarctic dissolved organic matter: study of chemical structure of fulvic acid

Study of the chemical structure of dissolved organic matter (DOM) has always been of great interest due to its contribution to global carbon cycle and its recognition as a source marker. Fulvic acids are a complicated mixture of organic compounds accounting for about half of Natural Organic Matter (NOM) in fresh water and 15-20 % of the NOM in marine waters. Fulvic acid composition and structure varies depending on the geographic locations and sources of inputs, but it is generally believed that it consists of weak aliphatic and aromatic organic acids soluble in water at all pH conditions. Chemical characterisation of fulvic material from the Antarctic with its cold climate and absence of higher order plants has the advantage of simpler mixture contribution and pure microbial input to the DOM. This relative simplicity helps facilitate the data interpretation and speculating origin and formation pathways of Antarctic fulvic acids. In this study, we have comprehensively examined fulvic acids from two different areas in Antarctica. Pony Lake fulvic acid (PLFA) from McMurdo Sound area, Western Antarctica as IHSS Standard Reference fulvic acid and three other fulvic acids from Vestfold Hills, Eastern Antarctica were chosen as samples. For the first time these fulvic acids were analysed using a range of derivatisation methods coupled to gas chromatography/mass spectrometry. These methods complemented each other to demonstrate the broad range of components present in these complex fulvic acids. Our findings indicated the presence of various chemical components (apart from carbohydrates and sugars); many of which have not been previously reported in fulvic acids; they mainly include aliphatic linear and branched carboxylic acids, fatty acids (saturated and unsaturated), heterocyclic nitrogen-containing compounds, aromatics and terpenoids. Our results are the first report of the presence of unbroken tricyclic terpenoids among the products of three out of four fulvic acids tabulated. It also highlighted the feasibility of characterisation of DOM using gas chromatography without the need for depolymerisation using hydrolysis or other harsh treatments. Our results also showed that using a fractionation step prior to gas chromatography can assist with providing additional information regarding molecular-level analysis of Antarctic fulvic acid as an important aquatic microbial end-member DOM. Furthermore, fractionation clearly revealed the presence of micelle-like structure in Antarctic fulvic materials supporting the theory of supramolecular structures for Humic substances.