Carbohydrates proteins and lipids in fulvic and humic acids of sediments and its relationships with mucilaginous aggregates in the Italian seas

Abstract

In this paper, the humic substance soluble in an alkaline medium (EHS) was extracted from three sediment samples from Tyrrhenian Sea and separated by precipitation at pH = 2 in the two fractions of fulvic acids (FAs) and humic acids (HAs). FAs were further fractionated in seven sub-samples of different molecular weight (mw) by means of ultrafiltration membranes. The qualitative composition of fractionated FAs and HAs was studied by means of Fourier Transform Infrared Spectroscopy in Diffuse Reflectance mode (FTIR-DRIFT). Carbohydrates and proteins were present in all the FA sub-samples and in the HA fractions, whereas some lipid classes such as fatty esters and fatty acids were differently distributed. In fact, fatty acids and fatty esters were detected in samples with mw < 1 kDa while they resulted negligible in samples with mw from 1 to 10 kDa. Fatty esters were detected again in samples with mw > 10 kDa and in all HA samples, whereas fatty acids were detected again in HA samples. These results suggest a different role played by each chemical class during the aggregation process of organic matter. Carbohydrates and proteins play a primary role in starting the aggregation process, from dissolved organic matter to the formation of particles and macromolecules by means of their specific interactions with cations and self-aggregation properties. On the opposite, lipid–cation interaction seems to become less significant as the aggregation proceeds. Differently, lipids seem to play a significant role in the formation of bridges among groups of aggregates by means of a mechanism called cross-linking action which, according to the results of FTIR-DRIFT analysis, becomes significant from 5∼10 kDa aggregates approximately. As the major relevance of carbohydrate–cation interactions with respect to the global lipid–cation interaction has been already supposed in the structure of mucilaginous macroaggregates of the Italian seas, we tried to find confirmations of cross-linking role of lipids in large sized aggregates such as marine snow, particles and mucilages. With this aim, the FITR spectra of HAs and FAs were included in a FTIR spectral database of marine mucilages and marine snow samples and examined by means of the multivariate statistic techniques Principal Component Analysis (PCA) and Cluster Analysis. The multivariate approach evidenced cases of structural similarity among different samples supporting the hypothesis of the different role played by carbohydrates and proteins with respect to lipids on the formation of aggregate dimension higher than FAs and HAs.