Humic Substances Research in Polar Waters

Abstract

A summary is presented of field and laboratory research on marine humic substances (HS) from polar regions. The contribution and the role of organic nitrogenous compounds in humic fractions of dissolved organic matter (DOM) have been investigated in seawater samples from Arctic and Antarctic regions. On average the HS contributed 40% to the pool of dissolved organic nitrogen (DON). Total dissolved amino acids (TDAA) accounted for about 35% and 7% of the nitrogen in the hydrophobic neutral and hydrophobic acid fractions of HS, respectively. Differences in amino acid distribution and composition patterns in dissolved and suspended particulate material suggest the selective preservation of certain amino acids, in particular glycine. In surface waters ca 60%, and at depths < 500 m almost 100% of TDAA are found in the “humic” fractions. A background value of TDAA of around 200 nM, mostly contained in the hydrophobic neutral fraction, is present throughout the water column, probably forming part of the refractory molecules. Culture experiments with Antarctic diatoms showed that relatively fresh dissolved compounds of algal origins were actively taken up by bacteria, regardless of their hydrophobic (“humic”) or hydrophilic (“non-humic”) nature as determined by sorption onto XAD-2 resin. Part of the particulate amino acid pool was transformed in hydrophilic DON, which was not adequate for supporting sustained bacterial growth. This suggests that humification may begin in the hydrophilic fraction of DOM. Further studies with Antarctic algae were carried out to test the efficiency of combinations of the XAD-2, -4 and -7 resins for extracting 14C-labelled algal derived DOM. The serial combinations XAD-7A2/-4 and XAD-7/-4/-2 allowed the isolation of a very high percentage of DOM (65%) from the seawater matrix, whereas a mixed bed of the same resins showed a significantly lower recovery (39%).