Formation of recalcitrant organic matter: humification dynamics of algal derived dissolved organic carbon and its hydrophobic fractions

The incorporation of radiolabeled carbon into XAD2-fractions of dissolved organic matter was followed during the growth, stationary and degradation phases of a batch culture of the Antarctic diatom Thalassiosira tumida. Changes in the fractions over the 267 day experiment were correlated with the production and decomposition of particulate organic matter. The bulk of the acid and neutral hydrophobic XAD-fractions, traditionally considered to be humic, were produced mainly during the diatom growth phase: 70% of this carbon was present in a relatively stable form by the end of the first month, representing 5% of the maximum carbon biomass. There was a progressive accumulation of a hydrophobic DOC fraction tightly bound to the XAD resin. This fraction is normally not considered following XAD2 extraction of aquatic humic material. However, it is evidently an important component of a resistant DOC pool, increasing during the experiment to levels between 40 and 50% of total DOC after 7–8 months. This fraction is thought to be mainly a product of particulate material degradation. At the end of the experiment the sum of all hydrophobic DOC fractions was 14% of the maximum particulate carbon.