Microbial transformation of labile dissolved organic matter into humic‐like matter in seawater
Microbial transformation of labile, low molecular weight dissolved organic matter (DOM) into dissolved humic matter (DHM) was studied in seawater. Surface water samples were amended with [14C into 14CO2, TO14C (total organic 14C), and PO14C (particulate organic 14C), was measured over time in confined samples. The humic and non-humic fractions of DO14C (dissolved organic 14C) were separated according to a common operational definition of DHM based on adsorption on XAD-8 macroporous resin. Both TO14C and non-humic DO14C decreased during the experiments. However, 14C-labelled DHM increased during the first week of the incubations, to a level where it comprised 15% of the TO14C remaining in the samples, or 3% of the initially added 14C. Towards the end of experiments (ca 70 days), the humic fraction of DO14C gradually approached the background level of poisoned control samples. Provided that the XAD-8 operational definition of DHM is accepted, this study indicates that humic matter may be formed in seawater within days from labile monomers such as glucose.