Microbial transformation of labile dissolved organic matter into humic-like matter in seawater

Abstract 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.