HPLC-SEC-FTIR characterization of the dissolved organic matter produced by the microbial carbon pump

The microbial carbon pump (MCP) concept regroups different processes that contribute to the formation of refractory dissolved organic matter (DOM) from labile substrates. In this study, long-term (1 yr) incubation experiments in fresh- and sea-waters were done with glucose as the only source of carbon for microorganisms. Bulk carbon, glucose and amino acid enantiomers analyses confirmed that glucose was transformed into microbial biomass and complex non-labile DOM. Dissolved organic carbon concentrations decreased during the first four months and then remained constant at similar to 20% of the initial level. After one year, the microbial DOM and DOM from natural waters were concentrated by evaporation and a low volume solid-phase extraction (SPE) that resulted in a very high (70 to 85%) extraction efficiency. These SPE-DOM samples were characterized by size-exclusion chromatography (HPLC-SEC) coupled with Fourier transform infrared spectroscopy (FTIR). These are the first HPLC-SEC-FTIR analyses of microbial DOM and of such high proportions of natural DOM. Results revealed that microbial DOM showed all the FTIR absorption bands seen in the DOM from natural waters. However, amino acids, carbohydrates, and aliphatic CH were more abundant in the microbial DOM, whereas phenols and esters were more visible in DOM from natural waters. It was difficult to identify consistent (in all samples) compositional trends according to molecular weight (MW). Cluster analyses showed that 72% of the variability in the relative absorbance for all the bands was explained by the origin and the type of the DOM extracts. The microbial DOM from freshwater had the most unique bulk composition while the microbial DOM from seawater was more similar to the DOM from natural waters. The DOM from natural fresh- and sea-waters exhibited the most similar composition when all the bands and MW are considered. The lack of consistent trends in the composition of the samples having the same origin (e.g., the two microbial DOM or the two freshwater DOM) shows the high heterogeneity and complexity of DOM, including the microbial non-labile DOM produced by the MCP. This suggests that the MCP contributes to the heterogeneity and complexity of the DOM found in natural waters.