Polymerized coumaric acid as a model substrate for terrestrial-derived dissolved organic carbon utilized by aquatic microorganisms

It is now widely accepted that many surface waters receive more terrestrial carbon than assumed in the past, and that aquatic food webs are largely based on the supply of external dissolved organic carbon. However, very little information is available on how efficiently external carbon is utilized by microorganisms and transported to consumers of higher trophic levels. To address this issue, we prepared and tested polymers of 14C-p-coumaric acid (PCA) as a model substrate for terrestrial organic carbon. Photodegradation products that can be considered potential substrates for microorganisms were identified using hyphenated techniques, including gas chromatography-mass spectrometry (GC/MS) and ion chromatography-electrospray ionization mass spectrometry (IC/MS). Photolysis of PCA released monomeric phenol derivatives, e.g. 4-hydroxybenzaldehyde. The photolysis products observed were similar to those characteristic for natural organic carbon. Both a heterotrophic bacteria assemblage and a cultured algae strain exhibiting heterotrophic capabilities proved capable of utilizing the model substrate. Irradiation of PCA increased the uptake rate approximately eight times for the bacteria, but no significant increase was observed for the algae. Potential sources of interferences, e.g. the uptake of 14CO2 released by photolysis, were addressed. It was concluded that PCA is a suitable substrate to study the metabolism of terrestrial DOC within aquatic communities.