Fluorescent characteristics of dissolved organic matter produced by bloom-forming coastal phytoplankton

Dynamics and sources of fluorescent dissolved organic matter (FDOM) are important for understanding biogeochemical processes in aquatic ecosystems. This study aimed to analyse direct production of FDOM by marine phytoplankton cultures and reveal fluorescent characteristics of exuded FDOM. Axenic cultures of eight species of bloom-forming marine phytoplankton, including two diatoms; a raphidophyte; two dinoflagellates; a chlorophyte; a cryptophyte and a haptophyte, were incubated in an artificial medium. Excitation emission matrices (EEMs) of FDOM in the culture medium were spectrofluorometrically measured. FDOM production was observed in all species, and fluorescent characteristics of the exudates varied considerably among species. Measured EEMs had peaks at 350/450 nm (excitation/emission) for the diatom Ditylum brightwellii and 370/450-470 nm for the raphidophyte Heterosigma akashiwo and the chlorophyte Oltmansiellopsis viridis, which have previously been regarded as the peaks of terrestrially derived humic-like substances. Direct production of FDOM by marine phytoplankton should be considered in future studies of FDOM dynamics in marine systems. Species-specific features of FDOM might be used for early detection of harmful blooms because this method is simple, rapid and suitable for monitoring.