Photo- and bioreactivity of chromophoric dissolved organic matter produced by marine bacterioplankton

The major source of oceanic dissolved organic matter (DOM) is organic compounds from phytoplankton released either directly during photosynthesis or via grazing. Bacterioplankton as a source of marine DOM have received considerably less attention. We determined the production of DOM during active bacterial growth in batch cultures with artificial seawater amended with glucose and inorganic nitrogen and phosphorus. During the initial growth of bacterioplankton, the fluorescence of bacterial-derived DOM increased steadily indicating release of chromophoric DOM. This bacterial-derived DOM was relatively rich in N and depleted in P as indicated by the C:N:P ratio of 147:29: 1. Exposure of this bacterial-derived DOM to artificial solar radiation provoked a rapid decline in fluorescence indicating its photoreactivity. Re-growth experiments with natural bacterial assemblages inoculated into the solar radiation-exposed bacterial-derived DOM indicated that the bioavailability remained essentially unaltered, i.e. is refractory to bacterial utilization. Our findings indicate that bacterioplankton release chromophoric, refractory DOM during active growth and that this DOM, albeit being photoreactive does not stimulate bacterial growth activity upon exposure to solar radiation.