Ultraviolet radiation induces reversal of the bioavailability of DOM to marine bacterioplankton

: The effect of solar radiation on the bioavailability of dissolved organic matter (DOM) was investigated during a cruise in the Southern Ocean (simil ar to 44.5 degreesS, 63 degreesE) in January and February 1999. Seawater wa s collected in the chlorophyll maximum layer (CML, 30 to 80 m depth) and in deep waters (DW, 200 to 5000 m depth), filtered through 0.2 pm pore-size f ilters, and exposed to surface solar irradiation for 8 h. The bisavailabili ty of the irradiated DOM and of that in the corresponding dark treatment wa s subsequently tested in bioassay experiments performed in the dark. Seawat er from the CML was also size-fractionated and the effect of solar radiatio n on the bioavailability of the <20 kDa DOM fraction was compared with that of the bulk DOM. Our results clearly demonstrate that the biological react ivity of DOM? from both the CML and DMI following exposure to solar radiati on is inversely related to the initial bioavailability of the DOM Frier to its exposure. The photochemically induced stimulation of the bacterial acti vity was <similar to>2-fold higher in the <20 kDa fraction than in the bulk DOM. These results clearly indicate that initially bioreactive DOM can be rendered biologically more recalcitrant by exposure to solar radiation, whi le DOM of initially low bioavailability can be, at least partly, photochemi cally transformed to compounds of higher biological reactivity.