Contrasting effects of solar radiation and nitrates on the bioavailability of dissolved organic matter to marine bacteria

We evaluated the role of nitrate (NO3−) as a potential photosensitizer and the bacterial responses to dissolved organic matter (DOM) phototransformation from coastal waters in the northwestern Mediterranean Sea. In spring, without any addition of NO3−, the exposure of 0.2 μm filtered seawater (DOM-solution) to natural solar radiation (i.e. Full Sun [FS], including photosynthetically available [PAR: 400–700 nm], ultraviolet-A [UVAR: 315–400 nm] and ultraviolet-B [UVBR: 280–315 nm] radiations) stimulated bacterial production (BP) and abundance (BA) in natural assemblages (0.8 μm filtered seawater) by 80 and 20% as compared to unexposed (Dark) DOM-solutions, respectively. This stimulation resulted primarily from the exposure to PAR. When NO3− (30 μM) was added to DOM-solution before irradiation, BP and BA increased by 150 and 65% in FS compared to Dark, respectively, due to both PAR and UVBR. By contrast, in summer, the exposure of DOM-solution caused a decrease in BP by 30% but an increase in BA by 23% in FS compared to Dark, regardless of the NO3− addition before irradiation. The inhibition of BP resulted mainly from UVAR, whereas the stimulation of BA resulted from PAR. These results suggest contrasting effects along seasons of solar radiation and NO3− on DOM bioavailability, depending on its initial chemical composition.