Photoreactivity of riverine and phytoplanktonic dissolved organic matter and its effects on the dynamics of a bacterial community from the coastal Mediterranean Sea

High solar radiation levels measured in the Mediterranean basin can result in photo-oxidation of dissolved organic matter (DOM) at the sea surface. The objective of the present study was to determine the effects of solar radiation on two contrasting sources of DOM (diatom-derived DOM and riverine DOM) and to evaluate its effect on the growth and the composition of a bacterial community from the Mediterranean Sea. DOM was sequentially exposed to artificial solar radiation or kept in the dark and then added to microcosms containing coastal seawater. For both sources of DOM, light exposure induced photo-alterations by decreasing absorbance and fluorescence of colored dissolved organic matter. However, resulting effects on bacteria were contrasted. Photo-alteration of phytoplankton-derived DOM resulted in a lower bacterial growth during the three first days, without modifying the total dissolved organic carbon consumption. Moreover, after 3 days of incubation, the bacterial diversity as determined by 16S rRNA gene pyrosequencing, revealed that DOM phototransformation led to a higher relative abundance of Alphaproteobacteria and a lower relative abundance of Gammaproteobacteria. A limited number of operational taxonomic units (OTUs) (n = 44) explained 80% of the diversity changes between dark and the light treatments, with an equal number of positively and negatively affected OTUs. In contrast, the photobleaching measured on riverine DOM after sunlight exposure had no effect on both bacterial carbon cycling and composition during a 7-days biodegradation experiment. Our results demonstrate that the effects of solar radiation on DOM greatly differ according to the initial composition of DOM and that changes in bacterial composition, when occurred, showed diverse responses inside a same class of bacteria, underlying the necessity to use a sufficiently resolutive approach to describe them. A better understanding of the interactive effects of the biological degradation and the photoinduced degradation of DOM are required in the context of the global change affecting Mediterranean Sea.