The UV responses of bacterioneuston and bacterioplankton isolates depend on the physiological condition and involve a metabolic shift.

Bacteria from the surface microlayer (bacterioneuston) and underlying waters (bacterioplankton) were isolated upon exposure to UV-B radiation, and their individual UV sensitivity in terms of CFU numbers, activity (leucine and thymidine incorporation), sole-carbon source use profiles, repair potential (light-dependent and independent), and photoadaptation potential, under different physiological conditions, was compared. Colony counts were 11.5-16.2% more reduced by UV-B exposure in bacterioplankton isolates (P < 0.05). Inhibition of leucine incorporation in bacterioneuston isolates was 10.9-11.5% higher than in bacterioplankton (P < 0.05). These effects were accompanied by a shift in sole-carbon source use profiles, assessed with Biolog(®) EcoPlates, with a reduction in consumption of amines and amino acids and increased use of polymers, particularly in bacterioneuston isolates. Recovery under starvation was generally enhanced compared with nourished conditions, especially in bacterioneuston isolates. Overall, only insignificant increases in the induction of antibiotic resistant mutant phenotypes (Rif(R) and Nal(R) ) were observed. In general, a potential for photoadaptation could not be detected among the tested isolates. These results indicate that UV effects on bacteria are influenced by their physiological condition and are accompanied by a shift in metabolic profiles, more significant in bacterioneuston isolates, suggesting the presence of bacterial strains adapted to high UV levels in the SML.