Effect of UV-A and UV-B on diel patterns of growth and metabolic activity in Nannochloris atomus cultures assessed by flow cytometry

The aim of this study was to assess the effect of UV-A (320 to 400 nm) and UV-B (280 to 320 nm) radiation on diel patterns of growth and metabolic activity of the marine picoplankter Nannochloris atomus using flow cytometry. N. atomus cells exposed to PAR (400 to 700 nm), PAR+UV-A and PAR+UV-A+UV-B showed clear diel patterns in cell size, chlorophyll fluorescence and metabolic activity, the latter being measured by a fluorescein diacetate-based cell esterase activity assay. For all spectral treatments, patterns increased during the day and decreased during the night, with minima near dawn and maxima near dusk. In addition, cell division was tightly phased to the light dark (L:D) cycle, occurring soon after dark. Exposure to UVR did not alter the synchrony of the parameters measured, but the extent of variation between dawn and dusk was dependent on the spectral conditions. Chlorophyll autofluorescence and metabolic activity decreased to a larger extent when cells were exposed to UV-B than in treatments where UV-B was excluded. In contrast, the cell size was larger under the treatment including UV-A+UV-B than under the treatment including only UV-A. These results show that UV-B damage can decrease growth and metabolic activity in N. atomus without altering the synchronization of the diel patterns, and contribute to a better understanding of phytoplankton behavior under UVR exposures.