Diel variations in the photosynthetic parameters of Prochlorococcus strain PCC 9511: Combined effects of light and cell cycle

International audienceWe examined the mechanisms related to the diel variations in the parameters of the relationship between the rate of carbon fixation of phytoplankton and irradiance (P vs. E curve). Our goal was to understand what determines the phase of these variations relative to that of the light cycle. We grew the marine prokaryote Prochlorococcus in an axenic cyclostat culture system under a light-dark cycle that mimicked natural conditions at sea surface and followed changes in cell physiology with a 2-h resolution. Individual cells divide mostly in phase with each other, once a day at the beginning of the dark period. The quantum yields of chlorophyll fluorescence, the maximum quantum yield of carbon fixation and the maximum rate of carbon fixation (P-max(B)) exhibited diel variations over about factors of 2, 4, and 4, respectively, with maxima at the beginning of the light period. The morning drop in phi(Cmax) and the quantum yield of fluorescence, which was accompanied by only a small decrease (< 15%) of photochemial efficiency of PSII (F-v/F-m), suggests regulation by light and preceded the drop in P-max(B) by 4 h. The decrease in P-max(B) during the day matched a decrease in the transcription level of Rubisco. The quantum yield of fluorescence, phi(Cmax), and P-max(B) increased again during the dark period, but this recovery was slowed at the time of cell division. Our results suggest that the pattern of diel variations in the photosynthetic parameters is determined both by photoacclimation and the cell-division cycle

Chromium isotope fractionations resulting from electroplating, chromating and anodizing: implications for groundwater pollution studies

Available online 27 March 2017A number of shallow aquifers in industrial regions have been polluted by toxic Cr(VI). At some sites, spontaneous reduction of dissolved Cr(VI) to insoluble Cr(III) has been observed. Precipitation of non-toxic Cr(III) is accompanied by a Cr isotope fractionation, with the residual Cr(VI) becoming enriched in the heavier isotope 53Cr, and depleted in the lighter isotope 52Cr. Thus far, δ53Cr values of the contamination source have been poorly constrained. These values are needed to quantify the extent of Cr(VI) reduction in the aquifers. We present δ53Cr values of solutions generated during Cr-electroplating, chromating and anodizing at nine industrial sites. The source chemical, CrO3, had a mean δ53Cr of 0.0‰. A small-to-negligible Cr isotope fractionation was observed between the solutions of the plating baths and the source chemical. Across all sample types, the mean δ53Cr(VI) value was 0.2‰. The mean δ53Cr(VI) value of contaminated groundwater in the same region, studied previously, was significantly higher (2.9‰), indicating Cr(VI) reduction. Based on low δ53Cr(VI) values of plating baths and rinsewaters as potential contamination sources, and their low variability, we suggest that most aquifer δ53Cr(VI) values higher than 1.0‰ are a result of in-situ Cr(VI) reduction.Martin Novak, Vladislav Chrastny, Ondrej Sebek, Eva Martinkova, Eva Prechova, Jan Curik, Frantisek Veselovsky, Marketa Stepanova, Barbora Dousova, Frantisek Buzek, Juraj Farkas, Alexandre Andronikov, Nikoleta Cimova, Marie Houskov