Interspecific variability in sensitivity to UV radiation and subsequent recovery in selected isolates of marine bacteria

Abstract

The interspecific variability in the sensitivity of marine bacterial isolates to UV-B (295- to 320-nm) radiation and their ability to recover from previous UV-B stress were examined. Isolates originating from different microenvironments of the northern Adriatic Sea were transferred to aged seawater and exposed to artificial UV-B radiation for 4 h and subsequently to different radiation regimens excluding UV-B to determine the recovery from UV-B stress. Bacterial activity was assessed by thymidine and leucine incorporation measurements prior to and immediately after the exposure to UV-B and after the subsequent exposure to the different radiation regimens. Large interspecific differences among the 11 bacterial isolates were found in the sensitivity to UV-B, ranging from 21 to 92% inhibition of leucine incorporation compared to the bacterial activity measured in dark controls and from 14 to 84% for thymidine incorporation. Interspecific differences in the recovery from the UV stress were also large. An inverse relation was detectable between the ability to recover under dark conditions and the recovery under photosynthetic active radiation (400 to 700 nm). The observed large interspecific differences in the sensitivity to UV-B radiation and even more so in the subsequent recovery from UV-B stress are not related to the prevailing radiation conditions of the microhabitats from which the bacterial isolates originate. Based on our investigations on the 11 marine isolates, we conclude that there are large interspecific differences in the sensitivity to UV-B radiation and even larger differences in the mechanisms of recovery from previous UV stress. This might lead to UV-mediated shifts in the bacterioplankton community composition in marine surface waters.

FIG. 1

Interspecific variation in the percentage of UV-B-mediated inhibition of thymidine and leucine incorporation of selected bacterial isolates originating from different environments of the northern Adriatic Sea (Table 1) compared to corresponding dark incubations. Samples were exposed to artificial UV-B radiation for 4 h. Symbols indicate the means ± SDs of triplicate measurements.

FIG. 2

Relation between the percentage of inhibition of thymidine (a) and leucine (b) incorporation in bacterial isolates due to UV-B exposure for 4 h (compared to the corresponding dark incubation) and the initial incorporation rates prior to exposure to UV-B. Error bars indicate the SDs of triplicate measurements; where they are missing, the SD is smaller than the symbol.

FIG. 3

Molar ratio of leucine to thymidine incorporation of bacterial isolates after exposure to UV-B for 4 h or after being held in the dark. Error bars indicate the SDs of triplicate measurements.

FIG. 4

Percentage of recovery of thymidine (a) and leucine (b) incorporation following incubation under PAR and UV-A radiation for 4 h compared to the respective incorporation rate measured after incubation in the dark for 4 h following UV-B exposure. The response measured in the PAR treatment was subtracted from that measured in the PAR plus UV-A treatment to distinguish between the photoreactivation induced by UV-A and that induced by PAR.

FIG. 5

Relation between the recovery of thymidine and leucine incorporation under PAR and UV-A irradiation (as percentage of the incorporation rate after incubation for 4 h in the dark subsequent to UV-B exposure) and the dark recovery (calculated as percentage of activity measured immediately after UV-B exposure and at the end of the dark incubation for 4 h). The response measured in the PAR treatment was subtracted from that measured in the PAR plus UV-A treatment to distinguish between the photoreactivation induced by UV-A and that induced by PAR. Dotted lines denote the boundaries among the four categories of recovery from UV stress.