Photoperiod effects on the UV-induced toxicity of fluoranthene to freshwater mussel glochidia: absence of repair during dark periods.

The effect of photoperiod on the ultraviolet radiation (UV)-induced toxicity of fluoranthene to glochidia of the freshwater mussel, Utterbackia imbecillis, was evaluated using a series of static renewal toxicity tests conducted using one of four different photoperiods (24 h light, 16 h light:8 h dark, 12 h light:12 h dark, 8 h light:16 h dark). Rates of acute mortality were dependent both upon fluoranthene dose and the photoperiod. Median lethal time (LT50) values calculated on the basis of accumulated UV exposure time (UV-LT50) were compared with LT50 values calculated from real time of exposure (R-LT50) to determine relative rates of photoactivated fluoranthene damage versus physiologic repair during periods of darkness. UV-LT50 values were only dependent on fluoranthene dose and not on photoperiod. The fact that UV-LT50 values did not increase with decreasing light cycle length indicates that physiologic repair during dark periods was not an important process in these experiments. These findings suggest that (1) species-specific and/or life history-specific factors, in part, determine the ability of an organism to repair photoactivated polycyclic aromatic hydrocarbon (PAH) damage during dark periods; and (2) predictions of the UV-induced toxic response of PAH in glochidia need only be based upon total UV dose (dose rate and duration) and PAH dose.