Determination of 13C concentration in biological materials using infrared absorption method

Tracer experiments on the transfer of carbon from culture solution to Daphnia magna through phytoplankton have been carried out using ¹³C and infrared absorption method. The purpose of this experiment is to investigate the experimental systems for the use of ¹³C in aquatic system. Daphnia magna was cultured in a 100 ml of solution containing phytoplankton and 20 mg of NaH¹³CO₃ for 24 hours in the light and dark conditions (2000 lux, 14 hour light and 10 hours dark at 25 C). The concentration of ¹³C in Daphnia magna under light condition was about 2.0% (atom% excess), while in the dark condition ¹³C tracer was not detected. The concentration of ¹³C in phytoplankton under light condition was around 20%. It was suggested that ¹³C added as carbonate in the culture solution was photosynthetically assimilated by phytoplankton and then transferred to Daphnia magna. Using this system, the effect of UV-B exposure on the ¹³C uptake of Daphnia magna has been examined. The UV-B intensity was 95 mW/cm² and exposure time was 15, 30, and 60 minutes. The concentration of ¹³C in Daphnia magna decreased with increasing exposure time. The Daphnia magna exposed to UV-B for 15 minutes intakes phytoplankton as much as control, while 30 minutes and 60 minutes exposures were lower than control. The lower concentrations of ¹³C observed in Daphnia magna were due to less intake of phytoplankton caused by the UV-B exposure.