Effects of Ultraviolet Radiation on Carbon Fixation in Antarctic Nanophytoflagellates

Carbon fixation in Antarctic nanoflagellates dominated by cryptomonads collected during a summer cruise in 1995 decreased after short-term exposition (3 h) under both UVA and UVA + UVB radiation compared to white light. The dose applied with artificial lamps was within the range of the natural UV radiation measured at the surface during the cruise. The depletion of C fixation was higher after UVA + UVB than after UVA alone. The inhibition of carbon fixation in the laboratory depended on the time of sample collection and, consequently, on the UV dose received in the natural environment before sampling. Thus, the cells collected in the morning showed 82% of inhibition by UVA + UVB but that collected at noon showed only 72%. The same effect was observed by UVA: 72% of inhibition in the morning samples and 62% at noon. Thus, photoprotection mechanisms seem to be operating during the day protecting the cells against a rise in UV radiation. Red fluorescence (attributed to chlorophyll) per cell, as determined by flow cytometry, was not affected by UV, however, orange fluorescence (attributed to phycoerythrin) increased clearly after UV radiation compared to that in white light. The increment of orange fluorescence was higher after UVA than after UVA + UVB radiation. The rapid increase in fluorescence emission could be due to an uncoupling of energy transfer and it is suggested as a protective mechanism against UV radiation by absorbing UV radiation.