IMPACT OF UV-B RADIATION ON N-15-AMMONIUM AND N-15-NITRATE UPTAKE BY HAEMATOCOCCUS-LACUSTRIS (VOLVOCALES) .1. DIFFERENT RESPONSE OF FLAGELLATES AND APLANOSPORES

The freshwater green alga Haematococcus lacustris accumulates N-15 given as N-15-NH4+ to a higher extend than with N-15-NO3-. Special attention was focused to investigate the influence of UV-B radiation on the N-15-uptake. UV-B (1.25 W m(-2)) was applied additionally to the standard light conditions (8 W m(-2)) and nitrogen incorporation was determined for 5 hours during and after the irradiation period. The response of two different developmental stages of the cells on the UV-B-load (green flagellates and red coloured aplanospores), each in two different metabolic conditions (nitrogen-starved dark precultivated and nitrogen-supplied light precultivated cells, respectively) were examined. Results indicate (1) UV-B inhibits the incorporation of N-15-nitrogen in both cell types. (2) The degree of inhibition in aplanospores is generally lower than in the flagellates. This principle is valid also in activated aplanospores. (3) Activated cells of both types show a higher degree of inhibition in nitrate and ammonium uptake than cells precultivated under starvation. (4) Nitrogen incorporation is also inhibited when UV-B-radiation is applied before addition of the N-15-labelled compounds; Different uptake kinetics derived from these experiments are discussed with respect to the possible target of UV-B-influence.