ACCLIMATION TO UV IRRADIANCE IN GRACILARIA-CHILENSIS BIRD, MCLACHLAN-AND-OLIVEIRA (GIGARTINALES, RHODOPHYTA)

Photoautotrophs can cope with an increase in ultraviolet (UV) irradian ce in the aquatic environment, through protection and acclimation mech anisms (i.e, synthesis of UV-absorbing compounds). This capacity has b een proven to vary according to the organism's sensitivity. To quantif y variations of this capacity between the different parts of macroalga e, an in vitro study was performed with the tips, cystocarps and thall i segments of Gracilaria chilensis. Whole algae incubated during 3 day s at high and low PAR, supplying UV-B (4.6 mu W cm(-2)) during 2 hours showed, as predicted, an increase in absorption (OD) at 320 nm of the different parts, after the first day of exposure to UV-B. The tips pr esented the highest increase in the standardized OD at 320 nm relative to cystocarps and thalli segments; their mean percentage of increase was 38% and 29% at low and high PAR, respectively. The lowest sensitiv ity was consistently found in the thalli segments, while the highest w as in the tips. The tips are important for growth and therefore they p lay a major role in the maintenance of the Gracilaria populations. Acc limation mechanisms that occurred in a short time scale, and mainly in the tips, may allow Gracilaria to have an almost immediate protection to increases in UV-B fluxes.