The algae Fucus vesiculosus (brown) and Enteromorpha compressa (green) were cultured in long-term experiments in which natural seawater without nutrient enrichment was used, or in which the plants were kept in complete darkness. Measurements of variable fluorescence (Fv/Fm), an indicator of photosynthetic competence, showed a gradual decrease of Fv/Fm in nutrient-deprivation experiments, varying from 0.64 to 0.25 (E. compressa) and from 0.65 to 0.40 (F. vesiculosus) after 6 months of cultivation. F. vesiculosus was virtually unaffected after more than 2 months in darkness, showing values of Fv/Fm around 0.65 throughout the experiments, but cultures of E. compressa collapsed after 45 days in darkness. In both species lower concentrations of chlorophyll a resulted of nutrient-deprivation experiments, decreasing 3 times in F. vesiculosus and 25 times in E. compressa by the end of the experiments. F. vesiculosus cultured in darkness showed 50% more chlorophyll a than the control, while plants of E. compressa kept in dark and in light showed similar values of chlorophyll a during the first 30 days of cultivation. The tissue-N content of both species decreased dramatically in nutrient-deprivation experiments, establishing variations in C:N ratio from 20:1 to 79:1 (F. vesiculosus) and from 8.6:1 to 75:1 (E. compressa). The algae showed small variations in C:N ratio throughout the experiments in darkness. Such as previously identified for phytoplanktonic species, the variations in the physiological responses exhibited by the seaweeds tested suggest that these conventional diagnostics of limitations may be misleading in natural algal assemblages, which are composed by mixed species. Gel electrophoretic is currently being performed to identify possible proteins that may respond specifically to these experimental conditions.

Key words: deprivation, Enteromorpha compressa, Fucus vesiculosus, light, nutrient, seaweeds