Nitrogen availability influences the biochemical composition and photosynthesis of tank-cultivated Ulva rigida (Chlorophyta)

Physiological and biochemical changes in relation to inorganic nitrogen ava ilability were studied for tank-cultivated Ulva rigida grown under nitrogen - enriched and nitrogen-depleted seawater. U. rigida was initially cultivat ed in nitrogen-enriched seawater (daily concentrations of NH4+ and NO3- + N O2- ranged between 0.5-1.7 and 0.06-0.15 mg L-1, respectively), then transf erred to nitrogen-depleted seawater where photosynthetic capacity decreased to zero after 23 d. At the time (14 d) when photosynthetic rates were lowe r than 2.0 mu mol O-2 g(-1) FW min(-1) and strong bleaching had occurred, s ome algae were returned to the initial nitrogen-enriched seawater to study recovery from N-limited growth. Data on biochemical composition (chlorophyl ls, ash, caloric content, fatty acids and dietary fibres) and colouration v aried significantly depending on the nitrogen conditions. C:N ratios correl ated significantly with biochemical parameters. Fatty acid (FA) synthesis c ontinued during the N-starvation period; saturated and mono-unsaturated FA increased to a maximun of 72.2%, while poly-unsaturated fatty acids (PUFA) decreased to 27.7%. During the N-enriched recovery period, the reverse was found. C:N ratios above 10 correlated with carbohydrate synthesis as shown by the dietary fibre level. Under nitrogen enriched conditions, C:N ratios decreased along with a decrease in fibre level. Under controlled conditions , nitrogen represents a major influence on the development of intensive tan k cultivation of Ulva rigida, not only by affecting parameters closely rela ted to nitrogen metabolism but also some clearly influenced by carbon uptak e.