Non-photosynthetic enhancement of growth by high CO2 level irm the nitrophilic seaweed Ulva rigida C. Agardh (Chlorophyta)

The effects of increased CO2 levels (10,000 mul 1(-1)) in cultures of the g reen nitrophilic macroalga Ulva rigida C. Agardh were tested under conditio ns of N saturation and N limitation, using nitrate as the only N source. En richment with CO2 enhanced growth, while net photosynthesis, gross photosyn thesis, dark respiration rates and soluble protein content decreased. The i nternal C pool remained constant at high CO2, while the assimilated C that was released to the external medium was less than half the values obtained under ambient CO2 levels. This higher retention of C provided the source fo r extra biomass production under N saturation. In N-sufficient thalli, nitr ate-uptake rate and the activity of nitrate reductase (EC 1.6.6.1) increase d under high CO2 levels. This did not affect the N content or the internal C:N balance, implying that the extra N-assimilation capacity led to the pro duction of new biomass in proportion to C. Growth enhancement by increased level of CO2 was entirely dependent on the enhancement effect of CO2 on N-a ssimilation rates. The increase in nitrate reductase activity at high CO2 w as not related to soluble carbohydrates or internal C. This indicates that the regulation of N assimilation by CO2 in U. rigida might involve a differ ent pathway from that proposed for higher plants. The role of organic C rel ease as an effective regulatory mechanism maintaining the internal C:N bala nce in response to different CO2 levels is discussed.