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

The effects of increased CO₂ levels (10,000 μl l⁻¹) in cultures of the green nitrophilic macroalga Ulva rigida C. Agardh were tested under conditions of N saturation and N limitation, using nitrate as the only N source. Enrichment with CO₂ enhanced growth, while net photosynthesis, gross photosynthesis, dark respiration rates and soluble protein content decreased. The internal 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 for extra biomass production under N saturation. In N-sufficient thalli, nitrate-uptake rate and the activity of nitrate reductase (EC 1.6.6.1) increased 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 production 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-assimilation rates. The increase in nitrate reductase activity at high CO₂ was not related to soluble carbohydrates or internal C. This indicates that the regulation of N assimilation by CO₂ in U. rigida might involve a different pathway from that proposed for higher plants. The role of organic C release as an effective regulatory mechanism maintaining the internal C:N balance in response to different CO₂ levels is discussed. Received: 27 March 2000 / Accepted: 9 October 2000