title: Non-photosynthetic enhancement of growth by high CO2 level in the nitrophilic seaweed Ulva rigida C. Agardh (Chlorophyta).
abstract: The effects of increased CO2 levels (10,000 microl 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.
authors.affiliation: Departamento de Ecología, Facultad de Ciencias, Universidad de Málaga, Spain. f.gordillo@qub.ac.uk
authors.forename: F J
authors.initials: FJ
authors.lastname: Gordillo
authors.forename: F X
authors.initials: FX
authors.lastname: Niell
authors.forename: F L
authors.initials: FL
authors.lastname: Figueroa
mesh_terms: D020418:Algal Proteins / Q000378:metabolism; D018533:Biomass; D002244:Carbon / Q000032:analysis / Q000378:metabolism; D002245:Carbon Dioxide / Q000008:administration & dosage / Q000378:metabolism*; D000460:Chlorophyta / Q000235:genetics* / Q000378:metabolism; D050901:Nitrate Reductase; D009565:Nitrate Reductases / Q000378:metabolism*; D009566:Nitrates / Q000032:analysis; D009573:Nitrites / Q000032:analysis; D009584:Nitrogen / Q000737:chemistry / Q000378:metabolism; D010101:Oxygen Consumption / Q000502:physiology; D010788:Photosynthesis / Q000502:physiology*; D012624:Seaweed / Q000254:growth & development* / Q000378:metabolism
chemical_list: D020418:Algal Proteins; D009566:Nitrates; D009573:Nitrites; D002245:Carbon Dioxide; D002244:Carbon; D009565:Nitrate Reductases; D050901:Nitrate Reductase; D009584:Nitrogen