The energy source for CO2 transport in the marine microalga Nannochloris atomus

CO2 fluxes in the marine microalga Nannochloris atomus were studied by mass spectrometry using inhibitors and artificial acceptors of photosynthetic electron transport to investigate the energy source for CO2 uptake. This algal species is capable of taking up CO2 from the external medium by active transport but lacks active HCO(3)(-) transport and extracellular carbonic anhydrase. The capacity of cells to take up CO2 was a function of photosynthetic photon flux density. Dark respiration rates were also dependent upon the light intensity during the preceding illumination period, indicating the presence of light-enhanced dark respiration. Addition of 3-(3',4'-dichlorophenyl)-1,1-dimethylurea to illuminated cell suspensions that had been allowed to concentrate inorganic carbon internally during photosynthesis caused a rapid burst of CO2, demonstrating that active CO2 transport had been abolished. A similar response was obtained when cell suspensions were treated with 2,5-dibromo-6-isopropyl-3methyl-1,4-benzoqinone or hydroxylamine. When methyl viologen was used to drain electrons from ferredoxin, cells were still able to take up CO2 from the external medium, although C-fixation decreased with time. These results demonstrate that active CO2 transport in N. atomus is supported by photosynthetic linear electron transport.