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

Abstract.

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 HCO₃ ^– 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.