Gas transfer controls carbon limitation during biomass production by marine microalgae

This study presents the first in-depth analysis of CO₂ limitation on the biomass productivity of the biofuel candidate marine microalga Nannochloropsis oculata. Net photosynthesis decreased by 60 % from 125 to 50 μmol L⁻¹ h⁻¹ over a 12 h light cycle as a direct result of carbon limitation. Continuous dissolved O₂ and pH measurements were used to develop a detailed diurnal mechanism for the interaction between photosynthesis, gas exchange and carbonate chemistry in the photo-bioreactor. Gas exchange determined the degree of carbon limitation experienced by the algae. Carbon limitation was confirmed by delivering more CO₂, which increased net photosynthesis back to its steady-state maximum. This study highlights the importance of maintaining replete carbon concentrations in photo-bioreactors and other culturing facilities, either by constant pH operation or preferably by designing a feedback loop based on the dissolved O₂ concentration.