Modeling the transformation of atmospheric CO2 into microalgal biomass

Abstract

Marine phytoplankton acts as a considerable sink of atmospheric CO₂ as it sequesters large quantities of this greenhouse gas for biomass production. To assess microalgae's counterbalancing of global warming, the quantities of CO₂ they fix need to be determined. For this task, it is mandatory to understand which environmental and physiological parameters govern this transformation from atmospheric CO₂ to microalgal biomass. However, experimental analyses are challenging as it has been found that the chemical environment has a major impact on the physiological properties of the microalgae cells (diameter typ. 5–20 μm). Moreover, the cells can only chemically interact with their immediate vicinity and thus compound sequestration needs to be studied on a microscopic spatial scale. Due to these reasons, computer simulations are a more promising approach than the experimental studies. Modeling software has been developed that describes the dissolution of atmospheric CO₂ into oceans followed by the formation of HCO₃⁻ which is then transported to individual microalgae cells. The second portion of this model describes the competition of different cell species for this HCO₃⁻, a nutrient, as well as its uptake and utilization for cell production. Two microalgae species, i.e. Dunaliella salina and Nannochloropsis oculata, were cultured individually and in a competition situation under different atmospheric CO₂ conditions. It is shown that this novel model's predictions of biomass production are in very good agreement with the experimental flow cytometry results. After model validation, it has been applied to long-term prediction of phytoplankton generation. These investigations were motivated by the question whether or not cell production slows down as cultures grow. This is of relevance as a reduced cell production rate means that the increase in a culture's CO₂-sinking capacity slows down as well. One implication resulting from this is that an increase in anthropogenic CO₂ may not be counterbalanced by an increase in phytoplankton production. Modeling studies have found that for several different atmospheric CO₂ levels provided to single-species cultures as well as to species in competing scenarios the cell production rate does slow down over time.