Effects of salinity on the CO₂ permeation across lipid bilayer for microalgae biofixation: a molecular dynamics study

The continuous threat of increasing CO₂ concentration in the atmosphere has altered the carbon balance of our planet causing global climate change. Biological fixation of atmospheric CO₂ by unicellular microorganisms such as microalgae is a promising technology pursued extensively by researchers as a means for carbon capture. The study aimed to provide an atomic level of study that will demonstrate the effect of the salinity on the mechanism of CO₂ absorption across microalgae lipid bilayer. Molecular dynamics simulations were utilized to calculate the free energies of CO₂ molecule as it permeates inside the microalgae cell. In thermodynamics, the transport process of a molecule can be demonstrated through its free energy gradient. Thus, calculating the free energies of CO₂ molecule across microalgae lipid bilayer can elucidate the mechanisms of permeation processes. Four microalgae lipid bilayer structures were constructed that contains 128-DPPC (dipalmitoylphosphatidylcholine) lipid bilayer with 3640 water molecules with different NaCl concentrations: 0, 3, 13, and 19 NaCl molecules which correspond to a salinity level of 0, 50, 200, and 300 mM, respectively. The cavity insertion Widom method was used to calculate the free energy of CO₂ molecule along the lipid bilayer. The results demonstrated that the salinity does not affect the free energies significantly, thus, it does not hamper CO₂ transport across microalgae lipid membrane.