pH control as an indicator of the microalgae culture density (model on the example of Arthrospira platensis)

by

R.P. Trenkenshu

 A.O. Kovalevsky Institute of Biology of the Southern Seas the Russian Academy of RAS, 2 Nakhimov ave., Sevastopol, Russian Federation

E-mail: r.trenkenshu@rambler.ru

DOI: 10.33075/2220-5861-2023-1-46-52

UDC 579.017.8:57.036                                                                                            

Abstract:

   The article focuses on the possibility of assessing the density of microalgae cultures by the pH value, which depends on the concentration and forms of nutrients necessary for cell growth. The simulation is based on the mechanism of carbon assimilation by the cells of the cyanobacterium Arthrospira platensis (spirulina), which is usually grown on the nutrient medium Zarrouk, containing 16 g/l NaHC03 at pH > 8.2.  Spirulina under photoautotrophic growth conditions assimilates carbon with the participation of cellular carbonic anhydrase, which converts one molecule of НСО3̅ into CO2 and OH ̅.  CO2 in the Calvin cycle is consumed for the biomass synthesis and ensures the culture growth. OH ̅ remains in the medium and while interacting with another molecule of НСО3̅ forms CO3 ̅ ̅. That is, the biomass synthesis is accompanied by the assimilation of one carbon molecule and the transfer of another molecule into a form nonabsorbed by cells, with an increase in the culture pH. Using the calculation results in silico (according to the ratio of various forms of carbon in solutions at different pH in the form of diagrams), a semi-empirical equation for the relationship of carbon forms available for assimilation by microalgae cells with the pH of the medium is proposed. The equation broadly describes the data from the diagrams published in the literature and enables to find the proportion of carbon absorbed by the cells on the basis of the balance. Given that the biomass of most microalgae species, expressed in absolute dry weight, contains 50% of carbon, the biomass concentration can be expressed in terms of the carbon content in the cells by pH value of the culture. The application of the model to describe experimental spirulina density data by pH value showed a good correspondence in a wide pH range (determination coefficient of 0.98).

Keywords: microalgae, cultivation, density, pH, carbon assimilation, modeling, Arthrospira.

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