A cleaner and smarter way to achieve high microalgal biomass density coupled with facilitated self-flocculation by utilizing bicarbonate as a source of dissolved carbon dioxide

Abstract

Microalgae feedstock-based bioprocesses are increasingly unfolding as promising futuristic technologies towards achieving some of the sustainable development goals, mostly pertaining to clean energy and environment. The current technologies that involve sparging flue-gas carbon dioxide (CO2) directly into closed or open photobioreactors suffer from very limited bioavailability of sparingly soluble CO2 in dissolved form leading to less algal biomass production. Thus, the present study was designed to assess the ability of Chlorella vulgaris as a model microalga to capture carbon from a dissolved inorganic carbon (DIC) source like sodium bicarbonate (NaHCO3) and grow in photobioreactors to high biomass density. With the aim to develop a proof of this concept, Chlorella vulgaris was grown in minimal medium supplemented with varying doses of initial bicarbonate (50–250 mM). The results demonstrated that with increasing initial bicarbonate (HCO3−) concentration, CO2 fixation rate and algal biomass productivity were enhanced. At 150 mM initial HCO3−, biomass concentration and CO2 fixation rate were increased by about 145% and 38%, respectively, as compared to minimal medium. Biomass concentration was further improved by implementing a smart intermittent bicarbonate feeding strategy, which resulted in an overall enhancement by about 255%, as opposed to the case of minimal medium. It was interesting to observe that Chlorella vulgaris facilitated its own harvesting by self-flocculation-cum-settling mechanism within a short period of time in presence of HCO3−. The efficiency of self-flocculation was found to increase by about 55% in presence of 150 mM initial HCO3−, when compared with minimal medium. High resolution microscopic analyses revealed that microalgal cells were embedded within a self-produced matrix of extracellular polymeric substances (EPS) and underwent alterations in topological complexity that may have facilitated nutrient uptake and gaseous exchange across the cell surface, thereby resulting in enhanced biomass production and self-settling. Therefore, strategic utilization of bicarbonate as a source of DIC convincingly establishes the novelty of our rational approach towards enhancing microalgal biomass coupled with efficient CO2 fixation and self-harvesting. This innovative and smart process may be scaled up and translated to efficiently sequester flue-gas CO2 released by thermal power plants and other fossil fuel-based industries through algal cultivation.