Role of carbonic utilization of microalgae on rock weathering and carbon cycle

The carbon sink on rock weathering is widely discussed for reducing global atmospheric carbon dioxide (CO₂). Two different views on karst carbon sink are proposed. One is that the karst carbon sink is huge because bicarbonate ion (${\rm{HCO}}_3^{-}$) is used by the photosynthesis of algae and photosynthetic bacteria in karst areas, which dynamically accelerates the process of karst weathering and subsequently promote the dissolution of atmospheric CO₂. Another is that the weathering of carbonate rock generates HCO$_3^{-}$, and then the equivalent calcium ions (Ca²⁺) and Magnesium ions (Mg²⁺) are produced for the deposition of carbonate rock on the sea floor as the river enters the ocean. This process only reflects the transport of carbonate rock instead of the carbon sink because only the weathering of silicate rock may generate the net carbon sink in the long term.

By literature review in this paper, the effects of microalgae (a typical aquatic organism) on rock weathering and its carbon sink are discussed based on the coupling between inorganic carbon utilization of microalgae in photosynthesis and ${\rm{HCO}}_3^{-}$ produced from rock-weathering. The facilitation on rock-weathering and its carbon sink by microalgae growth is demonstrated from two aspects, namely, the utilization mechanism of inorganic carbon and the action of carbonic anhydrase (the key enzyme of photosynthesis) in microalgae. Besides, the biomass of microalgae, in turn, is enhanced by the effects of weathering-environment, such as, higher pH value and higher ${\rm{HCO}}_3^{-}$. In this study, the following three arguments are proposed. Firstly, the weathering is accelerated because of the continuous consumption of ${\rm{HCO}}_3^{-}$ utilized by catalysis of extracellular carbonic anhydrase (CAex) in microalgae, which makes the weathering towards the direction on forming ${\rm{HCO}}_3^{-}$. Secondly, the microalgae can accelerate the weathering of calcium-magnesium silicate rocks, and Ca²⁺ and Mg²⁺ dissolved out by weathering may, in turn, facilitate the deposition of carbonate rock, hence a net carbon sink is generated. Thirdly, pure chemical weathering of carbonate rock cannot directly generate a net carbon sink at long time scale, but the ${\rm{HCO}}_3^{-}$ utilization from CO₂ in microalgae makes the weathering of carbonate rock proceed in the direction of HCO$_3^{-}$ conversion. In the process of calcium carbonate deposition involved by microalgae, inorganic carbon is converted into recalcitrant organic carbon and thus the carbon sink is generated.

Research findings can be concluded that through the CAex effect, the catalysis and acceleration of conversion of HCO$_3^{-}$ to CO₂ by microalgae will form the dynamic basis of water HCO$_3^{-}$ consumption. The utilization of inorganic carbon in microalgae can facilitate rock weathering, and hence the concentration of atmospheric CO₂ will be regulated. In this study, three aspects of prospect are also put forward. Firstly, to address the regional unbalance of carbon budget, it is crucial to assess the carbon sink of rock weathering under aquatic organism in karst areas. Besides, to improve the precision of calculating carbon sink of rock-weathering by hydrochemical runoff method, the mechanism and amount of biological carbon conversion in rock weathering should be determined. Finally, it is urgent to establish a new method to assess the time scale of carbon sink of rock weathering under the effects of aquatic organisms by water-cycle, which can clarify the contribution of carbon sink of rock weathering to the carbon budget.