363 / 2024-09-15 07:57:41

Study on the Impact of Spontaneous CaCO3 Precipitation and Diatom-induced Calcification on the Carbon Sequestration Effect in Alkaline Enhancement

CaCO3 Precipitation,Diatom,Calcification,Skeletonema costatum,Biological carbon pump,Counter carbonate pump

Alkaline enhancement stands out as a favored strategy for ocean carbon sequestration, attributed to its substantial capacity to augment carbon capture. However, heightened seawater alkalinity post-alkalization could elevate pH levels, potentially enhancing the saturation of CaCO₃. Spontaneous, substantial precipitation of CaCO₃ could undermine the alkaline enhancement's efficacy. To sustain the efficacy of alkaline enhancement, investigations into the effects of inorganic calcium carbonate precipitation, the stimulation of calcifying organisms' growth, and diatoms' extracellular calcification are imperative. While rapid inorganic precipitation parallels the carbonate counter pump (CCP) of calcifying organisms, potentially escalating seawater pCO₂, the high Mg content in seawater acts as a natural inhibitor against such precipitation. Current seawater requires CaCO₃ saturation levels above 12 for homogeneous nucleation and 18 for rapid precipitation. Our study identified that, Skeletonema costatum, a prevalent diatom, can significantly promote aragonite precipitation under photosynthetic conditions. Direct measurements of pH and Zeta potential on diatom cell surfaces revealed that diatoms facilitate CaCO₃ precipitation on the cell surface by fostering a microenvironment rich in carbonate and calcium ions, coupled with the dehydration of adsorbed calcium ions. This diatom-mediated calcification necessitates aragonite saturation in seawater exceeding 8.02, achievable during certain diatom bloom episodes. Notably, a marked reduction in total alkalinity was observed during a S. costatum bloom in the East China Sea, mirroring the non-calcifying algal bloom phenomena reported in the Gulf of Mexico's carbonate surveys, suggesting that diatom-induced calcification may be a genuine marine occurrence. The diatom-induced calcification process diverges from the conventional CCP, as it transpires in high pH seawater with inherently low pCO₂, precluding the seawater from becoming a carbon source. Instead, it serves to replenish CO₂ during bloom periods. Electron microscopy revealed that CaCO₃-algae aggregates, formed in conjunction with S. costatum, substantially boost the transport efficiency of particulate organic carbon (POC) and particulate inorganic carbon (PIC) via the ballast effect. Consequently, the diatom-induced calcification process, activated post-alkaline enhancement, is more likely to amplify the sequestration process rather than merely maintaining the status quo of carbon sequestration.