Abstract
Biomineralization by calcifying microalgae is a precisely controlled intracellular calcification process that produces delicate calcite scales (or coccoliths) in the coccolithophore Emiliania huxleyi (Haptophycea). Despite its importance in biogeochemical cycles and the marine environment globally, the underlying molecular mechanism of intracellular coccolith formation, which requires calcium, bicarbonate, and coccolith-polysaccharides, remains unclear. In E. huxleyi CCMP 371, we demonstrated that reducing the calcium concentration from 10 (ambient seawater) to 0.1 mM strongly restricted coccolith production, which was then recovered by adding 10 mM calcium, irrespective of inorganic phosphate conditions, indicating that coccolith production could be finely controlled by the calcium supply. Using this strain, we investigated the expression of differentially expressed genes (DEGs) to observe the cellular events induced by changes in calcium concentrations. Intriguingly, DEG analysis revealed that the phosphatidylinositol-specific phospholipase C (PI-PLC) gene was upregulated and coccolith production by cells was blocked by the PI-PLC inhibitor U73122 under conditions closely associated with calcium-induced calcification. These findings imply that PI-PLC plays an important role in the biomineralization process of the coccolithophore E. huxleyi.
Highlights
Coccolithophores play an essential role in the global carbon cycle through considerable carbon dioxide fixation and carbon sequestration as they are major producers of marine biogenic calcites and produce large blooms in the open ocean
This study provided experimental evidence supporting the fact that the calcification of E. huxleyi CCMP 371 is induced by the calcium supply and is independent of Pi conditions, since coccolith production was observed in both Pi-limited and -replete conditions (Figure 1)
Our finding in this study carefully suggests that phosphatidylinositol-specific phospholipase C (PI-PLC) functions to accumulate calcium in this reservoir and supports the stimulation of intracellular calcification that is promoted under calcium-rich conditions in E. huxleyi cells
Summary
Coccolithophores play an essential role in the global carbon cycle through considerable carbon dioxide fixation and carbon sequestration as they are major producers of marine biogenic calcites and produce large blooms in the open ocean. Emiliania huxleyi (E. huxleyi), a calcifying haptophyte, is widespread throughout the oceans worldwide [1,2], including polar [3] and coastal regions [4]. Due to their ability to form dense blooms at mid and high latitudes, they are intensively studied to understand the environmental implications of calcification [5,6]. They produce ornate calcium carbonate scales, known as coccoliths, as cell covering. Understanding the critical molecular mechanism underlying this complicated biological mineralization process will provide a profound insight into the biological function and advance our knowledge of such processes, facilitating mimicry of this complex biogenic morphology
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