2.5. Impermeability test The single cultures and bacterial mixtures grown in YA brothovernight were harvested by centrifugation at 6000 × g for 5 min,washed twice, and resuspended in a 0.9% sodium chloride solution at a final OD600of 1.0 (equivalent to 1 × 108cells/mL). Sterilesilica sand (200 g, 0.45–0.7 mm, Joomoonjin Sand Co. Ltd., Korea)was mixed with 10 mL urea (40 g/L) and calcium chloride dihydrate solution (25 g/L). The sand slurry (40 g) was packed into a25 mL plastic column (Corning Co. Ltd., Corning, NY, USA). Afterbeing dried for 48 h at 50◦C, the columns were run once by gravity with 10 mL of cell suspension. The columns were stored for 48 hto allow calcite crystal growth. Then, 2 mL of crystal violet (CV)was pipetted onto the packed sand column. The degree of imperme ability was determined by measuring the migration distance of CV