Calcium ion adsorption with extracellular proteins of thermophilic bacteria isolated from geothermal sites—A feasibility study

•A biological method for geothermal well anti-scaling was developed.•Thermophilic bacteria isolated from geothermal areas showed Ca adsorption ability.•Extracellular proteins of AT-A2 strain showed the highest Ca adsorption efficiency.•AT-A2′s Ca adsorption ability is better than other known metal-binding proteins.•An eco-friendly & energy efficient method for geothermal well antiscaling is shown.