A bacterial slime layer prevents the deposition of ______, and when the slime occasionally gets sloughed away, the slime carries away ______ and leaves the pipe surface bare.

• A. zinc orthophosphate, ferric hydroxide
• B. calcium carbonate, calcium carbonate
• C. calcium carbonate, calcium oxide
• D. zinc orthophosphate, ferrous hydroxide

Answer: (B)

The main idea here is how a bacterial slime layer, or biofilm, interacts with mineral deposition on pipe surfaces. In many water systems, calcium carbonate tends to precipitate and form scale on the inside of pipes. The slime layer acts as a barrier and microenvironment that reduces direct deposition of calcium carbonate onto the bare pipe surface, by altering local chemistry and providing a matrix where minerals can be trapped within the biofilm itself. When the biofilm sloughs off, the accumulated material—calcium carbonate—gets carried away with it, leaving a newly exposed, bare pipe surface that can then begin to redeposit or be challenged by corrosion processes again. This is why calcium carbonate is the substance described in both blanks. Other choices involve different minerals or treatment chemicals that aren’t typically described as being controlled in this exact biofilm-mediated way.