Explain the Corrosion by the Action of Bacteria?

Corrosion can also be accelerated by the action of bacteria.  In the absence of oxygen, sulphate-reducing bacteria can grow, and the sulphides produced by these can cause rapid attack on steel.  Some bottom sediments are oxygen deficient and sulphate-reducing bacterial attack can occur on buried steel in such sediments. The presence of limited growth of barnacles tends to increase the amount of pitting. There are opinions that heavy growth of barnacles tends to check corrosion.  However, there are contrary views also.