It looks like that the picture depicts bacterial cells forming a colony. The structure shown by the red arrow is a cell membrane. The principle function of the cell membrane is to protect the cell internal environment with respect to changes in the external environment. This is also called homeostasis.
For food capturing and mobility, the structure is not matching because there should be filaments or pseudopodia. Likewise, for DNA transfer, pili are required and the shown structure doesn't match that too.
Explanation: The arrow is pointing at the cell membrane, the cell membrane controls what is going in and out of the cell, it can also perform as a defensive barrier against things that are not good for the bacteria, therefore it is protection. Have a Good Day :)
