A series of processes that forms an integrated mechanism by which a cell or an organism detects the depletion of primary carbon sources and then activates genes to scavenge the last traces of the primary carbon source and to transport and metabolize alternative carbon sources such as carbon dioxide or carbonic acid. The utilization process begins when the cell or organism detects carbon levels, includes the activation of genes whose products detect, transport or metabolize carbon-containing substances, and ends when carbon is incorporated into the cell or organism's metabolism.