Each commissioner shall be appointed to serve a term of 6 years. At the expiration of the term of the commissioner appointed by the Governor, the Governor shall appoint a successor who shall hold office for 6 years. At the expiration of the term of a commissioner appointed by the Mid-America Intermodal Authority Port District Board, the Mid-America Intermodal Authority Port District Board shall appoint a successor who shall hold office for 6 years. Each commissioner shall hold office until his successor has been appointed.