Feb 15 (Reuters) - Utility firm Alliant Energy missed Wall Street estimates for fourth-quarter profit on Thursday, as lower retail electric and gas sales and higher asset depreciation expenses offset a drop in operating costs.

Through its subsidiaries - Interstate Power and Light Company and Wisconsin Power and Light Company - Alliant provides regulated energy service to 995,000 electric and 425,000 natural gas customers across Iowa and Wisconsin.

Retail electric sales fell 2.7% in the quarter ended Dec. 31, while retail gas sales slumped 18% from a year earlier. The company also saw a marginal increase in total number of utility customers.

The Madison, Wisconsin-based firm's quarterly operating expenses fell 13.4% from last year to $788 million. Unlike Alliant, its peers Southern Co and WEC Energy benefited from lower operating expenses in this quarter, beating profit estimates.

On an adjusted basis, Alliant earned 48 cents per share in the fourth quarter, compared with analysts' estimates of 55 cents per share, according to LSEG data. (Reporting by Mrinalika Roy in Bengaluru; Editing by Rashmi Aich)