The fungus employed in citric acid production is

The fungus employed in citric acid production is usually Aspergillus niger, a filamentous fungus that can grow on a wide range of substrates and produce high yields of citric acid. 

Citric acid is an organic acid widely used in food and beverage industry as a preservative, flavor enhancer, and acidulant. It is also used in pharmaceuticals, cosmetics, and cleaning agents. Citric acid is mainly produced by the fungal fermentation of molasses or other carbohydrate sources.