Oxygen enrichment (sparging bioreactors with oxygen-enriched air) ameliorates oxygen limitation in batch cultures of Aspergillus niger B1-D, and although nutrient utilization rates for both the carbon source and the nitrogen source are higher in oxygen-enriched cultures, excess oxygen does not enhance specific growth rate, instead the "extra" nutrient consumption is associated with defensive measures. Commencement of oxygen enrichment in early exponential phase leads to a transient rise in proteins showing oxidative damage (carbonylation), and to induction of enhanced proteolytic activity, which points to the antioxidant defense being temporarily overwhelmed, and that A. niger adapts to this oxidative environment by enhancing intracellular proteolytic activity to degrade damaged proteins which might otherwise accumulate under these conditions. Also the energy status of A. niger, reflected by intracellular ATP content, is found to be altered upon the commencement of oxygen enrichment and then reaches much lower levels than in the control. The reasons underlying this are discussed in the context of what is known regarding alternative respiration in industrial fungi. (c) 2007 Elsevier Ltd. All rights reserved.