It is now accepted that heat treatment can lead to oxidative stress events in microorganisms, but there are few detailed studies on protein oxidation and consequent recycling/elimination of oxidatively damaged proteins following such heat treatment. In batch cultures of a filamentous fungus, Aspergillus niger B1-D, raising culture temperature from 25 degrees C to 30 degrees C and 35 degrees C led to a general enhancement of intracellular metabolism, a higher specific growth rate, increased consumption of carbon and nitrogen sources, and raised intracellular ATP content in the exponential phase. By contrast, there was a transient accumulation of protein carbonyls, a widely used biomarker of protein oxidation, following such temperature increases, which could indicate that cellular antioxidant defences were being temporarily overwhelmed under these circumstances, despite the fact that enhanced activities in antioxidant enzyme activities have been reported in microorganisms during such heat treatment. Protein carbonyls can only be removed by proteolysis. The intracellular proteolytic activity in batch cultures of A. niger was found generally enhanced by temperature elevation, suggesting the role of proteolytic activity in protein quality control during heat treatment was dependant on the culture temperature. (C) 2008 Elsevier Ltd. All rights reserved.