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The excreting of exopolysaccharides (EPS) is thought as one of the main protection ways for cyanobacteria in desert algal crusts to survive desiccation. But how cyanobacteria adjust their carbohydrate metabolism to survive this stress is not elucidated. In this study, we treated Phormidium tenue, a cyanobacterium isolated from biological soil crusts with the changes of relative humidity (RH) to simulate different levels of desiccation and investigated its carbohydrate metabolism. It was found that photosynthetic activity (Fv/Fm) and cellular total carbohydrates production decreased significantly at low RH. But the production of EPS, reducing sugar, sucrose and the activity of sucrose phosphate synthase (SPS) increased significantly at low RH and reached maximum at 75% RH. Low RH could also cause the enhanced production of reactive oxygen species (ROS) generation, malondialdehyde (MDA) production and DNA strand breaks. However, when pretreated with exogenous 100 mg/L EPS, Fv/Fm and carbohydrate production were improved significantly, while ROS generation, MDA production and DNA strand breaks decreased significantly at various levels of RH in P tenue. These results indicated that P. tenue in arid regions could enhance desiccation tolerance by adjusting the carbohydrate metabolism to eliminate ROS and decrease oxidative damage. (C) 2011 Elsevier Masson SAS. All rights reserved.