Abstract:The biological soil crusts (BSCs) are commonly found in Gurbantunggut Desert, the largest fixed and semi-fixed desert in China. Four different BSCs developmental stages, including bare sand, algae crusts, lichen crusts and moss crusts were selected to determine successional changes in cyanobacterial and algal species composition, dominant species, and biomass. The results indicated that species composition differed between the four successional stages. There were some differences for common species. For example, Fragilaria sp., Oscillatoria willei, and Phormidium okenii were common species in bare sand. Species of Synechococcus parvus, Tychonema granulatum, and Phormidium retzli were common in algal crusts. Furthermore, some species were specific to particular successional stages. Dominant species varied from bare sand to mature crusts. The dominant species in bare sand, algae, lichen and moss crusts were Fragilaria sp., Microcoleus vaginatus, M. vaginatus and Scytonema ocellatum or Palmellococcus miniatus, respectively. Algal biomass had significant differences in different successional stages (p<0.01). Algal biomass in bare sand was the lowest and increased with BSCs development (such as algal crusts and lichen crusts). Algal biomass in lichen crusts was the highest, which was 8.3 times higher than in bare sand. However, algal biomass decreased with the development of moss crusts. Bare sand is mainly composed of uncompacted sand gains. Filamentous cyanobacteria, particularly M. vaginatus, were the dominant species early in crust succession. Additionally, fungal mycelium and moss rhizoids played a vital role in preventing water and wind erosion. Algal dominant species variation in different BSCs made the role and status of algae in BSCs change.