<p>In deserts that cover one-third of the Earth’s terrain, soil cyanobacteria and algae are key carbon dioxide (CO₂) fixers because few other plants can tolerate intense solar radiation, prolonged desiccation, and extreme temperature shifts. Extensively studied resilience of cyanobacteria and algae to environmental stresses explains their survival in deserts rather than their ubiquitous coexistence. To coexist, prokaryotic cyanobacteria and eukaryotic algae need to grow at similar rates on rare occasions when vital water becomes available. To test that, we incubated illuminated soil samples collected in the Negev desert after rainfall and dewfall events with¹⁴CO₂ and microdissected cyanobacteria and algae separately from the soil to determine group-specific CO₂ fixation potentials. We found that the mean biovolume-specific C-fixation rates of cyanobacteria were 2.5-3.0 times higher than the mean rates of algae, irrespective of the region. These reproducible results suggested that cyanobacteria could grow faster than algae and outcompete them. However, because the carbon content of the cyanobacterial biovolume is ~ 2.5 times higher than the carbon content of the algal biovolume, their mean doubling times are, in fact, similar. Therefore, once it rains, desert cyanobacteria and algae can grow at similar rates, and that explains their recurring coexistence in desert soils.</p>