A Minority of Desert Cyanobacteria and Algae Is Responsible for the Bulk of <scp> CO ₂ </scp> Fixation

Cyanobacteria and algae are the major photosynthetic organisms in deserts because they survive desiccation, high solar radiation and extreme temperature fluctuations better than other plants. Under favourable conditions, desert cyanobacteria and algae evidently photosynthesise. However, our understanding of whether each group modulates this metabolic process in response to preceding harsh conditions remains limited. To find out the effect of aridity on the photosynthetic activity of desert cyanobacteria and algae, we compared their cellular biovolume-specific carbon dioxide (CO₂) fixation in the hyper-arid and arid regions of a typical hot desert-the central Negev Desert. We found that the biovolume-specific CO₂ fixation of both cyanobacteria and algae was highly variable rather than being constant. The values ranged from 20 to 40 folds, with 3-4 values out > 80 being similarly high. In response to aridity, at least a third of cyanobacteria and algae had low CO₂ fixation activity (< 20% of the maximal rate) and contributed merely ~10% of the total CO₂ fixation. Less than a quarter of cyanobacteria and algae fixed CO₂ at a high rate (> 40% of the maximal rate) and, hence, were responsible for the bulk of CO₂ fixation in the desert. Because the taxonomic composition of cyanobacterial and algal assemblages in the Negev is typical of biocrusts of the drylands, the aridity-dependent dynamic of photosynthetic activity revealed in the Negev is likely a general trait for desert cyanobacteria and algae.