Guanidine fuels rapid resurrection of desert cyanobacteria

In desert ecosystems, microbial activity is driven by brief hydration pulses but severely constrained by persistent nutrient scarcity ^1–4 . Cyanobacteria, as essential pioneer photoautotrophs, sustain biogeochemical cycling and ecosystem stability in these arid landscapes, yet their rapid resuscitation following rewetting is critically limited by nitrogen availability ^3,4 . Although the nitrogen demand is met later by biological nitrogen fixation, it is irrelevant during early rehydration due to the high energy costs and delayed nitrogenase activation, creating a critical metabolic bottleneck ^5–10 . Here we demonstrate that the desert cyanobacterium Nostoc flagelliforme overcomes this constraint by activating a previously overlooked guanidine carboxylase pathway, which sustains the rapid remobilization of internal nitrogen reserves upon rehydration. Pathway activity correlates strongly with hydration and nitrogen status. Genetic experiments demonstrate its essential role in recovery from desiccation and characterize the key role of a guanidine-I riboswitch. Phylogenetic evidence indicates that pathway genes are transferred horizontally, correlating with habitat aridity. Our findings establish a novel ecological role for guanidine in desert ecosystems, and suggest its impact on microbial community assembly in nutrient-poor drylands. This work reveals a conserved adaptation mechanism that supports cyanobacterial resilience in xeric environments and provides a physiological basis for enhancing biocrust restoration strategies.