A fog-irrigated soil substrate system unifies and optimizes cyanobacterial biocrust inoculum production

Biological soil crusts (biocrusts) are globally important microbial communities inhabiting the top layer of soils. They provide multiple services to dryland ecosystems but are particularly vulnerable to anthropogenic disturbance from which they naturally recover only slowly. Assisted inoculation with cyanobacteria is held as a promising approach to promote biocrust regeneration. Two different methodologies have been developed for this purpose: mass cultivation of biocrust pioneer species (such as the cyanobacteria Microcoleus spp.) on cellulose supports, and polymicrobial cultivation of biocrusts in soils within greenhouse settings. Here, we aimed to test a novel method to grow cyanobacterial biocrust inoculum based on fog irrigation of soil substrates (FISS) that can be used with either culture-based or mixedcommunity approaches. We found that the FISS system presents clear advantages over previous inoculum production methodologies; overall, FISS eliminates the need for specialized facilities and decreases user effort. Specifically, there were increased microbial yields and simplification of design compared to those of the culture-based and mixed-community approaches, respectively. Its testing also allows us to make recommendations on underexplored aspects of biocrust restoration: (i) field inoculation levels should be equal to or greater than the biomass found in the substrate and (ii) practices regarding evaluation of cyanobacterial biomass should, under certain circumstances, include proxies additional to chlorophyll a.