[1] Hypolithic microbial communities are productive niches in deserts worldwide, but many facets of their basic
ecology remain unknown. The Namib Desert is an important site for hypolith study because it has abundant
quartz rocks suitable for colonization and extends west to east across a transition from fog- to rain-dominated
moisture sources. We show that fog sustains and impacts hypolithic ecology in several ways, as follows: (1) fog
effectively replaces rainfall in the western zone of the central Namib to enable high (≥95%) hypolithic abundance
at landscape (1–10 km) and larger scales; and (2) high water availability, through fog (western zone) and/or
rainfall (eastern zone), results in smaller size-class rocks being colonized (mean 6.3 ± 1.2 cm) at higher
proportions (e.g., 98% versus approximately 3%) than in previously studied hyperarid deserts. We measured
0.1% of incident sunlight as the lower limit for hypolithic growth on quartz rocks in the Namib and found that
uncolonized ventral rock surfaces were limited by light rather than moisture. In situ monitoring showed that
although rainfall supplied more liquid water (36 h) per event than fog (mean 4 h), on an equivalent annual basis,
fog provided nearly twice as much liquid water as rainfall to the hypolithic zone. Hypolithic abundance reaches
100% at a mean annual precipitation (MAP) of approximately 40–60 mm, but at a much lower MAP
(approximately 25 mm) when moisture from fog is available.This work was partially supported through NASA’s ASTEP Programhttp://agupubs.onlinelibrary.wiley.com/agu/jgr/journal/10.1002/(ISSN)2169-8961hb201
