Physical ecology of hypolithic communities in the central Namib Desert: The role of fog, rain, rock habitat, and light

Abstract

[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.

Requirements and limits for life in the context of exoplanets

“…Fig. 3B shows the green biofilm of cyanobacteria that live beneath translucent rocks in many deserts surviving on as little as a few days of rain or fog each year (32)(33)(34). The example shown is from an unusual carbonate rock from the Mojave Desert that is clear inside but covered with a red coating (35,36).…”

Diel-scale temporal dynamics recorded for bacterial groups in Namib Desert soil

“…This has been demonstrated for key edaphic bacterial phyla, such as Proteobacteria43, Actinobacteria44 and Cyanobacteria23. In coastal deserts, such as the Namib and Atacama Deserts, moisture inputs such as fog and dew precipitation are fundamental in sustaining hot desert insect and plant populations4546, as well as microbial communities4748495051. Fog events can occur when the water vapor concentration in the atmosphere reaches saturation, i.e., 100% relative humidity (RH)52.…”

Fogs: Physical Basis, Characteristic Properties, and Impacts on the Environment and Human Health

“…Investigations have been performed on the impacts of fog on plants, such as the red spruce [108] and beech [109], as well as on hypolithic microbial communities [101,110]. Percy et al [108] reported that red spruce exposure to pH 4.2 acid fog increased wax quantity for a certain period, but a longer exposure decreased it; reduced the rate of de novo wax synthesis; changed the composition of wax reducing production of secondary alcohols (pH 3.0 fog).…”

“…Most recently, Warren-Rhodes et al [110] investigated the role of fog and other factors in physical ecology of hypolithic microbial communities in Namib Desert and showed that fog sustains and impacts the hypolithic ecology, effectively replacing the rainfall, and enables high hypolithic abundance; it is also responsible for smaller size-class rocks to be colonized at a higher proportion, due to the higher availability of water.…”