Water regime history drives responses of soil Namib Desert microbial communities to wetting events

Abstract

Despite the dominance of microorganisms in arid soils, the structures and functional dynamics of microbial communities in hot deserts remain largely unresolved. The effects of wetting event frequency and intensity on Namib Desert microbial communities from two soils with different water-regime histories were tested over 36 days. A total of 168 soil microcosms received wetting events mimicking fog, light rain and heavy rainfall, with a parallel “dry condition” control. T-RFLP data showed that the different wetting events affected desert microbial community structures, but these effects were attenuated by the effects related to the long-term adaptation of both fungal and bacterial communities to soil origins (i.e. soil water regime histories). The intensity of the water pulses (i.e. the amount of water added) rather than the frequency of wetting events had greatest effect in shaping bacterial and fungal community structures. In contrast to microbial diversity, microbial activities (enzyme activities) showed very little response to the wetting events and were mainly driven by soil origin. This experiment clearly demonstrates the complexity of microbial community responses to wetting events in hyperarid hot desert soil ecosystems and underlines the dynamism of their indigenous microbial communities.

Namib Desert Soil Microbial Community Diversity, Assembly, and Function Along a Natural Xeric Gradient

“…Moisture source has previously been shown to influence Namib Desert hypolithic microbial community structures, assembly and colonization (Stomeo et al, 2013;Warren-Rhodes et al, 2013;, but studies on the effect of water/moisture source on Namib Desert edaphic community diversity and function are limited: In an V-shaped transect, with no inter-site replication, Namib Desert edaphic bacterial community structures were found influenced by water source (i.e. fog vs rain; Stomeo et al, 2013), and a more recent microcosm experiment established that water regime history was a critical factor in driving bacterial and fungal community structures as well as their adaptation to water stresses (Frossard et al, 2015).…”

“…Our working hypothesis was that climate and soil parameters across the xeric gradient should correlate with Namib Desert edaphic microbial community structures, as assessed by T-RFLP fingerprinting (Johnson et al, 2017) and shotgun metaviromics (Zablocki et al 2017). Similarly, gross functional capacities, as measured by extracellular enzymatic assays (Frossard et al, 2015), were also expected to respond quantitatively to water availability from the coast to the inland desert margin.…”

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

“…Stemming from this, the modern pulse-reserve ‘bioavailable water’ paradigm33 proposes a restriction of biological activity in arid soils to pre- and post episodic rainfall events, although this response can vary considerably depending on the duration and intensity of these events343536. By definition however, rainfall events are extremely infrequent in hot desert regions.…”

Nutrient acquisition, rather than stress response over diel cycles, drives microbial transcription in a dessicated Namib Desert soil

“…This conclusion has implications in terms of the perceived drivers of microbial community function, as our results suggest that the constant xeric stress is a more significant driver of in situ functionality than daily environmental variations (temperature, air moisture or light). We predict that this functional stability would only be substantially disrupted by stochastic events such as rainfall, which is recognised as a main driver of community assembly and activity in arid soil environments (Belnap et al , 2005; Pointing and Belnap, 2012; Frossard et al , 2015; Scola et al , 2017).…”

“…In recent years, the microbial ecology of various Namib Desert edaphic niches has been extensively studied (e.g. Scola et al , 2017; Johnson et al , 2017; Ronca et al , 2015; Frossard et al , 2015).…”