A significant proportion of the Earth's surface is desert or in the process of desertification. The extreme environmental conditions that characterize these areas result in a surface that is essentially barren, with a limited range of higher plants and animals. Microbial communities are probably the dominant drivers of these systems, mediating key ecosystem processes. In this review, we examine the microbial communities of hot desert terrestrial biotopes (including soils, cryptic and refuge niches and plant-root-associated microbes) and the processes that govern their assembly. We also assess the possible effects of global climate change on hot desert microbial communities and the resulting feedback mechanisms. We conclude by discussing current gaps in our understanding of the microbiology of hot deserts and suggest fruitful avenues for future research.

Microbial ecology of hot desert edaphic systems

The requirements for life on Earth, its elemental composition, and its environmental limits provide a way to assess the habitability of exoplanets Temperature is key both because of its influence on liquid water and because it can be directly estimated from orbital and climate models of exoplanetary systems Life can grow and reproduce at temperatures as low as −15 °C, and as high as 122 °C Studies of life in extreme deserts show that on a dry world, even a small amount of rain, fog, snow, and even atmospheric humidity can be adequate for photosynthetic production producing a small but detectable microbial community Life is able to use light at levels less than 10−5 of the solar flux at Earth UV or ionizing radiation can be tolerated by many microorganisms at very high levels and is unlikely to be life limiting on an exoplanet Biologically available nitrogen may limit habitability Levels of O2 over a few percent on an exoplanet would be consistent with the presence of multicellular organisms and high levels of O2 on Earth-like worlds indicate oxygenic photosynthesis Other factors such as pH and salinity are likely to vary and not limit life over an entire planet or moon

https://www.pnas.org/content/pnas/111/35/12628.full.pdf

Requirements and limits for life in the context of exoplanets

Non-rainfall water inputs (NRWIs) are the least studied hydrological components in most ecosystems. These NRWI components potentially play an important role in ecosystem dynamics and are particularly important for water-limited systems. In this review, we summarized recent advances investigating the effects of NRWIs on various ecosystem functions, including vegetation-water relations, biogeochemical cycling, groundwater recharge, as well as reptile and invertebrate adaptations. We also identified key knowledge gaps such as the mechanisms of NRWIs alleviating vegetation water stress, sources of the NRWI components and their quantitative contributions to ecosystem functions. To better predict the ecosystem responses to climate change especially in drylands, a better understanding and quantification of NRWI contributions is essential. WIREs Water 2017, 4:e1179. doi: 10.1002/wat2.1179

For further resources related to this article, please visit the WIREs website.

Effects of non-rainfall water inputs on ecosystem functions

Hypolithic microbial communities are specialized desert communities inhabiting the underside of translucent rocks. Here, we present the first study of the viral fraction of these communities isolated from the hyperarid Namib Desert. The taxonomic composition of the hypolithic viral communities was investigated and a functional assessment of the sequences determined. Phylotypic analysis showed that bacteriophages belonging to the order Caudovirales, in particular the family Siphoviridae, were most prevalent. Functional analysis and comparison with other metaviromes revealed a relatively high frequency of cell wall-degrading enzymes, ribonucleotide reductases (RNRs) and phage-associated genes. Phylogenetic analyses of terL and phoH marker genes indicated that many of the sequences were novel and distinct from known isolates, and the class distribution of the RNRs suggests that this is a novel environment. The composition of the viral hypolith fraction containing many Bacillus-infecting phages was not completely consistent with Namib hypolith phylotypic surveys of the bacterial hosts, in which the cyanobacterial genus Chroococcidiopsis was found to be dominant. This could be attributed to the lack of sequence information about hypolith viruses/bacteria in public databases or the possibility that hypolithic communities incorporate viruses from the surrounding soil.

/pdf/metagenomic-analysis-of-the-viral-community-in-namib-desert-1wo331m8nm.pdf

Metagenomic analysis of the viral community in Namib Desert hypoliths

In hyperarid deserts, endolithic microbial communities colonize the rocks' interior as a survival strategy. Yet, the composition of these communities and the drivers promoting their assembly are still poorly understood. We analysed the diversity and community composition of endoliths from four different lithic substrates - calcite, gypsum, ignimbrite and granite - collected in the hyperarid zone of the Atacama Desert, Chile. By combining microscopy, mineralogy, spectroscopy and high throughput sequencing, we found these communities to be highly specific to their lithic substrate, although they were all dominated by the same four main phyla, Cyanobacteria, Actinobacteria, Chloroflexi and Proteobacteria. Our finding indicates a fine scale diversification of the microbial reservoir driven by substrate properties. The data suggest that the overall rock chemistry and the light transmission properties of the substrates are not essential drivers of community structure and composition. Instead, we propose that the architecture of the rock, i.e., the space available for colonization and its physical structure, linked to water retention capabilities, is ultimately the driver of community diversity and composition at the dry limit of life.

/pdf/fundamental-drivers-for-endolithic-microbial-community-czi8dhri6a.pdf

Fundamental drivers for endolithic microbial community assemblies in the hyperarid Atacama Desert

[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.

/pdf/physical-ecology-of-hypolithic-communities-in-the-central-9ptt2qmxog.pdf

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

Disturbances elicit both positive and negative effects on organisms; these effects vary in their strength and their timing. Effects of disturbance interval (i.e., the length of time between disturbances) on population growth will depend on both the timing and strength of positive and negative effects of disturbances. Climate change can modify the relative strengths of these positive and negative effects, leading to altered optimal disturbance intervals (the disturbance interval at which population growth rate is highest) and changes in the sensitivity of population growth rate to disturbance interval. While we know that climate may alter impacts of disturbance in some systems, we have a poor understanding of which effects of disturbance and which vital rates might drive an altered response to disturbance interval in a changing climate. We use demographic monitoring of natural populations of Dionaea muscipula, the Venus flytrap, that have experienced natural and managed fires, combined with realistic past and future climate projections, to construct climate- and fire-driven integral projection models (IPMs). We use these IPMs to compare the effect of fire return interval (FRI) on population growth rate in past and future climates. To dissect the mechanisms driving FRI response, we then construct IPMs with demographic data from an experimental manipulation of fire effects (ash addition, neighbor removal) and an accidental fire. Our results show that an FRI of 10 years is optimal for D. muscipula in past climate conditions, but a longer FRI (12 years) is optimal in future climate conditions. Further, deviations from optimal FRI reduce population growth rate dramatically in the past climate, but this reduction is muted in a future climate (future minus past sensitivity = 0.006, 95% CI [0.002, 0.011]). Finally, our experimental work suggests that fire effects are driven in part by positive, additive effects of competitor removal and ash addition immediately following a fire; for one population, both these treatments significantly increased population growth rate. Our work suggests that climate change can alter the response of populations to disturbance, highlighting the need to consider the interacting effects of multiple abiotic drivers when projecting future population growth and geographical distributions. 

https://rss.onlinelibrary.wiley.com/doi/am-pdf/10.1002/ecm.1528

Climate change weakens the impact of disturbance interval on the growth rate of natural populations of Venus flytrap

Habitat restoration frequently focuses on reaching an idealized steady state, but this is unrealistic for disturbance‐dependent ecosystems where temporal variability is inherent and habitat conditions are expected to fluctuate. Understanding the ways in which the outcomes of restoration change over time in disturbance‐dependent ecosystems can better inform adaptive management plans and increase the likelihood that restoration efforts will be effective. We conducted a decade‐long restoration experiment to test how restoration efforts to increase disturbance levels impact habitat quality and populations of an endangered butterfly over time. We show that changes in plant communities as a response to disturbance vary depending on time since restoration, with target host plants initially increasing and peaking several years postrestoration but then declining. In the absence of further disturbance, butterfly population sizes follow a similar pattern, with population declines concurrent with declines in host plants. Due to this nonlinear response, management actions within disturbance‐dependent ecosystems need to include long‐term monitoring in order to accurately capture changes in habitat response, as well as active, adaptive planning that shifts according to current system stability. Restoration efforts within these dynamic habitats are more likely to succeed when temporal variability is explicitly tracked and multiple cycles of restoration are considered as part of management actions.

https://onlinelibrary.wiley.com/doi/pdfdirect/10.1111/rec.13883

Elsita M. Kiekebusch

Papers

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

Climate change weakens the impact of disturbance interval on the growth rate of natural populations of Venus flytrap

Restoration success varies based on time since restoration in a disturbance‐dependent ephemeral wetland ecosystem