The Bacterial Microbiome Associated With Arid Biocrusts and the Biogeochemical Influence of Biocrusts Upon the Underlying Soil.

Benjamin Moreira-Grez,Paul Nevill,Deepak Kumaresan,Andrew S Whiteley,Jean W H Yong,Mark Farrell,Adam T Cross,Kang Tam

doi:10.3389/fmicb.2019.02143

Abstract

Biocrusts are aggregated crusts that exist on the soil surface of arid environments. They are complex microbial communities comprised of cyanobacteria, lichens, mosses, algae and fungi. Recently, biocrusts have gained significant attention due to their ubiquitous distribution and likely important ecological roles, including soil stabilization, soil moisture retention, carbon (C) and nitrogen (N) fixation, as well as microbial engineers for semi-arid ecosystem restoration. Here, we collected three co-occurring types of biocrust (Cyanobacterial crust, Crustose lichen, and Foliose lichen) and their underlying soil from arid zones within Western Australia. Bacterial microbiome composition was determined through 16S rRNA gene amplicon sequencing to assess the extent of microbiome selection within the crusts versus underlying soil and biogeochemical measures performed to determine whether the crusts had significant impact upon the underlying soil for nutrient input. We determined that the bacterial communities of native biocrusts are distinct from those in their underlying soil, where dominant bacterial taxa differed according to crust morphologies. δ15N revealed that N-fixation appeared most evident in Foliose lichen crust (1.73 ± 1.04‰). Consequently, depending upon the crust type, biocrusts contained higher concentrations of organic C (2 to 50 times), total N (4 to 16 times) and available ammonium (2 to 4 times), though this enrichment did not extend to the soils underneath them. These findings demonstrate that biocrust communities are seemingly islands of biological activity in an arid landscape, uniquely different from their surrounding and underlying soil.

Highlights

Biological soil crusts (“biocrusts” hereafter) are complex communities comprised of cyanobacteria, lichens, bryophytes, fungi, and algae found on soil surfaces and are common and integral components of arid and semi-arid ecosystems, where plant cover is often sparse (Belnap et al, 2001; Rodriguez-Caballero et al, 2018)
Microbial communities in biocrusts were compositionally distinct from those found within the underlying soil for the 20 abundant most dominant families (Figure 2)
Microbial communities within cyanobacterial crust were highly correlated with Rhizobiales, Chroococcales, Nostocales one unknown family within the class Nostocophycideae, all belonging to the phylum Cyanobacteria

Summary

Introduction

Biological soil crusts (“biocrusts” hereafter) are complex communities comprised of cyanobacteria, lichens, bryophytes, fungi, and algae found on soil surfaces and are common and integral components of arid and semi-arid ecosystems, where plant cover is often sparse (Belnap et al, 2001; Rodriguez-Caballero et al, 2018). Biocrusts can cover up to 70% of the soil surface in Biocrust Microbiome desert areas (Yeager et al, 2004) and are estimated to cover over 12% of global terrestrial land surfaces (almost 18 million km2; Rodriguez-Caballero et al, 2018) Due to their ecological and microbial characteristics, aligned with their ubiquitous occurrence in arid environments, biocrusts have emerged as an important and emerging research focus in recent decades (Belnap and Lange, 2001; Belnap and Weber, 2013; Bowker et al, 2018), addressing many research themes, including stress survival, ecological assembly in harsh environments and biogeochemical influence on their surrounding soils. The release of nitrogenous products (e.g., ammonium, nitrate, amides, peptides, and amino acids) has been observed in some cyanobacteria and lichens that make up these biocrusts (see Barger et al, 2016 for review). Millbank (1978, 1982) and Silvester et al (1996) found labeled 15N in the extracellular environment of N-fixing cyanobacteria, showing that fixed N may be released into the surrounding soil environment, representing a potential pathway for N enrichment in topsoil directly underlying biocrusts and a vehicle to initiate soil N cycles for future plant colonization

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Discussion

Conclusion