Biocrust restoration: a key tool to recover degraded arid ecosystem functioning

peer reviewedBiocrusts are autotroph and heterotroph communities that cover 12% of the Earth land’s surface, where they act as ecosystem engineers. They are very vulnerable to climate change and disturbance caused by different anthropic activities. In this work, we revise the impacts of both disturbance types, which negatively affect biogeochemical cycles and water and energy balances, accelerate erosion processes and dust emissions, and decrease biodiversity, reducing ecosystem ability to provide services. We also explore the ability of these communities to recover after disturbance, which in general requires long time periods for the most developed communities. Because of this, new biotechnologies have emerged to accelerate their restoration, based on the inoculation of biocrust-forming organisms. Results from two main strategies according to the origin of the biocrust propagules used are revised: a) translocation of biocrust fragments from a donor area to the degraded area. This strategy is recommended for planned activities in which the existing biocrust before disturbance is applied; b) large-scale cultivation of biocrust-forming organisms (cyanobacteria, lichens, mosses or the whole community) in laboratory or greenhouse conditions for their later inoculation in the degraded area. Finally, we identify future challenges to maximize restoration success and biocrust conservation

Polyphasic evaluation of key cyanobacteria in biocrusts from the most arid region in Europe

Cyanobacteria are key microbes in topsoil communities that have important roles in preventing soil erosion, carbon and nitrogen fixation, and influencing soil hydrology. However, little is known regarding the identity and distribution of the microbial components in the photosynthetic assemblages that form a cohesive biological soil crust (biocrust) in drylands of Europe. In this study, we investigated the cyanobacterial species colonizing biocrusts in three representative dryland ecosystems from the most arid region in Europe (SE Spain) that are characterized by different soil conditions. Isolated cyanobacterial cultures were identified by a polyphasic approach, including 16S rRNA gene sequencing, phylogenetic relationship determination, and morphological and ecological habitat assessments. Three well-differentiated groups were identified: heterocystous-cyanobacteria (Nostoc commune, Nostoc calcicola, Tolypothrix distorta and Scytonema hyalinum), which play an important role in N and C cycling in soil; nonheterocystous bundle-forming cyanobacteria (Microcoleus steenstrupii, Trichocoleus desertorum, and Schizothrix cf. calcicola); and narrow filamentous cyanobacteria (Leptolyngbya frigida and Oculatella kazantipica), all of which are essential genera for initial biocrust formation. The results of this study contribute to our understanding of cyanobacterial species composition in biocrusts from important and understudied European habitats, such as the Mediterranean Basin, a hotspot of biodiversity, where these species are keystone pioneer organisms

Drivers of the terrestrial cyanobacterial community composition in the Sør Rondane Mountains, East Antarctica

The sparse ice-free regions of Antarctica harbor diverse microbial communities that can vary significantly between regions and micro-climatic conditions. The factors responsible for the diversity and community structure in inland nunataks of East Antarctica are still poorly understood. During the BELSPO MICROBIAN project, three sampling campaigns took place in the Sør Rondane Mountains during the austral summers of 2018, 2019 and 2020, resulting in more than 100 samples ranging from different types of barren bedrock to substrates covered by biofilms and well-developed biological soil crusts including lichens, mosses, microalgae and/or cyanobacteria. Cyanobacterial diversity was assessed by amplicon sequencing targeting the V3-V4 variable region of the 16S rRNA gene with cyanobacteria-specific primers using the Illumina MiSeq platform (2x300 bp). The recently developed CyanoSeq database was used for the taxonomic affiliation of the OTUs (99% similarity threshold). Whilst favorable habitats, such as sheltered spots in rocky areas, enhance the development of different kinds of cyanobacterial crusts, cyanobacteria were present even in the most extreme ones. Granitic soils were dominated by very diverse cyanobacterial crusts, mostly composed by filamentous cyanobacteria of the Leptolynbyaceae, Oculatellaceae and Microcoleaceae families, and by Nostocaceae. Most abundant OTUs on gneiss bedrock were from the Cyanothecaceae and Microcoleaceae. In contrast, marble soils were dominated almost exclusively by the Chroococcidiopsaceae family. Moraine samples from very dry areas were mainly characterized by members of the Gomontiellaceae family whereas moraine samples taken close to a lake were rich in filamentous taxa as well, mostly belonging to the Leptolynbyaceae, Oculatellaceae and Gomontiellaceae families. Next to bedrock type, other abiotic variables such as pH, NO3 and TOC were especially important drivers of the community composition in each sampled site.MICROBIAN15. Life on lan

Water Regulation in Cyanobacterial Biocrusts from Drylands: Negative Impacts of Anthropogenic Disturbance

Arid and semi-arid ecosystems are characterized by patchy vegetation and variable resource availability. The interplant spaces of these ecosystems are very often covered by cyanobacteria-dominated biocrusts, which are the primary colonizers of terrestrial ecosystems and key in facilitating the succession of other biocrust organisms and plants. Cyanobacterial biocrusts regulate the horizontal and vertical fluxes of water, carbon and nutrients into and from the soil and play crucial hydrological, geomorphological and ecological roles in these ecosystems. In this paper, we analyze the influence of cyanobacterial biocrusts on water balance components (infiltration-runoff, evaporation, soil moisture and non-rainfall water inputs (NRWIs)) in representative semiarid ecosystems in southeastern Spain. The influence of cyanobacterial biocrusts, in two stages of their development, on runoff-infiltration was studied by rainfall simulation and in field plots under natural rainfall at different spatial scales. Results showed that cover, exopolysaccharide content, roughness, organic carbon, total nitrogen, available water holding capacity, aggregate stability, and other properties increased with the development of the cyanobacterial biocrust. Due to the effects on these soil properties, runoff generation was lower in well-developed than in incipient-cyanobacterial biocrusts under both simulated and natural rainfall and on different spatial scales. Runoff yield decreased at coarser spatial scales due to re-infiltration along the hillslope, thus decreasing hydrological connectivity. Soil moisture monitoring at 0.03 m depth revealed higher moisture content and slower soil water loss in plots covered by cyanobacterial biocrusts compared to bare soils. Non-rainfall water inputs were also higher under well-developed cyanobacterial biocrusts than in bare soils. Disturbance of cyanobacterial biocrusts seriously affected the water balance by increasing runoff, decreasing soil moisture and accelerating soil water loss, at the same time that led to a very significant increase in sediment yield. The recovery of biocrust cover after disturbance can be relatively fast, but its growth rate is strongly conditioned by microclimate. The results of this paper show the important influence of cyanobacterial biocrust in modulating the different processes supporting the capacity of these ecosystems to provide key services such as water regulation or erosion control, and also the important impacts of their anthropic disturbance

Physiological performance under high salinity conditions of Nostoc commune from hot and cold deserts

Cyanobacteria are phototrophic bacteria that grow in extreme environments, such as those in drylands or polar zones, where they are the first photosynthetic colonizers of soils and sediments. Strong resistance traits underpin their occurrence in these extreme and contrasted environments, for example, the production of pigments (such as the UV-screening scytonemin) or exopolysaccharides help them to withstand hostile conditions. A better understanding of the ecophysiological mechanisms developed to survive to different stresses will contribute to clarify how these processes work. In this study, we analyzed the ecophysiological response to an increasing salinity of two cyanobacterial strains identified as belonging to the same species, Nostoc commune (16S rRNA gene similarity > 99%), but isolated from two contrasted environments: a microbial mat in a lake (Larsemann Hills, Prydz Bay, East Antarctica) and a biological soil crust from a semiarid region in Southeastern Spain (a limestone quarry (Gádor)). After culturing them in BG110 medium with different salinities (from 0 M to 2.5 M NaCl) under a light intensity of 5 µmol m-2 s-1, the pigment (chlorophyll, scytonemin and carotenoids) contents and the photosynthetic efficiency (Fv/Fm measured by Pulse-Amplified Modulation spectrophotometry) were determined after 1 hour, and 1, 7 and 43 days. Our results show that both strains were affected by salinity as the maximal chlorophyll and carotenoids yields were obtained after 43 days of cultivation without NaCl. The Antarctic strain could also grow in media with salinity concentrations up to 0.7 M NaCl, with an increase of physiological stress with salinity. In contrast, the strain from the hot desert could only grow till salinities of 0.05 M NaCl, with a similar increased stress. This work will contribute to a better understanding of the performance to salinity stress of related cyanobacterial strains growing in different extreme environments. However, further analyses relating these results to gene expression (RNASeq) would be desirable to obtain a more detailed understanding of these mechanisms.B.R.R. was supported by the University of Liège under Special Funds for Research, IPD-STEMA Programme

Different resistance to salinity and UVR of a pair of Nostoc commune strains from cold and warm deserts

Cyanobacteria can survive in extreme environments, such as drylands or polar regions, and are found in aquatic or terrestrial habitats around the world. For example, they live in associations with other organisms to form biological soil crusts. They have developed different resistance traits to cope with extreme and contrasted environments, such as the production of pigments to absorb UV radiation or a polysaccharidic matrix to withstand desiccation. However, a better understanding of their ecophysiological responses is necessary to clarify how these processes work. In the present study, we analyzed the ecophysiological performance of two Nostoc commune strains under two stress factors: salinity and UV radiation. This pair of strains has > 99% 16S rRNA gene similarity. Strain ULC002 and UAM817 were isolated from a microbial mat in an Antarctic lake (Larsemann Hills, East Antarctica) and from a biological soil crust in a semiarid region (limestone quarry in Gádor, SE Spain), respectively. To determine their capacity to withstand salinity, both strains were cultured in BG110 medium with increasing NaCl concentrations (from 0 M to 2.5 M) for 43 days. To analyze their resistance to UV radiation, both strains were incubated for 3 days under a UV-B and –A radiation of 2 W/m2 and 10 W/m2, respectively.The chlorophyll a, scytonemin and carotenoid contents were extracted from the cultures and measured with a spectrophotometer at different time points, as well as the photosynthetic efficiency (Fv/Fm, measured by Pulse-Amplified Modulation spectrophotometry). Our results show a higher resistance of the Antarctic strain to salinity, as it could grow in concentrations of up to 0.2 M NaCl, compared to the biocrust-forming strain that showed a negative growth rate at salinity concentrations above 0.05 M. Both strains showed a comparable increase in photosynthetic stress with salinity. Similarly, they also showed a decrease in Fv/Fm values under the UV radiation stressor, with only the biocrust-forming strain showing a significant decrease of chlorophyll a content after 2 and 3 days of UV radiation. In conclusion, our results show a higher resistance of the aquatic Antarctic strain of N. commune compared to the one isolated in warm dryland biocrusts. Further analyses of the gene expression by RNA-seq are desirable for a better understanding of the different mechanisms developed by these two strains.IPD-STEMA R.DIVE.0899-J-F-

Effect of water availability on induced cyanobacterial biocrust development

peer reviewedCyanobacteria inoculation has recently become an innovative biotechnological tool for restoring degraded arid soils. A major challenge for researchers, however, is the search for suitable species able to cope with water stress under field conditions. The aim of this study was to test the effect of water availability on induced biocrust growth in three different degraded soils from semiarid areas of Almeria (Spain). Three native N-fixing cyano-bacterial strains, Nostoc commune, Scytonema hyalinum and Tolypothrix distorta, were inoculated on soil samples from the study areas, individually and as a consortium. Two different irrigation treatments simulating the water availability in the selected areas, in a dry year (180 mm/year) and a wet year (380 mm/year), were applied for three months under laboratory conditions. Cyanobacteria cover, chlorophyll a spectral absorption (Chla spectral absorption), soil organic carbon (SOC) and total exopolysaccharide (EPS) gains were measured as indicators of biocrust development. Cyanobacteria crust cover, SOC and EPS gains were higher in inoculated soils than in uninoculated soils. Even though the hydration regime had a generally significant effect on cyanobacteria cover, Chla spectral absorption and EPS, similar biocrust development and improvement in edaphic conditions were observed under both hydration regimes for all treatments. Of the candidate inoculants, N. commune showed remarkably higher performance under dry conditions than the rest, providing evidence of high potential for growing under water-limited conditions and being a good candidate inoculant for restoration of arid degraded areas.RESUCI project (CGL2014-59946-R); REBIOARID project (RTI2018-101921-B-I00); Spanish National Plan for Research; RH2O-ARID (P18-RT-5130); FPU predoctoral fellowship (FPU14/05806); Special Funds for Research, IPD-STEMA Programme; HIPATIA-UAL postdoctoral fellowshi

Overcoming field barriers to restore dryland soils by cyanobacteria inoculation

Cyanobacteria inoculation to promote biocrust formation and improve soil properties has shown positive results in indoor studies, but limited success when applied under field conditions. Successful results of application of this technology in the field have been only found in desert sand dunes in China. Therefore, further development of this technique is necessary to extend its applicability to other soil types and regions. In this study, we inoculated a consortium of three N-fixing native cyanobacteria (Nostoc commune, Scytonema hyalinum and Tolypothrix dis-torta) on degraded soils from three semiarid study sites in south-eastern Spain with differing soil properties and soil development. After two years, chlorophyll a spectral absorption and albedo in inoculated and control plots were similar. Consequently, a second experiment was conducted to test the effectiveness of progressive cyano-bacterial preacclimation before inoculation as well as the use of habitat amelioration techniques, consisting of covering the inoculated soils with a vegetal fiber mesh or a plastic grid, to improve cyanobacteria performance. Our results showed that: 1) hardening these cyanobacterial strains did not enhance their colonization capability, and 2) covering inoculated soils with a vegetal mesh did promote soil colonization by cyanobacteria, as shown by higher chlorophyll a soil content and Chla spectral absorption and lower albedo than in the uncovered plots. Moreover, it promoted the presence of more condensed, tightly-bound exopolysaccharides (EPS) and higher molecular weight molecules in the more soluble loosely-bound EPS fraction, both of which could be involved in the improvement of soil aggregation. Finally, higher abundance of xylose and galactose was also found in this treatment, likely indicating a greater development of the induced biocrusts. The results of this study show that direct soil inoculation with cyanobacteria, whether subjected to desiccation hardening or not, did not promote the artificial formation of biocrusts in the field. However, more positive results were found when the inoculated soils were covered with a vegetal mesh to help cyanobacteria cope with abiotic stress and soil erosion. Therefore, developing diversified efficient habitat amelioration strategies might be key in the successful application of this restoration technique in the field

Ecophysiological and genomic approaches to cyanobacterial hardening for restoration.

peer reviewedCyanobacteria inhabit extreme environments, including drylands, providing multiple benefits to the ecosystem. Soil degradation in warm drylands is increasing due to land use intensification. Restoration methods adapted to the extreme stress in drylands are being developed, such as cyanobacteria inoculation to recover biocrusts. For this type of restoration method to be a success, it is crucial to optimize the survival of inoculated cyanobacteria in the field. One strategy is to harden them to be acclimated to stressful conditions after laboratory culturing. Here, we analyzed the genome and ecophysiological response to osmotic desiccation and UVR stresses of an Antarctic cyanobacterium, Stenomitos frigidus ULC029, which is closely related to other cyanobacteria from warm and cold dryland soils. Chlorophyll a concentrations showed that preculturing ULC029 under moderate osmotic stress improved its survival during an assay of desiccation plus rehydration under UVR. Additionally, its sequential exposure to these stress factors increased the production of exopolysaccharides, carotenoids, and scytonemin. Desiccation, but not osmotic stress, increased the concentrations of the osmoprotectants trehalose and sucrose. However, osmotic stress might induce the production of other osmoprotectants, for which the complete pathways were observed in the ULC029 genome. In total, 140 genes known to be involved in stress resistance were annotated. Here, we confirm that the sequential application of moderate osmotic stress and dehydration could improve cyanobacterial hardening for soil restoration by inducing several resistance mechanisms. We provide a high-quality genome of ULC029 and a description of the main resistance mechanisms (i.e., production of exopolysaccharides, osmoprotectants, chlorophyll, and carotenoids; DNA repair; and oxidative stress protection).Resistance traits developed by cyanobacteria colonizing two extreme environments: polar regions and drylands15. Life on lan

The influence of land degradation on cyanobacterial communities in semiarid biocrusts

Dryland soils are characterized by a patchy plant coverage coupled with biocrusts in the interplant spaces. Biocrusts are communities composed by cyanobacteria, mosses, lichens, bacteria, microalgae, fungi and other organisms, in a close relation with soil particles. They play key roles in ecosystem functioning by, for example, increasing soil fertility and stability. Among all the biocrust-forming organisms, cyanobacteria are of special interest because they are the first colonizers of soils and facilitate succession. Many studies have analysed how the cyanobacterial community composition is affected by climate or altitude. However, the influence of the ecosystem degradation has not yet been assessed, even if it has been reported that global change affects biocrusts, decreasing its worldwide coverage. Therefore, in this study we analysed the cyanobacterial community composition by extracting the DNA and sequencing the 16S rRNA gene from biocrusts at different developmental stages and collected from ecosystems within a degradation gradient. We selected three ecosystems in SE Spain with a decreasing land-condition level: a well-preserved area in Balsa Blanca (Cabo de Gata NP), a badland area under intense water erosive processes at El Cautivo (Tabernas desert), and a limestone quarry at Gádor. Our results show a decrease in richness (up to 28 OTUs) as degradation increases. The abundance’s decrease or increase of all the identified cyanobacteria, except for the unicellular ones, was significantly related with the site degradation. Among the species which abundance significantly increased with degradation, the most abundant in incipient biocrusts were Leptolyngbyafrigida and Trichocoleusdesertorum, while those with a higher relative abundance in more developed biocrusts were the heterocystousNostoc commune, Tolypothrixdistorta and Scytonemasp. Among the cyanobacteria which abundance significantly decreased with degradation, we found Microcoleusspp., and the heterocystousScytonemahyalinumand Macrochaetelichenoides. These results suggest an alternative developmental sequence for more degraded dryland ecosystems, where incipient biocrusts would be dominated by L. frigidaor T. desertoruminstead of traditional pioneer genera such asMicrocoleus. This information will help designing more efficient biocrust restoration approaches based on inoculating cyanobacteria on degraded soils by choosing those species that might be more resistant to the harsh conditions of degraded ecosystems