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.