Abstract

In drylands, soil surfaces in interplant spaces are usually covered by biocrusts, which consist of communities of heterotrophic and chemoautotrophic bacteria, cyanobacteria, mosses, lichens, microalgae, fungi and other organisms. Cyanobacteria are of special interest because of their capacity to promote biocrust succession or increase soil fertility and stability. Therefore, some studies have analyzed their communities in different ecosystems, focusing on how different factors, such as temperature or altitude, influence their composition. Nevertheless, to our knowledge, the relationship between ecosystem degradation and cyanobacterial community composition has not yet been studied in depth. This could be determinant for the successful development of tools for restoring degraded biocrusts by cyanobacterial inoculation. Thus, the objective of this study was to analyze the effect of the ecosystem degradation level on cyanobacteria composition from topsoil communities, where they are keystone pioneering organisms. To do this, we analyzed the cyanobacterial diversity by molecular sequencing (16S rRNA gene) of the DNA extracted from biocrusts at different developmental stages, which were collected from three ecosystems in southeastern Spain. The selected ecosystems represent different “land-condition” states as a result of degradation processes. In one of them soil was removed by mining (the Gádor quarry), the second is a natural badland area (El Cautivo) where water erosion is intense, and the third ecosystem is a well-preserved area (Balsa Blanca). Our findings show that cyanobacterial richness decreases (up to 28 OTUs) as degradation increases and biocrust developmental stage decreases. Also, the relative abundances of most of the species were significantly correlated with the degradation state of the sampling site, either showing a positive or negative trend. Two of the species which increased in abundance with site degradation, and were especially abundant in incipient biocrusts, were Leptolyngbya frigida and Trichocoleus desertorum, while other species, also showing an increase in abundance with degradation, but having a higher relative abundance in most developed biocrusts, were Nostoc commune, Tolypothrix distorta and Scytonema sp. The significant correlation of these species with degradation at different biocrusts developmental stages, suggests an alternative developmental sequence for drylands, at least in more degraded ecosystems. In less degraded ones, the composition of the major cyanobacterial groups followed the common pattern of bundle-forming cyanobacteria (54.7%) pertaining to the Microcoleus genus followed by other non-heterocystous filamentous (17.4%), unicellular/colonial (7.5%) and heterocystous cyanobacteria (1.1%). In comparison, the cyanobacterial groups dominating the most incipient biocrusts, colonizing the most degraded soil, were the filamentous non-heterocystous (50.7%) and the bundle-forming cyanobacteria (48.9%). Therefore, our results show that some cyanobacterial species, which do not belong to traditional pioneer genera, are frequent colonizers of degraded soils, and then, they could be potentially used for producing a more efficient inoculum for inducing biocrust formation and restoring degraded soils. Finally, it is also remarkable that L. frigida appears as dominant in some biocrusts from drylands (up to 74.9% of abundance) being therefore demonstrated its wide distribution in nonpolar biomes and its capacity to also inhabit degraded arid soils.

摘要

在干旱地区，植物间空间的土壤表面通常被生物结皮覆盖，生物结皮由异养和化学自养细菌，蓝细菌，苔藓，地衣，微藻，真菌和其他生物组成。蓝细菌由于其促进生物结皮演替或增加土壤肥力和稳定性的能力而特别受关注。因此，一些研究分析了它们在不同生态系统中的群落，重点研究了温度或海拔高度等不同因素如何影响其组成。然而，据我们所知，尚未深入研究生态系统退化与蓝细菌群落组成之间的关系。这可能是成功开发通过蓝细菌接种修复退化的生物结皮的工具的决定因素。从而，这项研究的目的是分析生态系统退化水平对表土群落中蓝细菌组成的影响，在那里它们是关键的开拓生物。为此，我们通过分子测序（16S rRNA基因）分析了在不同发育阶段从生物壳中提取的DNA的蓝细菌多样性，这些生物是从西班牙东南部的三个生态系统收集的。由于退化过程，选定的生态系统代表不同的“土地状况”状态。其中一个是通过采矿（Gádor采石场）去除了土壤，第二个是水蚀严重的自然荒地（El Cautivo），第三个生态系统是一个保存完好的区域（Balsa Blanca）。我们的研究结果表明，随着降解的增加和生物结壳发育阶段的减少，蓝细菌的丰富度降低（最多28个OTU）。而且，大多数物种的相对丰度与采样点的降解状态显着相关，呈正趋势或负趋势。随着物种退化而大量增加的物种中，尤其是在初期生物结壳中特别丰富的两个物种是扁豆斜纹夜蛾和沙门氏菌（Trichocoleus desertorum）等也显示出随着降解而增加的丰度，但在大多数发达的生物结壳中具有较高的相对丰度，分别是Nostoc公社，Tolypothrix distorta和Scytonema sp。这些物种与不同生物结壳发育阶段的退化之间的显着相关性表明，至少在退化程度更高的生态系统中，旱地的替代发展顺序。在降解程度较小的细菌中，主要的蓝细菌基团的组成遵循与成色小肠有关的成束蓝细菌的常见模式（54.7％）属，其次是其他非异型囊状丝状体（17.4％），单细胞/菌落状（7.5％）和异型囊状蓝细菌（1.1％）。相比之下，在最起步的生物结壳中占主导地位，在退化程度最大的土壤中定居的蓝细菌基团是丝状非杂囊性细菌（50.7％）和成束的蓝细菌（48.9％）。因此，我们的结果表明，不属于传统先驱属的某些蓝细菌种类是退化土壤的频繁定居者，因此，它们有可能被潜在地用于生产更有效的接种物，以诱导生物结皮的形成和恢复退化土壤。最后，同样令人印象深刻的是，弗氏乳杆菌 因此，在干旱地区的某些生物结皮中似乎占主导地位（占丰度的74.9％），因此证明了其在非极性生物群落中的分布广泛，并且还可以在退化的干旱土壤中栖息。