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Universität Wien (2016)

Diazotrophic activity and community composition in regionally different biological soil crusts from the Negev Desert, Israel

Schmidt, Stefan

Titre : Diazotrophic activity and community composition in regionally different biological soil crusts from the Negev Desert, Israel

Auteur : Schmidt, Stefan

Université de soutenance : Universität Wien

Grade : Masterarbeit, 2016

Résumé
Biological soil crusts (BSCs) cover vast areas in the Negev desert in Israel. The- se crusts not only support soil aggregation and stabilization but also harbour hetero- cystous cyanobacteria and other diazotrophic bacteria that are able to fix atmospheric nitrogen (N2) and therefore contribute significantly to the nitrogen (N) budget in this arid ecosystem. We have compared two methods to detect N2-fixation activity in Neg- ev desert crusts by using samples from an arid site of two different seasons and also evaluated the impact of carbon (C) addition on N2-fixation activity. We only observed N2-fixation activity with 15N2 tracer method and showed that spring samples had high- er activity than fall samples. The addition of a C source caused significantly higher N2-fixation activity rates compared to non-carbon treated samples but did not lead to similar activities of crusts from both seasons. Based on this information we performed a 15N2 tracer assay and phylogenetic analysis of the general microbial community (by sequencing the 16S rRNA subunit from DNA and RNA) and the diazotrophic community (by sequencing the marker gene for N2-fixation – dinitrogenase reductase, nifH) of crusts along a transect in the Negev desert. We compared the diazotrophic activity and microbial community com- position of three crusts, which vary in precipitation (20 mm, 100 mm or 250 mm an- nual rainfall), and observed the highest N2-fixation activity (maximum δ15N of 14,300‰) in the hyper-arid crusts amended with C, followed by the arid (δ15N= 1,300‰) and semi-arid crusts (δ15N 380‰). However, the crusts without C addition showed the opposite picture with highest activity in semi-arid crusts (δ15N= 200‰) and lowest activity in hyper-arid crusts (δ15N= 40‰). The statistical analysis of se- quence data showed strong correlation of the general microbial composition with lo- cation and the results demonstrate that the semi-arid and arid BSCs are commonly dominated by Cyanobacteria, whereas the hyper-arid BSCs are dominated by hetero- trophic bacteria, in particular by Firmicutes. The same was observed for the diazo- trophic community composition, where these two groups are the dominant diazo- trophs with the capability to fix N2. This study gives insight how the microbial community of BSCs is influenced by climate, highlights the role of heterotrophic bacteria in N2-fixation in hyper-arid regions and the positive influence/dependance of readily available C on N2-fixation activity.

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Page publiée le 1er octobre 2017