The Amazon rainforest is the Earth’s largest reservoir of plant and animal diversity, and it has been subjected to especially high rates of land use change, primarily to cattle pasture. This conversion has had a strongly negative effect on biological diversity, reducing the number of plant and animal species and homogenizing communities. We report here that microbial biodiversity also responds strongly to conversion of the Amazon rainforest, but in a manner different from plants and animals. Local taxonomic and phylogenetic diversity of soil bacteria increases after conversion, but communities become more similar across space. This homogenization is driven by the loss of forest soil bacteria with restricted ranges (endemics) and results in a net loss of diversity. This study shows homogenization of microbial communities in response to human activities. Given that soil microbes represent the majority of biodiversity in terrestrial ecosystems and are intimately involved in ecosystem functions, we argue that microbial biodiversity loss should be taken into account when assessing the impact of land use change in tropical forests.

The Antarctic is a pristine environment that contributes to the maintenance of the global climate equilibrium. The harsh conditions of this habitat are fundamental to selecting those organisms able to survive in such an extreme habitat and able to support the relatively simple ecosystems. The DNA of the microbial community associated with the rhizospheres of Deschampsia antarctica Desv (Poaceae) and Colobanthus quitensis (Kunth) BartI (Caryophyllaceae), the only two native vascular plants that are found in Antarctic ecosystems, was evaluated using a 16S rRNA multiplex 454 pyrosequencing approach. This analysis revealed similar patterns of bacterial diversity between the two plant species from different locations, arguing against the hypothesis that there would be differences between the rhizosphere communities of different plants. Furthermore, the phylum distribution presented a peculiar pattern, with a bacterial community structure different from those reported of many other soils. Firmicutes was the most abundant phylum in almost all the analyzed samples, and there were high levels of anaerobic representatives. Also, some phyla that are dominant in most temperate and tropical soils, such as Acidobacteria, were rarely found in the analyzed samples. Analyzing all the sample libraries together, the predominant genera found were Bifidobacterium (phylum Actinobacteria), Arcobacter (phylum Proteobacteria) and Faecalibacterium (phylum Firmicutes). To the best of our knowledge, this is the first major bacterial sequencing effort of this kind of soil, and it revealed more than expected diversity within these rhizospheres of both maritime Antarctica vascular plants in Admiralty Bay, King George Island, which is part of the South Shetlands archipelago.

Land use change in the Amazon rainforest alters the taxonomic structure of soil microbial communities, but whether it alters their functional gene composition is unknown. We used the highly parallel microarray technology GeoChip 4.0, which contains 83,992 probes specific for genes linked nutrient cycling and other processes, to evaluate how the diversity, abundance and similarity of the targeted genes responded to forest-to-pasture conversion. We also evaluated whether these parameters were reestablished with secondary forest growth. A spatially nested scheme was employed to sample a primary forest, two pastures (6 and 38 years old) and a secondary forest. Both pastures had significantly lower microbial functional genes richness and diversity when compared to the primary forest. Gene composition and turnover were also significantly modified with land use change. Edaphic traits associated with soil acidity, iron availability, soil texture and organic matter concentration were correlated with these gene changes. Although primary and secondary forests showed similar functional gene richness and diversity, there were differences in gene composition and turnover, suggesting that community recovery was not complete in the secondary forest. Gene association analysis revealed that response to ecosystem conversion varied significantly across functional gene groups, with genes linked to carbon and nitrogen cycling mostly altered. This study indicates that diversity and abundance of numerous environmentally important genes respond to forest-to-pasture conversion and hence have the potential to affect the related processes at an ecosystem scale.

Bacteria and functional genes associated with biphenyl (BP) degradation in the root zone of an Austrian pine (Pinus nigra L.) growing naturally in polychlorinated-BP (PCB)-contaminated soil were identified using stable isotope probing (SIP) integrated with comprehensive functional gene analyses. SIP revealed 75 different genera that derived carbon from 13C-BP, with Pseudonocardia, Kribella, Nocardiodes and Sphingomonas predominating carbon acquisition. Rhodococcus spp. were not detected with SIP, despite being the most abundant BP utilizers isolated from agar plates. Only one organism, an Arthrobacter spp., was detected as a BP utilizer by both cultivation and SIP methods. Time-course SIP analyses indicated that secondary carbon flow from BP-utilizing bacteria into other soil organisms may have occurred largely between 4 and 14 days incubation. Functional gene contents of the BP-utilizing metagenome (13C-DNA) were explored using the GeoChip, a functional gene array containing 6465 probes targeting aromatic degradative genes. The GeoChip detected 27 genes, including several associated with catabolism of BP, benzoate and a variety of aromatic ring hydroxylating dioygenase (ARHD) subunits. Genes associated with the beta-ketoadipate pathway were also detected, suggesting a potential role for this plant aromatic catabolic pathway in PCB degradation. Further ARHD analyses using targeted polymerase chain reaction primers and sequence analyses revealed novel dioxygenase sequences in 13C-DNA, including several sequences that clustered distantly from all known ARHDs and others that resembled known Rhodococcus ARHDs. The findings improve our understanding of BP degradation and carbon flow in soil, reveal the extent of culture bias, and may benefit bioremediation research by facilitating the development of molecular tools to detect, quantify and monitor populations involved in degradative processes.

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