In ARGs.

In  the opposite, human gut, ocean and cow gut metagenomes, exhibit the lowest diversity values (19, 17 and 12 different type of b-lactamase genes respectively). Richness was other important diversity index analyzed, and our results indicate that not always a high level of diversity correlated with richness. Thus, soil metagenomes, glaciers and wastewater metagenomes were the most b-lactamase diverse environments; however the most rich b-lactamase environments were agricultural soils, human gut, non-agricultural soils and wasterwater (in average 0.0106, 0.0105, 0.0093 and 0.0092% of reads identified like b-lactamases from the total number of read per environment). In contrast, glacier metagenomes that exhibited a high b-lactamase diversity, show in average an intermediate b-lactamase richness (0.0068% of total number of reads).

Differences in b-lactamase content of agricultural and non-agricultural soil metagenomes could be attributed to agricultural practices such as manure amendment, fertilization, irrigation and more. Thus,  ...
author = {Nikolina Udikovic-Kolic and Fabienne Wichmann and Nichole A. Broderick and Jo Handelsman},  title = {Bloom of resident antibiotic-resistant bacteria in soil following manure fertilization},  journal = {Proceedings of the National Academy of Sciences}  }" data-bib-key="Udikovic_Kolic_2014" contenteditable="false">Udikovic-Kolic 2014 et al. (format) recently demonstrate an increase in ARG (specially in b-lactamase content) in soils amendment with manure compared to the ARG content in soil fertilized with NPK. Interestingly, the cattle that produced the manure had not been treated with antibiotics. The authors concluded that manure application produced a bloom of certain bacterial resistant populations without connection to antibiotic exposure. 

Thepresence of b-lactamase genes in low anthropogenic impacted environment such glaciers, have been reported previously. 

The  analysis of the supporting information accompanying the metagenome sequences, clearly indicate that different factor affect the b-lactamase diversity values here obtained. In this context is important indicate that despite that all the metagenomes share the same type of technology used in the sequencing process, differences in methodology can be expected. In the same context, despite that metagenomes can be grouped according different environments, some differences can affect the sequencing results; thus, in the ocean environment, metagenomes related to the Sydney harbour metagenome project (see Table S1) were obtained at 0,3 m in depth, in the estuary of Sydney, and metagenomes related to Amazon continuum metagenomes project were obtained at depths ranged from 3.63 to 4.47 meters, and the samples were obtained at different points, from seawater close to the cost line until seawater far from the cost line (open ocean). For instance, some studies have been show the differences in the AR profile across different water layers in different lake samples ( ...
author = {Andrea Di Cesare and Ester M. Eckert and Alessia Teruggi and Diego Fontaneto and Roberto Bertoni and Cristiana Callieri and Gianluca Corno},  title = {Constitutive presence of antibiotic resistance genes within the bacterial community of a large subalpine lake},  journal = {Molecular Ecology}  }" data-bib-key="Di_Cesare_2015" contenteditable="false">Cesare 2015). In relation to soil metagenomes, agricultural metagenomes were exposed to different agricultural practices that can affect the microbiological content on soil and therefor, affect the genomic content on soil. Ilustrating Illustrating  the last point,   point,    class="ltx_cite" data-bib-text="@article{Agga_2015, doi = {10.1371/journal.pone.0132586},  url = {http://dx.doi.org/10.1371/journal.pone.0132586},  year = 2015, 
...
author = {Jos{\'{e}} L. Mart{\'{\i}}nez},  title = {Bottlenecks in the Transferability of Antibiotic Resistance from Natural Ecosystems to Human Bacterial Pathogens},  journal = {Front. Microbio.}  }" data-bib-key="Mart_nez_2012" contenteditable="false">Mart' 2012, Forsberg 2014, Udikovic-Kolic 2014).

In the same way that in our study, Fondi et al (2016) found that geography does not influence the microbial gene pool distributions, indicating that the dispersal potential of microorganisms is affected by environmental factors more than by geographical distances.