Regional and global elevational patterns in two-facets of aquatic microbial diversity
Elevational gradients in biodiversity are highly attractive concerns in ecology both for macro- and microorganisms, especially the latter in recent years. However, the aquatic environments are still understudied. Here, we present bacteria and diatom biodiversity from stream biofilm on 6 mountains in China, Spain and Norway. We used wet combustion with hydrogen peroxide to clean diatom frustules of organic material, and diatoms were identified to species level as possible using phase-contrast light microscopy. Bacteria were examined with high-throughput sequencing of 16S rRNA gene amplicons (pyrosequencing and Illumina Miseq). Obtained sequences were analyzed with software packages, such as Qiime and Mothur, and bacterial species were defined at the sequence similarity of 0.97. Correlation analyses and mix model were applied to explore the relationships between biodiversity and elevations or environmental variables.
Our results revealed that even within the same ecosystem type (e.g. stream) or same region (e.g. Yunnan, China), both organisms showed contrasting patterns in species richness (monotonically increasing and decreasing, for instance). The richness elevational patterns for bacteria varied among monotonically increasing (2 mountains), decreasing (2) and U-shaped (2). For the bacterial evenness, however, we observed significant elevational trends in increasing (2) and decreasing (1) on three mountains, while no significant patterns for the other three cases. For diatoms, the elevational patterns in richness were significant for only three mountains. The elevational patterns in evenness, however, were significantly increasing (2), decreasing (3) or hump-shaped (1). These results showed that the aquatic microbes tend to show significant elevational patterns in richness, especially for bacteria, which is different from soil microbes for the reported elevational gradients, showing 38% with no diversity patterns, and 33% with decreasing patterns. For Chinese streams, the species richness was lowest and nutrient concentrations (total phosphorus and nitrogen) were highest among the three regions. Detailed inspection with mixed models showed that the human effects, such as nutrient inputs and dams, are potentially important in explaining the observed patterns within the regional and global scales. More details are discussed.