Abstract
Understanding the response of bacterial communities to environmental change is extremely important in predicting the effect of biogeochemical modifications in ecosystem functioning. The Cuatro Cienegas Basin is an ancient oasis in the Mexican Chihuahuan desert that hosts a wide diversity of microbial mats and stromatolites that have survived in extremely oligotrophic pools with nearly constant conditions. However, thus far, the response of these unique microbial communities to long-term environmental disturbances remains unexplored. We therefore studied the compositional stability of these bacterial mat communities by using a replicated (3x) mesocosm experiment: a) Control; b) Fluct: fluctuating temperature; c) 40C: increase to 40 ºC; d) UVplus: artificial increase in UV radiation; and f) UVmin: UV radiation protection. In order to observe the changes in biodiversity, we obtained 16S rRNA gene clone libraries from microbial mats at the end of the experiment (eight months) and analyzed them using multivariate and phylogenetic tools. Sequences were assigned to 13 major lineages, among which Cyanobacteria (38.8%) and Alphaproteobacteria (25.5%) were the most abundant. The less extreme treatments (Control and UVmin) had a more similar composition and distribution of the phylogenetic groups with the natural pools than the most extreme treatments (Fluct, 40C, and UVplus), which showed drastic changes in the community composition and structure, indicating a different community response to each environmental disturbance. An increase in bacterial diversity was found in the UVmin treatment, suggesting that protected environments promote the establishment of complex bacterial communities, while stressful environments reduce diversity and increase the dominance of a few Cyanobacterial OTUs (mainly Leptolyngbya sp) through environmental filtering. Mesocosm experiments using complex bacterial communities, along with multivariate and phylogenetic analyses of molecular data, can assist in addressing questions about bacterial responses to long-term environmental stress.
Highlights
It is predicted that global environmental change will severely affect aquatic ecosystems due to the projected alteration of UV irradiance and temperature regimes [1,2,3]
At the end of this experiment, we recovered a total of 250 operational taxonomic units (OTUs) out of 600 sequences from the “microbial mat catchers”, including both the natural pools and the mesocosm experiment (S1 Fig., S1 Table)
A large proportion of OTUs resembled organisms associated with freshwater environments (44%; Table 1 and S2 Fig.), from where a majority had a close affinity with bacterioplankton from this mesocosm experiment, especially Cyanobacteria belonging to the Pseudanabaenaceae and Noctocaceae families, as well as Alphaproteobacteria belonging to the Rhizobiales and Rhodobacterales orders [33]
Summary
It is predicted that global environmental change will severely affect aquatic ecosystems due to the projected alteration of UV irradiance and temperature regimes [1,2,3]. Studying the phylogenetic structure of communities can provide valuable insights into the evolutionary and ecological processes that drive community assembly as well as the response of communities to environmental changes [11,12]. These techniques have been further enriched by multivariate statistical analyses, which can be applied to address fundamental and complex questions in microbial ecology [13], such as whether microbial diversity responds to the same factors as macroorganism diversity [14,15]. The full potential of 16S rRNA gene analyses in combination with multivariate statistics provides the appropriate tools for describing variations in bacterial community composition while linking such variations to changes in the environment
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