Community homeostasis of coastal microbial mats from the Camargue during winter (cold) and summer (hot) seasons

Microbial mats are complex layered biofilms under the effect of continuous daily and seasonal fluctuations in abiotic factors such as nutrients, pH, light, temperature, and salinity. In this study, community variability of microbial mats from the Camargue wetlands (Rhone Delta, southern France) was determined in winter and summer over 2 years, analyzing thin vertical layers at three different depths (0-2, 2-4, and 4-6 mm). Amplicons of the V1-V2 region of the 16S rRNA gene were sequenced to assess changes in the bacterial diversity of the mats. Functionality was inferred according to PICRUSt bioinformatics analysis. The microbial composition of the three layers included more than 30 phyla of bacteria, the major phyla being Proteobacteria, Bacteroidetes, Cyanobacteria, and Spirochaetes. The microbial composition of the layers differed significantly, especially between layer 1 (0-2 mm) and the other two, regardless of the season. Cyanobacteria and Rhodobacteraceae (Alphaproteobacteria) were more abundant in layer 1. Chromatiaceae (Gammaproteobacteria) and Deltaproteobacteria were detected in layers 2 (2-4 mm) and 3 (4-6 mm). Flavobacteriaceae was the predominant family of the phylum Bacteroidetes, being especially abundant in layer 3. Spirochaetes were distributed in all three layers, but above all in layer 2. Pathways related to energy and carbohydrate metabolism indicated that the community was more active in summer than in winter. Carbon fixation in the mats was predominantly through the Wood-Ljungdahl pathway. Heterotrophic diazotrophs appeared to be responsible for a substantial part of nitrogen input, even more than phototrophic microorganisms. Layers 1 and 2 represented a stressed environment, measured by relative abundances of KEGG Orthologs associated with oxidation, starvation, and osmotic and xenobiotic stress, and suffered unbalanced growth in which there is no increase in cell numbers, but it allows the accumulation of polymers, such as polyhydroxybutyrate. The Camargue mats showed a community homeostasis, its populations adapting to fluctuating conditions while maintaining the characteristic structure of vertically stratified functional groups of microorganisms.

Automated summary

In this study, the microbial mats from the Camargue wetlands in southern France were analyzed to determine their community variability and functionality. The microbial composition of the mats varied significantly between different layers and seasons, with certain phyla of bacteria being more abundant in specific layers. The bioinformatics analysis revealed that the mats were more active in summer and had a predominant carbon fixation pathway. The microbial mats showed a community homeostasis, adapting to fluctuating conditions while maintaining their characteristic structure.