Thermophilic bacteria dwell in phyllosphere
Résumé
The thermophilic microbe biotopes are rare and far-away environments as deep sediments or hot springs but at the same time thermophilic microbes seem present in abundance in our daily environments. Indeed, engineered processes such as thermophilic anaerobic digestion, composting or some fermentation (e.g. cheese) rely on thermophiles and are easily inoculated from common environments. So the question is: where is the natural habitat of these thermophilic microbes? We hypothesize that the phyllosphere (surface of leaves) exposed to the sun is this natural biotope. To confirm the hypothesis, we question the presence and potential activity of thermophile microbes in the phyllosphere. Leaves were collected from different tree species from various biomes (rainforest, chaparral, deciduous forest) and various locations (France, Réunion Island, Madeira Island, Thailand, Korea) and incubated at 55°C under aerobic and anaerobic conditions. At the end of the incubations, we measured the change in bacterial communities. In all experiments, comparison between fresh and fermented leaves based on 16S rDNA gene fingerprint show a complete switch of the microbial community structures. 16S rDNA sequencing reveals the presence of 1 to 5% of thermophilic bacteria on fresh leaves and 5 to 95% after aerobic or anaerobic. The increase of the abundance of thermophilic bacteria was confirmed by the activity under anaerobic conditions (production of hydrogen, volatile fatty acids and carbon dioxide). This study reveals: (i) the presence of aerobic and anaerobic thermophilic microbes on leaf surfaces; (ii) surface of leaves are potentially the biotope for many thermophilic microbes; (iii) microbes used in engineered processes such as anaerobic digestion, composting or food fermentation were probably originated from leaf biotope. But this study reveals also the great diversity of thermophilic microbes which are currently no fully identified as thermophilic by the 16S rDNA approach. Despite their important role in many processes the ecology of these thermophilic organisms is not really understood compared to hyperthermophilic ones. Taking into account the phyllosphere and the associated parameters can allow a better understanding of the physiology and ecology of these thermophiles and can provide guidelines to improve thermophilic processes.