Contrasting trends in distribution of four major planktonic betaproteobacterial groups along a pH gradient of epilimnion of 72 freshwater habitats

摘要

The distribution and abundance of Betaproteobacteria and three of its genera: Limnohabitans genus (R-BT065 lineage), Polynucleobacter genus (including two subclusters P. necessarius and P. acidiphobus/difficilis), and Methylophilus genus across the epilimnion of 72 limnologically diverse freshwater habitats was investigated using fluorescence in situ hybridization. Moreover, seasonal development of Betaproteobacteria subgroups along the axis of a longitudinally heterogeneous reservoir was followed. Betaproteobacteria comprised on average 29.1%, Polynucleobacter 11.6%, P. necessarius 10.1%, P. acidiphobus/difficilis 0.5%, Limnohabitans 8.9%, and Methylophilus 0.9% of total bacterioplankton cells in the investigated habitats. P. necessarius and Limnohabitans coexisted in the majority of habitats but showed contrasting abundance patterns along the pH gradient of habitats (pH 3.8–8.5). The observed distribution patterns could be explained by different preferences for substrate sources, i.e. substances of humic origin in acidic waters and algal-derived substances in alkaline waters. However, substrate utilization patterns observed in laboratory experiments indicate no coherent group-specific differences in substrate preferences. Interestingly, similar distribution patterns were revealed for Limnohabitans and P. acidiphobus/difficilis, suggesting similar ecological adaptations of these only distantly related taxa. Our findings further emphasize that at least two major taxa of freshwater Betaproteobacteria represent ecologically diversified groups. Investigations at higher phylogenetic resolution are required for obtaining further insights into the ecology of these important taxa.