Contrasting trends in distribution of four major planktonic betaproteobacterial groups along a p H gradient of epilimnia of 72 freshwater habitats

Abstract The distribution and abundance of B etaproteobacteria and three of its genera – L imnohabitans ( R ‐BT065 lineage), P olynucleobacter (including subclusters P olynucleobacter necessarius and P olynucleobacter acidiphobus / P olynucleobacter difficilis ), and M ethylophilus – across the epilimnia of 72 limnologically diverse freshwater habitats were investigated using fluorescence in situ hybridization. Moreover, seasonal development of B etaproteobacteria subgroups along the longitudinal axis of a reservoir was followed. B etaproteobacteria comprised on average 29.1%, P olynucleobacter 11.6%, P . necessarius 10.1%, P .  acidiphobus / difficilis 0.5%, L imnohabitans 8.9%, and M ethylophilus 0.9% of total bacterioplankton cells in the investigated habitats. P olynucleobacter necessarius and L imnohabitans coexisted in the majority of habitats but showed contrasting abundance patterns along the p H gradient of habitats (p H , 3.8–8.5). The observed distribution patterns could theoretically be explained by different preferences for substrate sources, that is, 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 L imnohabitans and P .  acidiphobus / difficilis , suggesting similar ecological adaptations of these distantly related taxa. Our findings further emphasize that at least two taxa of freshwater B etaproteobacteria represent ecologically diversified groups. Investigations at higher phylogenetic resolution are required for obtaining further insights into their ecology.