Unbalanced predatory communities and a lack of microbial degraders characterize the microbiota of a highly sewage-polluted Eastern-Mediterranean stream.

Wastewater pollution of water resources takes a heavy toll on humans and on the environment. In highly polluted water bodies, self-purification is impaired, as the capacity of the riverine microbes to regenerate the ecosystem is overwhelmed. To date, information on the composition, dynamics, and functions of the microbial communities in highly sewage-impacted rivers is limited in particular in arid and semi-arid environments. In this year-long study of the highly sewage-impacted Al-Nar/Kidron stream in the Barr al-Khalil/Judean Desert east of Jerusalem we show, using 16S and 18S rRNA gene-based community analysis and targeted QPCR, that both the bacterial and micro-eukaryotic communities, while abundant, exhibited low stability and diversity. Organic compounds hydrolyzers, and nitrogen and phosphorus recyclers were lacking, pointing at a reduced potential for regeneration. Furthermore, facultative bacterial predators were almost absent, and the obligate predators Bdellovibrio-and-like-organisms were found at very low abundance. Finally, the micro-eukaryotic predatory community differed from those of other freshwater environments. The lack of essential biochemical functions may explain the stream's inability to self-purify while the very low levels of bacterial predators and the disturbed assemblages of micro-eukaryote predators present in Al-Nar/Kidron may contribute to community instability and disfunction.