## 3 Results
### Prokaryotic Community Composition of the Studied Fluvial Sediment  
The NG-TAS analysis provided a complete and accurate picture of the prokaryotic community structure (Bacteria and Archaea) of the studied sediments. An interactive visualization is provided as a Krona html chart in the supplementary material (Figure S2). This chart has embedded the taxonomic hierarchy of the sample, in such way that taxonomic categories are subdivided into sectors of a pie-type diagram. Accordingly, taxonomy nodes are arranged from the top level of the hierarchy at the center of the pie and progressing outwards. The analysis showed a bacterial assemblage dominated by obligate or facultative anaerbes, most of them belonging to microbial guilds related to the anaerobic degradation of organic matter.  
A total of 16,437 sequence reads were recorded, where those assigned to Bacteria represented nearly 99% of the mapped sequences. Among them, Proteobacteria was the dominant phylum, accounting for c.a. 58% of the reads. It was by far followed by other phyla such as Epsilonbacteraeota (10%) and Bacteroidetes (6%). The Gammaproteabacteria class dominated among Proteobacteria and mainly consisted on unclassified members of families Steroidobacteraceae, Burkholderiaceae, and Rhodanobacteraceae, whereas sulfur oxidizers (_Thiobacillus_), sulfate reducers (_Desulforhabtus_), and denitrifiers (_Dechloromonas_) were also relatively abundant. The Alphaproteabacteria, the other  
Figure 2: Analyses of the settling characteristics of the suspended solids in the water of the studied site: (**a**) sequence of material settling in the sediment traps installed inside the control mesocosms, (**b**) aspect of the particles retained in the AH-934 filter, and (**c**) settled material in the Imhoff cone.  
significant though less abundant class of Proteobacteria, also displayed the occurrence of denitrifiers such as _Rubrimonas_.  
Among the Epsilonbacteraeota, abundant groups related to the sulfur cycle also occurred, as the Campylobacteraless genera _Sulfuroum_ and _Sulfuricurum_, and to a lesser extent, also _Arobacter_. The Bacteroidetes cluster was composed by diverse groups, dominated by _Asinibacterium_ and unclassified members of the family Flavobacteriaceae. The former belongs to the facultatively anaerobic members of the family Chitinophagaceae. Other members of this family were also found, though less abundant, such as _Sediminibacterium_, _Dinghuibacter_, and _Flavihunibacter_. Another moderately abundant phylum, Firmicutes, was predominantly represented by strict anaerobes as well, where diverse families of the order Clostridales were clearly dominant. Among them, fermentative and proteolytic bacteria such as _Youngiibacter_ and _Proteiniclasticum_ composed mainly the contribution of this cluster.  
The archaeal assemblage represented nearly 1% of the prokaryotic community in the sediments and was dominated by methanogens of phylum Euryarchaeota. All metabolic pathways for methane production were potentially present, which means that either acetate, formate, or H\({}_{2}\) could be used as a methanogenic substrate. The genus _Methanobacterium_ and several members of the family Methanoregulaceae were the dominant members of the archaeal community. The later, that particularly uses H\({}_{2}\) and formate as substrates to produce methane but requires acetate for growth, included the genera _Methanoregula_, _Methanolinea_, and _Methanosaeta_. Other Archaea present in the sample that are able to use acetate are those of the family Methanofastidiosaceae. On the other hand, the less abundant group of methanogens recorded in the sample, Methanomassiliicoccales, was phylogenetically distant from all other orders of methanogens.