The detection of methanogens by FISH analysis also showed the presence of rRNA, which is expressed in active cells. However, high rRNA levels may be maintained despite inactivity. this website In conclusion, the activity of the Methanosaeta-like organisms is an open question.

If the Methanosaeta-like species do not grow fast enough to avoid washout, their constant presence requires that they are constantly added to the sludge. Possible sources are the influent wastewater and recycled water from an anaerobic bioreactor. By FISH analysis, Archaea was confirmed to be present in high numbers in both the anaerobic bioreactor and in the reject water (Figure  9). Thus the bioreactor might seed the activated sludge with Archaea . This is supported by the fact that a majority of the detected 16S rRNA sequences cluster with sequences from anaerobic sludge (Figure  4). Furthermore, no sequences matched typical methanogens in human fecal matter, such as Methanobacter smithii and Methanosphaera stadtmanae[45], indicating that fecal matter from the influent water was not an important input to the methanogens in the activated sludge. The second largest group in the clone library was Thermoplasmatales-related sequences affiliated with Rice TH-302 cell line Cluster III (RC-III). No cultured representative of RC-III

Archaea exists, but a study of a methanogenic enrichment culture suggests that RC-III Archaea are mesophilic anaerobes growing heterotrophically on peptides with a

doubling time of approximately three days [27]. RC-III has been detected in soil [27], anaerobic bioreactors [46] and groundwater [47]. This study shows that RC-III Archaea can also be present in activated sludge. Thermoplasmatales-related sequences of Cluster B and C were also found in the clone library. There are currently no cultured representatives or proposed phenotypes for these groups. Cluster B and C sequences have been retrieved from environments with methanogenic communities and complete or partially anoxic zones, such as water [48], landfill leachates [49], sediments [50], bioreactors only [51] and the digestive tract of animals [52]. This study adds activated sludge to that list. One sequence, clone G15, belongs to a yet undescribed lineage of Archaea: ARC I[29]. The ARC I lineage is well-represented in anaerobic bioreactors and in reactors with a high abundance of ARC I, the abundance of species related to M. CB-5083 concilii is low and vice versa [53], which could indicate a competition for acetate between these two lineages. The clone library in this study followed the same pattern with low abundance of ARC I and high abundance of M. concilii. The same pattern was also seen in the TRF profiles since the only time that the TRFs corresponding to sequence G15 was observed (January 28, 2004) the relative abundance of the TRFs associated with M. concilii had decreased to around 80%.