## 4 Discussion
Thorough examinations of the functioning of the assayed BAP have shown that this product did not stimulate the production of hydrogen sulfide because of the absence of thiosulfate reductase activity, but instead promoted methane production [11; 41]. These assumptions on the role of the different microbial guilds would still require further demonstration supported by additional sequencing studies. Even so, our experiments may shed some light on this subject. The separate addition of molybdate and BES was done to differentially suppress the activity of sulfate-reducing bacteria [18] and methanogens [18; 24], respectively. Both inhibitors significantly affected the performance of the BAP, at least regarding the net removal of organic matter, thereby demonstrating that both microbial guilds were involved in the process triggered by the BAP. It is worth mentioning again the likely co-existence rather than competition of both acetoclastic groups [17; 18], which is possible even in sulfate-rich environments when shared substrates are abundant [42]. The regulation of methanogenesis might also originate from other causes. A poor performance of methane production has been commonly observed in anaerobic digesters with elevated ammonium concentrations [43; 44]. This agrees with the results found in our study, since the higher increases of ammonium were found in those treatments being less effective in reducing OM. Electron acceptors as nitrate, and its denitrification products such as NO\({}_{2}\), can also have adverse effects on methanogenesis [45]. This is because nitrogen oxides increase the redox potential above that suitable for methane production, which otherwise could be additionally related with a harmful effect of these compounds [46]. This is consistent with the results of our experiments, as the experimental additions of nitrate and, in some cases, the increases of nitrite, occurred in those treatments that less efficiently reduced the OM. Hence, treatments supplemented with either nitrate or iron to be used as electron acceptors were conducted, aiming to facilitate anaerobic metabolic pathways other than sulfate-reduction and methanogenesis. This was part of the prospective study and intended to explore the possibility of taking advantage of the biostimulation of native denitriying or iron-reducing bacteria, respectively. However, our results were not promising in that sense, as no significant reductions of organic matter were achieved in any of these cases. Only a partial increase in the mineralization of nutrients was deduced in view of the increasing concentrations observed for some of them. Similarly, the additions, in other treatments, of nitrogen, phosphorus, or oligoelements, did not improve perceptibly the reduction of OM, which indicates that neither the action of the BAP nor that of the native microbial guilds dominating the studied sediments were strongly limited by these nutrients. Only the slight improvement of the BOD\({}_{5}\) relative to the COD observed in these treatments, which occurred in other treatments as well, as commented above, led to consider that these amendments could be covering some nutritional requirements for the aerobic heterotrophic microorganisms. On the other hand, variations of inorganic N in the experiments were strongly related to the pH of sediments, which is also an important factor in the degradation of OM [37]. The pH correlated positively to ammonium and negatively with nitrite concentrations, respectively, as displayed by the correspondent vectors in the PCO (Figure 5). This is because an alkalization of the environment firstly occurs by the release of ammonium via the microbially catalyzed urea hydrolysis [47]. However, pH decreases again as ammonium is later removed during nitrification. Moreover, anaerobic ammonium oxidations can also occur in anoxic environments [48].