This encouraged the river basin authority to explore the use of bioremediation techniques to decrease the undesirable organic loads in the riverine sediments, provided that they should be adapted to the specific characteristics of the polluted site [7]. Minimally invasive in situ solutions for bioremediation of polluted riverine sediments may include bioaugmentation and biostimulation techniques [8]. The former implies the addition of specialized degradative microbes, whereas the latter is intended to enhance the pollutant degradative capacity of the native microbial populations by adjusting the environmental conditions. Both are comparatively low-cost technologies that can be assayed even in sites with reduced accessibility, such as the upper-middle course of many rivers. A widely used commercial product for reducing pollution is the bacterial-enzymatic lyophile complex commercially known as Acti-zymeTM (also licensed as HycuraTM). This biologically active product (BAP) is manufactured by Acti-Zyme Products, Ltd. This BAP has been commonly used to improve the functioning of wastewater treatment plants for the removal of chemical pollutants and pathogens [9; 10], as well as to promote the generation of biogas [11; 12]. However, utilization of this BAP for the remediation of polluted natural riverine ecosystems has been rare so far, despite some promising results having been reported [5]. The anaerobic degradation of the OM via methanogenesis has been advised as practical for bioremediation purposes in highly reduced aquatic sediments with high OM content [13]. In these environments, CO2 is almost the only inorganic electron acceptor available for energy conservation. This restricts the options to methanogenesis and fermentation, the former completing the final step in the decay of OM. However, there are several environmental constraints that may prevent the growth of methanogens and other pollution-degrading anaerobic microorganisms. One of them is the availability of their metabolic precursors [14]. Among these, acetate (CH3 COO-) is a key intermediate compound for the degradation of OM that drives a predominant pathway of methane production in freshwater ecosystems [14; 15]. Sulfate-reducing bacteria, some of which can use acetate, outcompete methanogens when sulfate is available, these interactions being a major concern for microbial environmental engineering [16]. Nevertheless, both microbial guilds can co-exist if concentrations of shared substrates are high enough to avoid a competitive exclusion [17; 18], which appears to be feasible if there is a high content of OM and enough acetate availability [19]. An analogous competitive scenario might occur between denitriying bacteria and methanogens [20], though this would depend on the availability of nitrate, which, however, can be limited under the usual low-redox conditions of OM-rich sediments. Experimental additions of acetate have been occasionally carried out to address these concerns and enhance the OM degradative activity [21]. All these issues deserve consideration for the design of bioremediation strategies, in order to adjust them to the conditions of the site. Accordingly, our study focused on the investigation of the performance of a BAP as a remediation solution to reduce the exceeding OM loads of heavily contaminated streambeds, also suitable for the sediments of other types of aquatic ecosystem. This was examined under different experimental conditions. A first set of treatments included a prospection for an optimized use of the BAP (i.e., amount, temporal dosage, spatial dosage). On the other hand, other treatments consisted in combination of the BAP with other compounds that can act as nutrient amendments or as electron acceptors, or even their use without BAP. This second set of treatments aimed for the assessment of its suitability for complementing the application of the BAP under eventual natural availability limitations of nutrients and/or electron acceptors, and consequently, they were amended at a non-limiting concentration.