الفهرس 
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Abstract
Increasing microalgae carbohydrate content through nutrient limitation has been regarded as a potential strategy to optimize microalgae biogas production since the anaerobic digestion of protein-rich substrate, like microalgae, might mediate anaerobic microorganisms’ inhibition by high ammonium concentration levels. This study assessed three different media in which nitrogen, phosphorus and sulfur concentration were decreased by 10-fold with regard to the control media. The performance of two robust microalgae, namely Chlorella vulgaris and Scenedesmus sp., were evaluated in terms of growth and biochemical composition. At the stationary growth phase, sulfur and phosphorus limited media supported the highest carbohydrate accumulation on Chlorella vulgaris and Scenedesmus sp., respectively (approximately 4-fold). Under these scenarios, Scenedesmussp. biomass did not exhibit arrested growth, while Chlorellavulgarisdecreased its concentration by 10%. This reduction was attributed to the decreased photosynthetic performance that concomitantly affected nutrients uptake. After 15 days of cultivation, the biochemical analysis revealed that out of the total carbohydrates obtained under those limited media (30% DW for Chlorella vulgaris and 22% DW for Scenedesmus sp.), only 5% was starch and therefore the main fraction of carbohydrates was contained in the cell wall. Likewise, the carbohydrate profile presented fast changes in short periods of time and therefore the importance of an appropriate harvesting schedule to obtain carbohydrate rich biomass was highlighted.On the other hand, the impact of iron oxide nanoparticle (Fe3O4 NPs) on Chlorella vulgaris biomass solubilization and biogas production was also investigated. The maximum hydrolysis efficiency (30%) was achieved in the presence of 10 mg/L of Fe3O4NPs, however, this degree decreased to 23.4 and 22% at 15 and 20 mg/L, respectively. Concurrently, the highest biogas yield (595 mL/g VSin) was attained in reactors fed with pretreated biomass at 10 mg/L. A key threshold of 40 minutes for Fe3O4NPs pretreatment was established to increase organic materials solubilization in reactors. The effort to expand biomass loads over 16 g/L resulted in lower hydrolysis efficiency and a simultaneous DROP in the augmentation of biogas production. Therefore, Fe3O4NPs applicationprior to microalgae anaerobic digestion might be a potential strategy to reduce energy input needed for cell wall disruption with the added benefit of biogas production enhancement.