Energy is a crucial entity for the development and it has various alternative forms of energy sources. Recently, the synthesis of nanoparticles using benign biocatalyst has attracted increased attention. In this study, silver nanoparticles were synthesized and characterized using Azadirachta indica plant-derived phytochemical as the reducing agent. Biomass of the microalga Chlorella sp. cultivated in BG11 medium increased after exposure to low concentrations of up to 0.48 mg L -1 AgNPs. In addition, algal cells treated with 0.24 mg L -1 AgNPs and cultivated in BG11 0 medium which contained no nitrogen source showed the highest hydrogen yield of 10.8 mmol L -1 , whereas the untreated cells under the same conditions showed very low hydrogen yield of 0.003 mmol L -1 . The enhanced hydrogen production observed in the treated cells was consistent with an increase in hydrogenase activity. Treatment of BG11 0 grown cells with low concentration of green synthesized AgNPs at 0.24 mg L -1 enhanced hydrogenase activity with a 5-fold increase of enzyme activity compared to untreated BG11 0 grown cells. In addition, to improve photolytic water splitting efficiency for hydrogen production, cells treated with AgNPs at 0.24 mg L -1 showed highest oxygen evolution signifying improvement in photosynthesis. The silver nanoparticles synthesized using phytochemicals derived from plant enhanced both microalgal biomass and hydrogen production with an added advantage of CO 2 reduction which could be achieved due to an increase in biomass. Hence, treating microalgae with nanoparticles provided a promising strategy to reduce the atmospheric carbon dioxide as well as increasing production of hydrogen as clean energy., Competing Interests: Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper., (Copyright © 2024 Elsevier Ltd. All rights reserved.)