Contents lists available at ScienceDirect Ecotoxicology and Environmental Safety journal homepage: www.elsevier.com/locate/ecoenv Impact of heavy metal on activity of some microbial enzymes in the riverbed sediments: Ecotoxicological implications in the Ganga River (India) Deepa Jaiswal, Jitendra Pandey ⁎ Ganga River Ecology Research Laboratory, Environmental Science Division, Centre of Advanced Study in Botany, Institute of Science, Banaras Hindu University, Varanasi 221005, India ARTICLE INFO Keywords: Bioavailability Ecological risk index Extracellular enzyme activity Ganga River Heavy metal Microbial metabolic quotient ABSTRACT We studied the extracellular enzyme activity (EEA) in the riverbed sediment along a 518 km gradient of the Ganga River receiving carbon and nutrient load from varied human sources. Also, we tested, together with substrate-driven stimulation, if the heavy metal accumulated in the sediment inhibits enzyme activities. Because pristine values are not available, we considered Dev Prayag, a least polluted site located 624 km upstream to main study stretch, as a reference site. There were distinct increases in enzyme activities in the sediment along the study gradient from Dev Prayag, however, between-site differences were in concordance with sediment carbon(C), nitrogen (N) and phosphorus (P). Fluorescein diacetate hydrolysis (FDAase), β-glucosidase (Glu) and protease activities showed positive correlation with C, N and P while alkaline phosphatase was found negatively correlated with P. Enzyme activities were found negatively correlated with heavy metal, although ecological risk index (E R i ) varied with site and metal species. Dynamic fit curves showed significant positive correlation between heavy metal and microbial metabolic quotient (qCO 2 ) indicating a decrease in microbial activity in response to increasing heavy metal concentrations. This study forms the first report linking microbial enzyme activities to regional scale sediment heavy metal accumulation in the Ganga River, suggests that the microbial enzyme activities in the riverbed sediment were well associated with the proportion of C, N and P and appeared to be a sensitive indicator of C, N and P accumulation in the river. Heavy metal accumulated in the sediment inhibits enzyme activities, although C rich sediment showed relatively low toxicity due probably to reduced bioavail- ability of the metal. The study has relevance from ecotoxicological as well as from biomonitoring perspectives. 1. Introduction Knowledge of ecosystem status is central in designing conservation planning and approaches to surface water management. In the past decades, a number of studies contributed to assessment of the fate of carbon and nutrients in river and streams. These studies were seen critical to monitor the effect of fluxes of carbon and nutrients, linking eutrophication as one of the key factors of river status. More recent studies establish sediment microbial enzyme activities as an indicator of carbon and nutrient limitation, which can be used to uncover the in- fluence of regional scale anthropogenic stressors (Sinsabaugh et al., 2009; Hill et al., 2010; Gibbons et al., 2014; Yadav and Pandey, 2017; Pandey and Yadav, 2017). The substrate specific nature of the extra- cellular enzymes make them an important tool for investigations on functional profile of microbial communities as influenced by anthro- pogenic activities (Hill et al., 2010). Rivers receive, in addition to other inputs, a number of heavy metals of natural and anthropogenic origin, and riverbed sediments are important depositories of these metals. The wide distribution and persistent nature of these metals make them potentially toxic to aquatic biota influencing the ecological functioning. Factors such as the magnitude of external loading, source partitioning, mineralogical composition, adsorption, river flow, sediment delivery and urban-industrial discharge influence metal distribution in riverbed sediment. Depending upon their concentration and bioavailability, these may lead toxic effect on river biota including microbial activity in the sediment (Baran and Tarnawski, 2015; Dell’Anno et al., 2003). Organic carbon in sediment forms complexes and often buffer the bioavailability of heavy metals (Griscom et al., 2000). Thus, the data on microbial enzyme activities linking bioavailable concentrations along with total content of heavy metals provide important cues on the river status and eco-toxicology that research should focus on. Understanding the biological effects of heavy metals on microbial activities in the riverbed sediment is an important scientific challenge, because the microbial community in riverbed sediment plays a key role regulating nutrient cycling and breakdown of organic matter. Detrimental effects of heavy metals on sediment microbial community might have https://doi.org/10.1016/j.ecoenv.2017.12.015 Received 15 July 2017; Received in revised form 29 November 2017; Accepted 8 December 2017 ⁎ Corresponding author. E-mail address: jiten_pandey@rediffmail.com (J. Pandey). Ecotoxicology and Environmental Safety 150 (2018) 104–115 0147-6513/ © 2017 Elsevier Inc. All rights reserved. T