Estuarine sediments are important sites for denitrification, which is microbially mediated reduction of nitrate to dinitrogen that also influences global climate change by co-production of nitrous oxide, a potent greenhouse gas. Physicochemical properties and nutrients of sediment samples that influence denitrification rate were studied in Ashtamudi estuarine sediments. They were pH, electrical conductivity (EC), salinity, nitrate-nitrogen (NO-N), exchangeable ammonia (NH-N), total kjeldahl nitrogen (TKN) and organic carbon (Corg). Sediment samples were collected from six stations during summer, monsoon of 2013 and 13 stations from monsoon 2014 and summer 2015. The sedimentary denitrification potential ranged from 0.49 ± 0.05 to 4.85 ± 0.782 mmol NO m h. Maximum denitrification was observed in S4, which is attributed to a local anthropogenic source coupled with intense rainfall episode preceding the sampling season of monsoon 2013. However, this trend was not repeated in the subsequent monsoon samples. This shows that in Ashtamudi, monsoonal effects do not influence sedimentary denitrification. Among the various environmental variables, NO-N, Corg and NH-N were the key factors that influence denitrification in the Ashtamudi estuarine sediments. Among these key factors, NO-N was the limiting factor for denitrification, and hence, it is of prime importance to understand the source of NO-N that fuel denitrification in the sediments. In Ashtamudi, the concentration of NO-N in overlying water was very less, which suggests reduced nitrogen yield in the estuary from the fluvial input of Kallada River and agricultural runoff. Sedimentary NO-N correlated with denitrification which reveals that denitrification is coupled with nitrification in the sediments. This is further explained by the fact that NH-N positively correlated with denitrification. The anoxic sediments were the source of ammonia for nitrous oxide production by nitrogen mineralisation. Also, the Corg in sediment samples were sufficient to support denitrification and Corg was an important factor favouring but not limiting denitrification. The results of sediment denitrification in Ashtamudi can be a model for tropical estuaries experiencing unpredictable rainfall as well as high temperature than temperate systems.

Estuarine sediments are best suited for bioprospecting cellulose degrading microorganisms because of continuous input of cellulosic carbon from rivers and terrestrial runoff, and such sediments act as a substrate for decomposition by microbes. Sediment samples were collected from thirteen stations of Ashtamudi estuary, a tropical Ramsar site during April 2016 and January 2017 and analysed for environmental variables such as temperature, pH, electrical conductivity, oxidation-reduction potential, sulphate, total organic carbon (C org ), carbohydrate, protein, lipid and labile organic matter. Microcosm experiments were conducted in the sediment samples to compare native and substrate-induced cellulase enzyme activities in mesophilic and thermophilic conditions added with crystalline cellulose and cellobiose as substrates. Abundance of cellulolytic anaerobes in the roll tubes was higher with cellobiose than crystalline cellulose. Substrate induced enzyme activity was more than native enzyme activity [0.0012±0.0001-0.004±0.002 (April 2016) and 0.004±0.001-0.161±0.002 mg glucose h -1 (January 2017)] in the sediment samples and cellulolytic activity was more pronounced in thermophilic conditions during April 2016. Redundancy analysis indicated that salinity was the highest determining factor for explaining variations among bacterial abundance and activity during April 2016 and sediment lipid content during January 2017. The study reveals that estuarine sediments can act as a potential source of thermophilic cellulase enzyme producing bacteria, which needs to be further explored owing to their vast industrial applications.

Aim: To investigate the influence of environmental variables on the abundance and activity of methanogenic archaea (MA) in Akkulam-Veli and Vellayani Lake sediments. Methodology: Sediment and overlying water samples (n=5 each) were collected from Veli and Vellayani lakes of Thiruvananthapuram, Kerala. Samples were analysed for environmental variables using standard protocols. Multivariate analysis was done to study the influence of environmental variables on abundance and activity of MA. Results: Environmental variables of overlying water and sediment showed significant variation between the two lakes. Salinity and sulphate were more in Akkulam-Veli than Vellayani, as Akkulam-Veli is a brackish lake and temporarily connected with Arabian Sea. Highly reduced sediments of Akkulam-Veli favoured more population of methylotrophic and acetoclastic MA than Vellayani. A distributional difference of MA with depth was observed in both lakes, which is attributed to availability of more labile organic matter. The methylotrophic MA activity was not significantly different between the two lakes; however, their abundance was significantly different. Nevertheless, methane production was higher in Vellayani than in Akkulam-Veli Lake. PCA revealed that Corg and labile organic matter (LOM) were the important environmental variables influencing methane production potential. Interpretation: Anthropogenic activities like sewage and waste disposal results in increased input of organic matter in lake sediments. The labile organic matter fraction in the sediments favours methanogenic activity thereby resulting in methane production and release from the lakes. Key words: Labile organic matter, Methanogenic archaea, Methane production, Sulphate reduction, Tropical lakes

Estuarine sediments are best suited for bioprospecting cellulose degrading microorganisms because of continuous input of cellulosic carbon from rivers and terrestrial runoff, and such sediments act as a substrate for decomposition by microbes. Sediment samples were collected from thirteen stations of Ashtamudi estuary, a tropical Ramsar site during April 2016 and January 2017 and analysed for environmental variables such as temperature, pH, electrical conductivity, oxidation- reduction potential, sulphate, total organic carbon (Corg), carbohydrate, protein, lipid and labile organic matter. Microcosm experiments were conducted in the sediment samples to compare native and substrate-induced cellulase enzyme activities in mesophilic and thermophilic conditions added with crystalline cellulose and cellobiose as substrates. Abundance of cellulolytic anaerobes in the roll tubes was higher with cellobiose than crystalline cellulose. Substrate induced enzyme activity was more than native enzyme activity [0.0012±0.0001- 0.004±0.002 (April 2016) and 0.004±0.001- 0.161±0.002 mg glucose h-1 (January 2017)] in the sediment samples and cellulolytic activity was more pronounced in thermophilic conditions during April 2016. Redundancy analysis indicated that salinity was the highest determining factor for explaining variations among bacterial abundance and activity during April 2016 and sediment lipid content during January 2017. The study reveals that estuarine sediments can act as a potential source of thermophilic cellulase enzyme producing bacteria, which needs to be further explored owing to their vast industrial applications.