Denitrification activity in mangrove sediments varies with associated vegetation

International audienceTo test the hypothesis that mangrove vegetation associated sediment characteristics could influence an ecologically important function such as denitrification, a study was carried out at the Divar mangrove ecosystem in Goa, India. The composition of intertidal mangroves was assessed based on which the down-core (0-10 cm) variation in environmental variables and denitrification activity (DNT) associated with dominant vegetation types was examined. Our observations revealed a distinct zonation pattern of mangroves with the dominance of Rhizophora mucronata (RM) in the lower intertidal zone followed by Avicennia marina (AM) in the mid region. Acanthus ilicifolius (AI) was restricted to the upper intertidal zone (UIZ) which was characterized by relatively lower sediment temperatures, higher porewater salinity and intense reducing conditions. A zonation pattern in occurrence of DNT was also observed. Denitrification activity generally decreased with depth and increased in intensity from the seaward to the landward side. Maximum DNT of 4.06 +/- 0.44 nmol N2O g(-1) h(-1) was recorded in the surficial AI sediments followed by AM and RM sediments. Surface plant litter content also increased from the lower to UIZ. A negative relationship of NH4+ and total organic carbon content (n=15, r=-0.388, p<0.05) in AI sediments was observed. This suggested that re-mineralization of accumulated organic matter (up to 3.07 + 0.58% at 0-2 cm) enhanced NH4+ availability in the porewater which in turn could support nitrification-denitrification. Thus, degradation of vegetation-derived organic matter was important in sustaining nutrients mainly towards the landward side proving our hypothesis that vegetation could have an influence on DNT, albeit indirectly. (C) 2016 Published by Elsevier B.V

Microbial activity promotes the enrichment of cobalt over nickel on hydrogenetic ferromanganese crusts

<p>The different mineral phases of the ferromanganese (Fe–Mn) crusts stem from the interaction of biotic and abiotic components. It is therefore vital to study the activity of these components to decipher their contribution to the enrichment/depletion of metals in the crust. Thus, the present study examined sorption and release of Co and Ni by Fe-Mn crusts with associated microbial communities in the presence and absence of the metabolic poison sodium azide (15 mM). The study was conducted in the presence (G⁺) and absence (G⁻) of added glucose (0.1%) at temperatures of 4 ± 1°C and 28 ± 2°C. Results showed that the microbial community had maximal sorption of Co of 66.12 µg g⁻¹ at 4 ± 1°C in the absence of added glucose and 479.75 µg g⁻¹ at 28 ± 2°C in the presence of added glucose. Maximum sorption of Ni in the absence of added glucose was 1.89 µg g⁻¹ at 4 ± 1°C and release of Ni was 51.28 µg g⁻¹ in the presence of added glucose. Under abiotic conditions with 15 mM sodium azide as a metabolic inhibitor, significant amounts of Co and Ni were released in the G⁺ medium. Total cell counts on the Fe-Mn crust in the presence of added glucose increased by an order of magnitude from 10⁶ to 10⁷ cells g⁻¹ and in the absence of added glucose remained within the order of 10⁶ cells g⁻¹ irrespective of temperature of incubation. Microscopic observation of the samples from biotic incubations showed numerous bacterial cells, exopolysaccharides, and structures resembling secondary minerals formed by bacteria. The results indicate that bacteria promote the enrichment of Co and Ni on the hydrogenetic Fe-Mn crusts by sorption processes and release of Ni by reductive dissolution of the oxides. The higher enrichment of Co than Ni is attributed to the way in which microbes interact with the metals.</p