Nitrate reduction functional genes and nitrate reduction potentials persist in deeper estuarine sediments. Why?

Papaspyrou, S., Smith, C. J. , Dong, L. F., Whitby, C., Dumbrell, A. J. and Nedwell, D. B. (2014) Nitrate reduction functional genes and nitrate reduction potentials persist in deeper estuarine sediments. Why? PLoS ONE, 9(4), e94111. (doi: 10.1371/journal.pone.0094111) (PMID:24728381) (PMCID:PMC3984109)

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Denitrification and dissimilatory nitrate reduction to ammonium (DNRA) are processes occurring simultaneously under oxygen-limited or anaerobic conditions, where both compete for nitrate and organic carbon. Despite their ecological importance, there has been little investigation of how denitrification and DNRA potentials and related functional genes vary vertically with sediment depth. Nitrate reduction potentials measured in sediment depth profiles along the Colne estuary were in the upper range of nitrate reduction rates reported from other sediments and showed the existence of strong decreasing trends both with increasing depth and along the estuary. Denitrification potential decreased along the estuary, decreasing more rapidly with depth towards the estuary mouth. In contrast, DNRA potential increased along the estuary. Significant decreases in copy numbers of 16S rRNA and nitrate reducing genes were observed along the estuary and from surface to deeper sediments. Both metabolic potentials and functional genes persisted at sediment depths where porewater nitrate was absent. Transport of nitrate by bioturbation, based on macrofauna distributions, could only account for the upper 10 cm depth of sediment. A several fold higher combined freeze-lysable KCl-extractable nitrate pool compared to porewater nitrate was detected. We hypothesised that his could be attributed to intracellular nitrate pools from nitrate accumulating microorganisms like Thioploca or Beggiatoa. However, pyrosequencing analysis did not detect any such organisms, leaving other bacteria, microbenthic algae, or foraminiferans which have also been shown to accumulate nitrate, as possible candidates. The importance and bioavailability of a KCl-extractable nitrate sediment pool remains to be tested. The significant variation in the vertical pattern and abundance of the various nitrate reducing genes phylotypes reasonably suggests differences in their activity throughout the sediment column. This raises interesting questions as to what the alternative metabolic roles for the various nitrate reductases could be, analogous to the alternative metabolic roles found for nitrite reductases.

Item Type:Articles
Additional Information:SP acknowledges the support from a Marie-Curie Intra-European Fellowship (EU 024108 – DEFUNIREG) and a Marie-Curie Reintegration Grant (EU 235005 – NITRICOS), and CW the financial support from the University of Essex.
Glasgow Author(s) Enlighten ID:Smith, Professor Cindy
Authors: Papaspyrou, S., Smith, C. J., Dong, L. F., Whitby, C., Dumbrell, A. J., and Nedwell, D. B.
College/School:College of Science and Engineering > School of Engineering > Infrastructure and Environment
Journal Name:PLoS ONE
Publisher:Public Library of Science
ISSN (Online):1932-6203
Published Online:11 April 2014
Copyright Holders:Copyright © 2014 Papaspyrou et al.
First Published:First published in PLoS ONE 9(4):e94111
Publisher Policy:Reproduced under a creative commons license

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