Denitrification and environmental factors influencing nitrate removal in Guaymas Basin hydrothermally altered sediments

Abstract

We measured potential nitrate removal and denitrification rates in hydrothermally altered sediments inhabited by Beggiatoa mats and adjacent brown oil stained sediments from the Guaymas Basin, Gulf of California. Sediments with Beggiatoa maintained slightly higher rates of potential denitrification than did brown sediments at 31.2 ± 12.1 versus 21.9 ± 1.4 µM N day(-1), respectively. In contrast, the nitrate removal rates in brown sediments were higher than those observed in mat-hosting sediments at 418 ± 145 versus 174 ± 74 µM N day(-1), respectively. Additional experiments were conducted to assess the responses of denitrifying communities to environmental factors [i.e., nitrate, sulfide, and dissolved organic carbon (DOC) concentration)]. The denitrifying community had a high affinity for nitrate (K(m) = 137 ± 91 µM NO3-), in comparison to other environmental communities of denitrifiers, and was capable of high maximum rates of denitrification (V(max) = 1164 ± 153 µM N day(-1)). The presence of sulfide resulted in significantly lower denitrification rates. Microorganisms with the potential to perform denitrification were assessed in these sediments using the bacterial 16S rRNA gene and nitrous oxide reductase (nosZ) functional gene libraries. The bacterial 16S rRNA gene clone library was dominated by Epsilonproteobacteria (38%), some of which (e.g., Sulfurimonas sp.) have a potential for sulfide-dependent denitrification. The nosZ clone library did not contain clones similar to pure culture denitrifiers; these clones were most closely associated with environmental clones.

Keywords: Beggiatoa; denitrification; nitrogen cycle.

FIGURE 1

Time series incubations of mat (A) and brown (B) sediments with the substrate (${\text{NO}}_3^{-}$, µM) and products (²⁹N₂ and ³⁰N₂, µM) plotted against time (h).

FIGURE 2

Environmental influences of nitrate (A), sulfide (B), and DOC (C) on denitrification and nitrogen species end products.

FIGURE 3

16S rRNA bacterial gene phylogeny of Guaymas Basin environmental clones relative to pure culture and other environmental clones. Environmental clones from this work appear as bold text, pure cultures are italicized, and environmental clones from other environments appear as normal text. Neighbor joining method was used to generate a tree with a Jukes–Cantor correction of evolutionary distance. Bootstrap values for branches occurring for >50% of 1000 iterations are reported. Scaling of the phylogenetic tree is based on an evolutionary distance of 0.03.

FIGURE 4

nosZ phylogeny of Guaymas Basin environmental clones relative to environmental clones from continental margin sediments and pure culture data. Minimum evolution method was used to generate the phylogeny, with a Poisson correction of evolutionary distance. Bootstrap values for branches occurring for >50% of 1000 iterations are reported. Scaling of the phylogenetic tree is based on an evolutionary distance of 0.10.