Widespread nitrogen fixation in sediments from diverse deep‐sea sites of elevated carbon loading

Abstract

Nitrogen fixation, the biological conversion of N to NH , is critical to alleviating nitrogen limitation in many marine ecosystems. To date, few measurements exist of N fixation in deep-sea sediments. Here, we conducted > 400 bottle incubations with sediments from methane seeps, whale falls and background sites off the western coast of the United States from 600 to 2893 m water depth to investigate the potential rates, spatial distribution and biological mediators of benthic N fixation. We found that N fixation was widespread, yet heterogeneously distributed with sediment depth at all sites. In some locations, rates exceeded previous measurements by > 10×, and provided up to 30% of the community anabolic growth requirement for nitrogen. Diazotrophic activity appeared to be inhibited by pore water ammonium: N fixation was only observed if incubation ammonium concentrations were ≤ 25 μM, and experimental additions of ammonium reduced diazotrophy. In seep sediments, N fixation was dependent on CH and coincident with sulphate reduction, consistent with previous work showing diazotrophy by microorganisms mediating sulphate-coupled methane oxidation. However, the pattern of diazotrophy was different in whale-fall and associated reference sediments, where it was largely unaffected by CH , suggesting catabolically different diazotrophs at these sites.

Evidence for phylogenetically and catabolically diverse active diazotrophs in deep-sea sediment

“…In the present study, we investigate the identity and activity of diazotrophs within deep-sea sediment collected at the distal end of Monterey Canyon, CA, USA (0-3 and 9-12 cm below seafloor [cmbsf]; 2893 m water depth). Active diazotrophy was previously demonstrated within this sediment via 15 N 2 tracer assays [12], however, the responsible organisms were unidentified. Here, we identify the active diazotrophs using a combination of density-gradient 15 N-DNA stable isotope probing ( 15 N-SIP) and a novel nifH amplicon analysis pipeline.…”

“…However, despite its well-documented ecological and biogeochemical importance in the pelagic euphotic zone [6], and its continued investigation in shallow marine sediments [7][8][9][10], nitrogen fixation in deep marine sediments (>200 m water depth) remains relatively unexplored. This may be a significant oversight, given that deep-sea sediments cover nearly two-thirds of Earth's surface, have high microbial densities (up to 1000× greater than surface waters) [11], and can have relatively low concentrations of bioavailable nitrogen at the surface (<25 μM) [12].…”

“…Recently, nitrogen fixation has been detected or implicated in several geochemically anomalous deep-sea habitats, including methane seeps [12][13][14][15], hydrothermal vents [16], whale falls [12], and oxygen minimum zones [17,18]. The emerging perspective from nifH sequencing [19], inhibition experiments [20], and geochemical correlation analyses [12,15,17,18] suggests that the diazotrophs across these sites are phylogenetically and catabolically diverse. However, only two deep-sea taxa have been directly identified as diazotrophs to date, the methanogenic Methanocaldococcus sp.…”

“…FS406-22 isolated from hydrothermal vent fluid [21] and the methanotrophic ANME-2 archaea found at methane seeps [22]. Even less is known about the identity of deep-sea diazotrophs outside these geochemically anomalous regions, despite diverse nifH sequences [19] and bulk 15 N 2 assimilation [12] suggesting their presence and activity. Identifying the organisms that fix nitrogen in widely representative deep-sea sediments would likely reveal novel nitrogen-fixing taxa, prescribe ecosystem function to uncultivated lineages, and help evaluate the biogeochemical impacts of and controls on diazotrophy in the greater marine benthos.…”

Community composition and functional genes as well as ecological role of bacteria attached to different bloom-forming cyanobacteria

“…Sulfate is known to inhibit microbial uptake of molybdate, which is an element essential for the activity of the nifH nitrogenase [55], expressed speci cally in Dolichospermum [56]. Phylogenetic evidence suggests that sulfur and sulfate reducers are responsible for the N 2 xation in marine sediment [57,58]. Sul des produced in the reduction process will bene t the synthesis of nitrogenase and the optimization of N 2 -…”