Denitrifying community in coastal sediments performs aerobic and anaerobic respiration simultaneously

Abstract

Nitrogen (N) input to the coastal oceans has increased considerably because of anthropogenic activities, however, concurrent increases have not occurred in open oceans. It has been suggested that benthic denitrification in sandy coastal sediments is a sink for this N. Sandy sediments are dynamic permeable environments, where electron acceptor and donor concentrations fluctuate over short temporal and spatial scales. The response of denitrifiers to these fluctuations are largely unknown, although previous observations suggest they may denitrify under aerobic conditions. We examined the response of benthic denitrification to fluctuating oxygen concentrations, finding that denitrification not only occurred at high O concentrations but was stimulated by frequent switches between oxic and anoxic conditions. Throughout a tidal cycle, in situtranscription of genes for aerobic respiration and denitrification were positively correlated within diverse bacterial classes, regardless of O concentrations, indicating that denitrification gene transcription is not strongly regulated by O in sandy sediments. This allows microbes to respond rapidly to changing environmental conditions, but also means that denitrification is utilized as an auxiliary respiration under aerobic conditions when imbalances occur in electron donor and acceptor supply. Aerobic denitrification therefore contributes significantly to N-loss in permeable sediments making the process an important sink for anthropogenic N-inputs.

Effect of urease and nitrification inhibitors on ammonia volatilization and abundance of N‐cycling genes in an agricultural soil

“…S5). These results agree with previous reports ( Ji et al., ; Marchant et al., ; Castellano‐Hinojosa et al., ) and lend support to the existence of denitrification under aerobic conditions. In contrast to DMPP, no increases in the abundance of norB and nosZ I during the incubation time were observed in soils treated with urea + NBPT, which suggest an effective temporal effect of NBPT on denitrifiers.…”

Metabolic specialization of denitrifiers in permeable sediments controls N2O emissions

“…Few trends could be seen across the tidal cycle when the different genes were examined individually ( Fig. 4 and see Marchant et al, 2017). This suggests that there is no strong response at the transcriptional level to the large fluctuations that occur over a day at the sand flat.…”

“…Previous studies have shown that oxygen can penetrate more than 5 cm while the tide is in, and while the tide is out only penetrates 1-2 cm (see Marchant et al, 2017, Jansen et al 2009. Sediment was collected from the upper 5 cm of the sand flat and stored in washed plastic containers for transport before use in steady state core incubations, dynamic core incubations and microsensor experiments.…”

“…Sediment samples for 454-based metagenomics were taken in October 2009 from 0-5 cm depth, and in March 2010 from 0-2 cm depth, a detailed description of the samples is given in Marchant et al, (2017). Briefly, three samples were taken in March, two of which were pooled and divided again into two technical replicates, named MarA and MarB, the third sample (the biological replicate) is referred to as MarC.…”

Does denitrification occur within porous carbonate sand grains?

“…This observation cannot easily be explained by the presence of denitrification under oxic conditions. We note that our maximum rate of denitrification under bulk oxic conditions measured in the FTR reactors (4.6 µmol N L −1 h −1 ) is at the lower end of denitrification rates reported under oxic conditions in silicate sands of ∼ 6-17 µmol N L −1 h −1 (Gao et al, 2010;Marchant et al, 2017), suggesting that denitrification under oxic conditions, where it occurs, has a greater enhancement effect on total denitrification than denitrification in anoxic microniches.…”