Abstract

The nitrogen cycle is an indispensable part of the biogeochemical cycle, and the reactions that occur in the ocean oxygen minimum zone (OMZ) mediate much of the loss of nitrogen from oceans worldwide. Here, nitrate-utilizing bacteria were isolated from the water column at 17 stations within the OMZ of the Changjiang (Yangtze River) Estuary using selective media and a culture-dependent method. The microbial diversity, nitrogen metabolism and nitrate reduction test of culturable heterotrophic bacteria were examined. A total of 164 isolates were obtained; they were mostly affiliated with Proteobacteria (81.1%), Actinobacteria (5.5%), Bacteroidetes (12.3%), and Firmicutes (0.6%). Pseudomonas aeruginosa, Sphingobium naphthae, and Zunongwangia profunda were found at most stations. Among 24 tested representative strains, 8 were positive for nitrate reduction; they belonged to genera Aurantimonas, Halomonas, Marinobacter, Pseudomonas, Thalassospira, and Vibrio. Pseudomonas aeruginosa contained the genes (napAB, norBC, nirS, and nosZ) for complete denitrification and may be responsible for mediating denitrification. 66% representative isolates (16/24) contained genes for reducing nitrate to nitrite (nasA, napAB, or narGHI) and 79% representative isolates (19/24) possessed genes for converting nitrite to ammonia (nirA or nirBD), suggesting that nitrate and nitrite could act as electron acceptors to generate ammonium, subsequently being utilized as a reduced nitrogen source. This study improves our understanding of the microbial diversity within the OMZ of Changjiang Estuary and may facilitate the cultivation and exploitation of bacteria involved in the nitrogen cycle.

Highlights

  • METHODOLOGYIn the environment, the oxidation state of nitrogen ranges from -3 in ammonium (NH4+) to + 5 (NO3−), and is governed by several processes

  • Studies on surface sediments of the hypoxic zone near the Changjiang Estuary and in the East China Sea revealed that the bacterial community composition mainly comprised Gammaproteobacteria (Ye et al, 2016)

  • Pseudomonas aeruginosa, Sphingobium naphthae, and Zunongwangia profunda were found at most stations with different dissolved oxygen (DO), NO3-N and NO2-N values

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Summary

METHODOLOGY

The oxidation state of nitrogen ranges from -3 in ammonium (NH4+) to + 5 (NO3−), and is governed by several processes. We used a culture-dependent method to investigate microorganisms that participate in the nitrogen cycle in the seawater column of the Changjiang Estuary OMZ, aiming to: (1) explore the diversity of the culturable nitrate-utilizing bacteria; (2) explore nitrogen metabolism based on their genomes data; (3) test nitrate reduction of the isolated strains; (4) investigate the relationship between their distribution and environmental parameters (dissolved oxygen, nitrate, and nitrite). If the test sample was red, pink, or orange, there was nitrite in the tube and the isolate was scored as positive for nitrate reduction and marked as “ + ” If no color appeared, zinc dust was added. The lack of any color change at this point was taken as indicating that there was no residual nitrate in the liquid medium, meaning that the strain could reduce nitrate completely; this was marked as “ ++ ” If the liquid medium changed from colorless to red, pink, or orange after the addition of the zinc dust (as did the control), indicating the continued lack of nitrite, the strain was considered to be unable to reduce nitrate, and was marked as“−”

RESULTS AND DISCUSSION
CONCLUSION
DATA AVAILABILITY STATEMENT
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