Ammonia oxidation to nitrite and its subsequent oxidation to nitrate provides energy to the two populations of nitrifying chemoautotrophs in the energy-starved dark ocean, driving a coupling between reduced inorganic nitrogen (N) pools and production of new organic carbon (C) in the dark ocean. However, the relationship between the flux of new C production and the fluxes of N of the two steps of oxidation remains unclear. Here, we show that, despite orders-of-magnitude difference in cell abundances between ammonia oxidizers and nitrite oxidizers, the two populations sustain similar bulk N-oxidation rates throughout the deep waters with similarly high affinities for ammonia and nitrite under increasing substrate limitation, thus maintaining overall homeostasis in the oceanic nitrification pathway. Our observations confirm the theoretical predictions of a redox-informed ecosystem model. Using balances from this model, we suggest that consistently low ammonia and nitrite concentrations are maintained when the two populations have similarly high substrate affinities and their loss rates are proportional to their maximum growth rates. The stoichiometric relations between the fluxes of C and N indicate a threefold to fourfold higher C-fixation efficiency per mole of N oxidized by ammonia oxidizers compared to nitrite oxidizers due to nearly identical apparent energetic requirements for C fixation of the two populations. We estimate that the rate of chemoautotrophic C fixation amounts to ∼1 × 1013to ∼2 × 1013mol of C per year globally through the flux of ∼1 × 1014to ∼2 × 1014mol of N per year of the two steps of oxidation throughout the dark ocean.

Abstract. Nitrification is a series of processes that oxidizes ammonia to nitrate, which contributes to hypoxia development in coastal oceans, especially in eutrophicated regions. The nitrification rate of bulk water (NR b ) and particle free water (NR pf , particle > 3 µm eliminated) were determined along the Chang Jiang River plume in August 2011 by nitrogen isotope tracer technique. Measurements of dissolved oxygen (DO), community respiration rate (CR), nutrients, dissolved organic nitrogen (DON), total suspended matter (TSM), particulate organic carbon/nitrogen (POC / PON), acid-leachable iron and manganese on suspended particles and both archaeal and β-proteobacterial ammonia monooxygenase subunit A gene (amoA) abundance on size-fractioned particles (> 3 µm and 0.22-3 µm) were conducted. The NR b ranged from undetectable up to 4.6 µmol L −1 day −1 , peaking at a salinity of ∼ 29. NR b values were positively correlated with ammonium concentration, suggesting the importance of substrate in nitrification. In the river mouth and the inner plume, NR b was much higher than NR pf , indicating that the nitrifying microorganism is mainly particle associated, which was supported by its significant correlation with amoA gene abundance and TSM concentration. The estimated oxygen demands of nitrification accounted for 0.32 to 318 % of CR, in which 50 % samples demanded more oxygen than that predicted by by the Redfield model (23 %), indicating that oxygen might not be the sole oxidant though DO was sufficient (> 58 µmol kg −1 ) throughout the observation period. The excess nitrification-associated oxygen demand (NOD) showed a tendency to occur at lower DO samples accompanied by higher acid-leachable Fe / Mn, which implied reactive Fe 3+ / Mn 4+ may play a role as oxidant in the nitrification process. Stoichiometric calculation suggested that reactive Fe on particles was 10 times the oxidant demand required to complete ammonia oxidation in the entire plume. The potential involvement of reactive iron and manganese in the nitrification process in oxygenated water further complicated nitrogen cycling in the turbid river plume.

Shallow-water hydrothermal vent ecosystems are distinctly different from deep-sea vents, as other than geothermal, sunlight is one of their primary sources of energy, so their resulting microbial communities differ to some extent. Yet compared with deep-sea systems, less is known about the active microbial community in shallow-water ecosystems. Thus, we studied the community compositions, their metabolic pathways, and possible coupling of microbially driven biogeochemical cycles in a shallow-water hydrothermal vent system off Kueishantao Islet, Taiwan, using high-throughput 16S rRNA sequences and metatranscriptome analyses. Gammaproteobacteria and Epsilonbacteraeota were the major active bacterial groups in the 16S rRNA libraries and the metatranscriptomes, and involved in the carbon, sulfur, and nitrogen metabolic pathways. As core players, Thiomicrospira, Thiomicrorhabdus, Thiothrix, Sulfurovum, and Arcobacter derived energy from the oxidation of reduced sulfur compounds and fixed dissolved inorganic carbon (DIC) by the Calvin-Benson-Bassham (CBB) or reverse tricarboxylic acid cycles. Sox-dependent and reverse sulfate reduction were the main pathways of energy generation, and probably coupled to denitrification by providing electrons to nitrate and nitrite. Sulfur-reducing Nautiliaceae members, accounting for a small proportion in the community, obtained energy by the oxidation of hydrogen, which also supplies metabolic energy for some sulfur-oxidizing bacteria. In addition, ammonia and nitrite oxidation is another type of energy generation in this hydrothermal system, with marker gene sequences belonging to Thaumarchaeota/Crenarchaeota and Nitrospina, respectively, and ammonia and nitrite oxidation was likely coupled to denitrification by providing substrate for nitrate and nitrite reduction to nitric oxide. Moreover, unlike the deep-sea systems, cyanobacteria may also actively participate in major metabolic pathways. This study helps us to better understand biogeochemical processes mediated by microorganisms and possible coupling of the carbon, sulfur, and nitrogen cycles in these unique ecosystems.

Abstract. Coupled nitrification–denitrification plays a critical role in the removal of excess nitrogen, which is chiefly caused by humans, to mitigate estuary and coastal eutrophication. Despite its obvious importance, limited information about the relationships between nitrifying and denitrifying microbial communities in estuaries, and their controlling factors have been documented. We investigated the nitrifying and denitrifying microbial communities in the estuary of turbid subtropical Yangtze River (YRE), the largest river in Asia, by analyzing the ammonia monooxygenase gene amoA, including archaeal and bacterial amoA, and the dissimilatory nitrite reductase gene nirS using clone libraries and quantitative PCR (qPCR). The diversity indices and rarefaction analysis revealed a quite low diversity for both β-proteobacterial and archaeal amoA genes, but qPCR data showed significantly higher amoA gene copy numbers for archaea than β-proteobacteria. Compared with the amoA gene, a significantly higher level of diversity but lower gene copy numbers were found for the nirS gene. Nitrification and denitrification rates based on 15N incubation experiments supported gene abundance data as denitrification rates were below detection limit, suggesting lower denitrification than nitrification potential. In general, the abundances of the amoA and nirS genes were significantly higher in the bottom samples than the surface ones, and in the high-turbidity river mouth, were significantly higher in the particle-associated (> 3 μm) than the free-living (0.2 ~ 3 μm) communities. Notably, positive correlations between the amoA and nirS gene abundances suggested potential gene-based coupling between nitrification and denitrification, especially for the particle-associated assemblages. Statistical analysis of correlations between the community structure, gene abundances and environmental variables further revealed that dissolved oxygen and total suspended material might be the key factors controlling community spatial structure and regulating nitrification and denitrification potentials in the YRE ecosystem.

Abstract. The niche differentiation of ammonia and nitrite oxidizers is controversial because they display disparate patterns in estuarine, coastal, and oceanic regimes. We analyzed diversity and abundance of ammonia-oxidizing archaea (AOA) and β-proteobacteria (AOB), nitrite-oxidizing bacteria (NOB), and nitrification rates to identify their niche differentiation along a salinity gradient from the Pearl River estuary to the South China Sea. AOA were generally more abundant than β-AOB; however, AOB more clearly attached to particles compared with AOA in the upper reaches of the Pearl River estuary. The NOB Nitrospira had higher abundances in the upper and middle reaches of the Pearl River estuary, while Nitrospina was dominant in the lower estuary. In addition, AOB and Nitrospira could be more active than AOA and Nitrospina since significantly positive correlations were observed between their gene abundance and the nitrification rate in the Pearl River estuary. There is a significant positive correlation between ammonia and nitrite oxidizer abundances in the hypoxic waters of the estuary, suggesting a possible coupling through metabolic interactions between them. Phylogenetic analysis further revealed that the AOA and NOB Nitrospina subgroups can be separated into different niches based on their adaptations to substrate levels. Water mass mixing is apparently crucial in regulating the distribution of nitrifiers from the estuary to open ocean. However, when eliminating water mass effect, the substrate availability and the nitrifiers' adaptations to substrate availability via their ecological strategies essentially determine their niche differentiation.

A novel bacterium, strain JL1095(T), was isolated from the surface water of the Yangtze Estuary, China (31° N, 122° E). Cells were Gram negative, aerobic, oval-shaped with one peak end and motile by gliding. Cells divided by binary fission. Growth occurred at 15-50 °C (optimum at 35 °C), 2-10 % (w/v) NaCl (optimum at ~3 %) and pH 6.0-9.0 (optimum at pH ~ 7.6). Strain JL1095(T) was able to utilize various sole-carbon-source, such as Tween 40, Tween 80, acetic acid, L-arabinose, D,L-lactic acid, urocanic acid, methyl-pyruvate, α-hydroxy butyric acid, β-hydroxy butyric acid, and γ-hydroxy butyric acid. The major cellular fatty acids were C16:0, C18:0, C19:0 ω8c cyclo, C20:1 ω7c, and Summed Feature 8. The whole respiratory ubiquinone was Q-10. The genomic DNA G+C content of strain JL1095(T) was 51.5 %. According to the phylogenetic analysis, strain JL1095(T) formed a monophyletic branch at the periphery of the evolutionary radiation occupied by the genera Labrenzia, Pannonibacter, Stappia, Wenxnia, and Amaricoccus. The sequence similarity was 92.8 % with the most closely relating strain Stappia indica B106(T), and 92.6 % with the type species Stappia stellulatum IAM 12621(T). Based on the biochemical characteristics, chemotaxonomy and phylogenetic analysis, strain JL1095(T) is considered to be a novel genus which belongs to the family Rhodobacteraceae. The novel strain is named Acuticoccus yangtzensis gen. nov., sp. nov. The type strain of Acuticoccus yangtzensis is JL1095(T) (=CGMCC 1.12795 = DSM 28604).

Abstract. Coupled nitrification-denitrification plays a critical role in the removal of excess nitrogen, which is chiefly caused by humans, to mitigate estuary and coastal eutrophication. Despite its obvious importance, limited information about the relationships between nitrifying and denitrifying microbial communities in estuaries, and their controlling factors have been documented. By analyzing the ammonia monooxygenase gene amoA, including archaeal and bacterial amoA, and the dissimilatory nitrite reductase gene nirS using clone libraries and quantitative PCR (qPCR), we investigated the nitrifying and denitrifying microbial communities in the estuary of turbid subtropical Yangtze River (YRE), the largest river in Asia. The diversity indices and rarefaction analysis revealed a quite low diversity for both β-proteobacterial and archaeal amoA genes, but qPCR data showed significantly higher amoA gene copy numbers for archaea than β-proteobacteria, suggesting that the archaea might play a dominant role in nitrification in the YRE. Compared with the amoA gene, a distinctly higher level of diversity but lower gene copy numbers were found for thenirS gene suggesting lower denitrification than nitrification potential. 15N incubation experiments indicated that nitrification rates were strongly correlated with amoA gene abundances while denitrification rates were below detection limit. In general, the abundances of the amoA and nirS genes were significantly higher in the bottom samples than the surface ones, and in the high-turbidity river mouth, were distinctly higher in the particle-associated (> 3 μm) than the free-living (0.2 ~ 3 μm) communities. Notably, analysis of correlations between the gene abundances suggested potential gene-based coupling between nitrification and denitrification, especially for the particle-associated assemblages. Statistical analysis of correlations between the community structure, gene abundances and environmental variables further revealed that dissolved oxygen and total suspended material might be the key factors controlling community spatial structure and regulating nitrification and denitrification potentials in the YRE ecosystem.

<p><strong>Abstract.</strong> The niche differentiation between ammonia and nitrite oxidizers are controversial because they display disparate patterns in estuarine, coastal, and oceanic regimes. We analyzed ammonia-oxidizing archaea (AOA) and <i>β</i>-proteobacteria (AOB) <i>amo</i>A genes, nitrite-oxidizing bacteria (NOB) <i>nxr</i>B and 16S rRNA genes, and nitrification rates to identify their niche differentiation along a salinity gradient from the Pearl River estuary to the South China Sea. The archaeal <i>amo</i>A genes were generally more abundant than the <i>β</i>-AOB <i>amo</i>A genes; however, AOB more clearly attached to particles compared with AOA in the upper reaches of the Pearl River estuary. The NOB <i>Nitrospina</i> had higher abundances in the upper and middle reaches of the Pearl River estuary, while <i>Nitrospina</i> was dominant in the lower estuary. In addition, AOB and <i>Nitrospina</i> could be more active than AOA and <i>Nitrospina</i> since significantly positive correlations were observed between their gene abundance and the nitrification rate in the Pearl River estuary. There is a coupling of ammonia and nitrite oxidizers in the hypoxic waters of the estuary, suggesting metabolic interactions between them. Phylogenetic analysis further revealed that the AOA and NOB <i>Nitrospina</i> subgroups can be separated into different niches based on their adaptations to substrate levels. Water mass mixing is apparently crucial in regulating the distribution of nitrifiers from the estuary to open ocean. However, when eliminating water mass effect, the substrate availability and the nitrifiers’ adaptations to substrate availability via their ecological strategies essentially determine their niche differentiation.</p>