papers

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Jan Reent Köster1,*, 

Reinhard Well2, 

Béla Tuzson3, 

Roland Bol4, 

Klaus Dittert1,5,

Anette Giesemann2, 

Lukas Emmenegger3,

Albert Manninen3, 

Laura Cárdenas6,

Joachim Mohn3

RATIONALE

Nitrous oxide (N2O), a highly climate-relevant trace gas, is mainly derived from microbial denitrification and nitrification processes in soils. Apportioning N2O to these source processes is a challenging task, but better understanding of the processes is required to improve mitigation strategies. The N2O site-specific 15 N signatures from denitrification and nitrification have been shown to be clearly different, making this signature a potential tool for N2O source identification. We have applied for the first time quantum cascade laser absorption spectroscopy (QCLAS) for the continuous analysis of the intramolecular 15 N distribution of soil-derived N2O and compared this with state-of-the-art isotope ratio mass spectrometry (IRMS).

METHODS

Soil was amended with nitrate and sucrose and incubated in a laboratory setup. The N2O release was quantified by FTIR spectroscopy, while the N2O intramolecular 15 N distribution was continuously analyzed by online QCLAS at 1 Hz resolution. The QCLAS results on time-integrating flask samples were compared with those from the IRMS analysis.

RESULTS

The analytical precision (2σ) of QCLAS was around 0.3 ‰ for the δ15Nbulk and the 15 N site preference (SP) for 1-min average values. Comparing the two techniques on flask samples, excellent agreement (R2 = 0.99; offset of 1.2 ‰) was observed for the δ15Nbulk values while for the SP values the correlation was less good (R2 = 0.76; offset of 0.9 ‰), presumably due to the lower precision of the IRMS SP measurements.

CONCLUSIONS

These findings validate QCLAS as a viable alternative technique with even higher precision than state-of-the-art IRMS. Thus, laser spectroscopy has the potential to contribute significantly to a better understanding of N turnover in soils, which is crucial for advancing strategies to mitigate emissions of this efficient greenhouse gas. 

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Julie B. Olson1,*, R. Wayne Litaker2, Hans W. Paerl1,**

1Institute of Marine Sciences, University of North Carolina at Chapel Hill, Morehead City, North Carolina 28557, USA 2Program in Molecular Biology and Biotechnology, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599, USA

*Present address: Harbor Branch Oceanographic Institution, Fort Pierce, Florida 34946, USA **Addressee for correspondence. E-mail: [email protected]

ABSTRACT: Cyanobacteria are a dominant structural component of N2-fixing microbial mats in diverse marine ecosystems. As a result, much of the measured N2 fixation activity has been attributed to the cyanobacterial component. Until recently, the contribution of heterotrophic N2 fixers has received much less attention. In this study, the presence, diversity, and ubiquity of heterotrophic N2 fixers were investigated in 3 cyanobacteria (Microcoleus)-dominated, intertidal microbial mats obtained from Tomales Bay, California, Sippewissett Salt Marsh, Massachusetts, and Bird Shoal, North Carolina. Using PCR techniques, a diverse array of heretofore uncharacterized heterotrophic nifH (gene encoding the Fe-protein subunit of nitrogenase) sequences were found in these geographically disparate microbial mats, suggesting that heterotrophic diazotrophs may play a larger role in N2 fixation dynamics than previously thought. Phylogenetically similar heterotrophic diazotrophic sequences were obtained from the 3 sites, potentially indicating that similar heterotrophic N2-fixing communities are responsible for this process at diverse locations.

KEY WORDS: Microbial mat · Heterotrophic bacteria · N2 fixation · Cyanobacteria

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Silvio Pantojaa,

Daniel J Repetaa, 

Julian P Sachsb, 

Daniel M Sigmanc

a Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA

b Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

c Department of Geosciences, Princeton University, Princeton, NJ 08544, USA

ABSTRACT

We used the nitrogen isotope ratio of algae, suspended particles and nitrate in the water column to track spatial variations in the marine nitrogen cycle in the Mediterranean Sea. Surface PON (5–74 m) was more depleted in 15N in the eastern basin (−0.3±0.5‰) than in the western basin (+2.4±1.4‰), suggesting that nitrogen supplied by biological N2 fixation may be an important source of new nitrogen in the eastern basin, where preformed nitrate from the Atlantic Ocean could have been depleted during its transit eastward. Theδ15N of nitrate in the deep Mediterranean (∼3‰ in the western-most Mediterranean and decreasing toward the east) is significantly lower than nitrate at similar depths from the North Atlantic (4.8–5‰), also suggesting an important role for N2 fixation. The eastward decrease in the δ15N of surface PON is greater than the eastward decrease in the δ15N of the subsurface nitrate, implying that the amount of N2 fixation in the eastern Mediterranean is great enough to cause a major divergence in the δ15N of phytoplankton biomass from the δ15N of the nitrate upwelled from below. Variations in productivity associated with frontal processes, including shoaling of the nitracline, did not lead to detectable variations in the δ15N of PON. This indicates that no differential fertilization or productivity gradient occurred in the Almerian/Oran area. Our results are consistent with a lack of gradient in chlorophyll-a (chl-a) and nitrate concentration in the Alboran Sea. 15N enrichment in particles below 500 m depth was detected in the Alboran Sea with respect to surface PON, reaching an average value of +7.4±0.7‰. The δ15N in sinking particles caught at 100 m depth (4.9–5.6‰) was intermediate between suspended surface and suspended deep particles. We found a consistent difference in the isotopic composition of nitrogen in PON compared with that of chlorophyll (Δδ15N[PON-chlorin]=+6.4±1.4‰) in the surface, similar to the offset reported earlier in cultures for cellular N and chl-a. This indicates that δ15N of phytoplankton biomass was retained in surface PON, and that alteration of the isotopic signal of PON at depth was due to heterotrophic activity.