Exchange of ¹⁵NO₃⁻ and ¹⁴NO₃⁻ over time in summer incubations.

<p>The dashed line indicates the initial amount of ¹⁵NO₃⁻ added to the incubation</p

Respiration of stored intracellular nitrate by eukaryotes and prokaryotes.

<p>A) Example of the appearance of ¹⁵N labeled compounds stored in the sediment during the secondary incubation to which no label was added (example from spring incubations) B) Comparison of the overall fraction of ¹⁵N recovered as either N₂ or NH₄⁺ in both the ¹⁵NO₃⁻ incubation and the subsequent incubation with no additional ¹⁵NO₃⁻ and the partitioning between eukaryotes and prokaryotes C) Eukaryotic denitrification rates E) prokaryotic denitrification rates D) eukaryotic DNRA rates F) prokaryotic DNRA rates. Error bars are SD (n = 3).</p

Seasonal patterns in DNRA rates mediated by the prokaryotic and eukaryotic community.

<p>A) Eukaryotic DNRA rates B) Prokaryotic DNRA rates C) Overall DNRA rates. Total rates were determined from the incubation amended with ¹⁵NO₃⁻. Prokaryotic rates were determined after addition of the eukaryote inhibitor cycloheximide. Eukaryotic rates were calculated by subtracting prokaryotic rates from total rates. Error bars are SD (n = 3)</p

Rates of nitrogen cycling processes determined over 3 seasons.

<p>Anaerobic dissimilatory nitrate reduction to ammonium (DNRA) and denitrification (Denit.) rates from the incubation amended with ¹⁵NO₃⁻. Rates were determined from linear production slopes after oxygen had been consumed. Error bars are SD (n = 3)</p

Comparison of incubation methods.

<p>Incubations were carried out using either membrane inlet mass spectrometry (MIMS) which allowed for continuous online measurements or by discrete porewater sampling into exetainers and subsequent measurement on a GC-IRMS. The example shown is taken from a replicate during the spring incubations directly after the addition of ¹⁵NO₃⁻.</p

Average percentage of ¹⁵N recovered in various pools.

<p>Denitrification denotes ¹⁵N that was present as ²⁹⁺³⁰N₂, DNRA denotes ¹⁵N that was present as ¹⁵NH₄⁺ and intracellular storage represents ¹⁵N which was present as either ²⁹⁺³⁰N₂ or ¹⁵NH₄ at the termination of the secondary incubation to which no additional ¹⁵NO₃ was added. The ‘missing label’ fraction refers to the discrepancy between added ¹⁵NO₃⁻, measured ¹⁵NO₃⁻ and ¹⁵N₂ production (shown in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0104517#pone-0104517-g004" target="_blank">Fig. 4</a>). Error bars are overall SD (n = 3)</p

Gene expression analysis shown as enrichment factor of relative transcript abundance.

<p>The genes are indicated in the top right corner of each panel. Filled circles represent the experimental data during the 48 h phase. The grey bars indicate the regular dark phase and the striped grey bars indicate the subjective dark phase. Symbols and error bars represent mean ± SE of triplicate cultures.</p

Enrichment in ¹⁵N (color scale: ¹⁵N/¹⁴N) due to N₂ fixation by individual <i>C.</i><i>watsonii</i> cells.

<p>A. Regular dark phase. B. Regular light phase. C. Subjective dark phase. D. Subjective light phase. (Scale bars: 5 µm in A, C and D, 2 µm in B). E–H. Secondary electron images (complementary to A–D.) which are simultaneously recorded during the measurement and showing the individual cells. The aggregation of cells was an artifact of filtration; the <i>C. watsonii</i> cells are unicellular (as per microscopic observation), however, gather in trenches upon filtration due to the unevenness of the filtration devices at the micrometer scale.</p

Summary of single-cell photosynthesis (DIC uptake) and N₂ fixation rates.

<p>Values are in fmol C and N cell⁻¹ h⁻¹ for DIC uptake and N₂ fixation, respectively, and represent mean ± SD.</p>*<p>significantly different from all other phases, all other phases are not significantly different from each other.</p>†<p>not significantly different from each other, but from ^‡.</p>‡<p>not significantly different from each other, but from ^†.</p

¹⁵N₂ fixation and photosynthesis (NaH¹³CO₃ uptake) rates as calculated from the isotopic enrichment of individual cells (each symbol represents one individual cell).

<p>The corresponding dark or light phase is indicated in the upper right corner of each panel. The large variability in the ¹³C signal/photosynthesis for the regular dark phase is due to the precision of the nanoSIMS measurement combined with the low labeling during the non-photosynthetic phase.</p