Presentation1.pdf

<p>Recently, the method widely used to determine ¹⁵N₂ fixation rates in marine and freshwater environments was found to underestimate rates because the dissolution of the added ¹⁵N₂ gas bubble in seawater takes longer than theoretically calculated. As a solution to the potential underestimate of rate measurements, the usage of the enriched water method was proposed to provide constant ¹⁵N₂ enrichment. Still, the superiority of enriched water method over the previously used bubble injection remains inconclusive. To clarify this issue, we performed laboratory based experiments and implemented the results into an error analysis of ¹⁵N₂ fixation rates. Moreover, we conducted a literature search on the comparison of the two methods to calculate a mean effect size using a meta-analysis approach. Our results indicate that the error potentially introduced by an equilibrium phase of the ¹⁵N₂ gas is −72% at maximum for experiments with very short incubation times of 1 h. In contrast, the underestimation was negligible for incubations lasting 12–24 h (error is −0.2%). Our meta-analysis indicates that 84% of the measurements in the two groups will overlap and there is a 61% chance that a sample picked at random from the enriched water group will have a higher value than one picked at random from the bubble group. Overall, the underestimation of N₂ fixation rates when using the bubble method relative to the enriched water method is highly dependent on incubation time and other experimental conditions and cannot be generalized.</p

Metadata for stations sampled in the ARP.

<p>Measurements taken in conjunction with the metatranscriptomes are listed here. Asterisks highlight where concentration of the variable was below limit of detection.</p

Sample size-normalized gene counts for the 31 biogeochemically-relevant genes.

<p>Values are the average of the duplicate samples, per 10 million sequences. Bolded/underlined numbers highlight the highest expression for that gene.</p

Transcriptomic versus biogeochemical data.

<p>Panel A: The correlation between diatom microscope counts and log RuBisCO Form ID transcripts counts. Panel B: The inverse relationship of carbonic anhydrase transcript abundance to DIC concentration. Panel C: The inverse relationship between polyphosphate kinase transcript abundance and phosphate concentration. Station 2 and 25 had little or no phosphate, due to the diatom bloom, however <i>ppk</i> was not upregulated.</p

Salinity map of the May/June 2010 Amazon River Plume cruise aboard the RV Knorr.

<p>Salinity (PSU) from the underway system along the ship track was augmented with National Oceanographic Data Center profiles in regions of low coverage then interpolated and contoured.</p

Ratios of transcript abundance at stations 10:2 (black bars) and 25:2 (white bars).

<p>Station 10 has very high levels of eukaryotic nitrate transporter as well as chitin synthase compared to station 2. Note log scale. Stations 2 and 25 perform similar functions in the ARP. Thus the plot of the ratio of Station 25: Station 2 has smaller values than the ratio of stations 10 and 2.</p