Dataset: Trichodesmium_AE1409

All data collected as described in Van Mooy et al (2015).

Sampling - Trichodesmium colonies were collected with surface water net tows along a cruise transect in the western North Atlantic aboard the R/V Atlantic Explorer (AE1409) during May 2014. Sampling occurred at the same time each day (~7:30-8:30 am) using nets with a mesh size of 130 um. Nets were deployed and hauled through the surface water column 6 times before recovery. Individual Trichodesmium colonies were isolated and washed three times by successive transfer through fresh 0.2 um sterile-filtered local surface seawater. A pooled sample of colonies was isolated and processed from each station. For each sample, an average of ~30 cleaned colonies were transferred onto 47 mm 5 um pore size polycarbonate filters, gently vacuum filtered to remove excess liquid, flash frozen and stored in liquid nitrogen until extraction and sequencing. There were no discernable changes in average colony size from one station to another across the transect. In order to broadly assess the microbiome composition of the North Atlantic Trichodesmium populations, colony composition was sampled to reflect the distribution of Trichodesmium colony morphology found in net tows. At all stations raft type colonies were much more abundant than puff or bowtie variants with approximately 30 rafts to 2 puff/bowtie colonies. As such, the data largely reflect the dominant raft morphology.

Nitrogen Fixation Rates - N2 fixation was measured using the acetylene reduction technique as previously described (Capone, 1993; Paerl, 1994). Briefly, approximately 20 Trichodesmium colonies were placed in a 60 mL polycarbonate bottle containing 60 mL of filtered seawater. A 1 mL aliquot of acetylene was injected into the bottle through a septum cap, the bottle was gently inverted, and allowed to incubate in an on-deck incubator at ambient temperature and light. The headspace of the bottle was analyzed for ethylene approximately every 30 minutes and the rate of ethylene production through acetylene reduction was determined by linear regression. All incubations were conducted in triplicate between approximately local noon and 2 PM.

Phosphate uptake rates - The incubation bottles were carried to a laboratory van that was designated solely for work with radioactive isotopes. Each incubation bottle was spiked with approximately 1.5 uCi of 33P-phosphoric acid. The final concentration of 33P-phosphate in the incubations was approximately 6 pmol L-1, which was likely approximately two orders of magnitude smaller than ambient phosphate concentrations. The bottles were capped and mixed by gently inverting. At each station, three incubations were dedicated to measuring 33P-phosphate uptake and three incubations were dedicated to measuring the chemical reduction of 33P-phosphate to P(III) compounds. The bottles were placed in a flow-through on-deck incubator that was maintained at surface seawater temperatures by continually flushing with the surface seawater from the ship’s pumping system. Temperature in the incubators was occasionally monitored with a waterproof temperature logger (Onset), and found to be within 1C of surface water temperature. The incubators used a combination of neutral density screening and blue transparent film to achieve a light intensity of mimicking PAR at roughly 20m, as confirmed using an underwater spherical quantum sensor (Li-Cor). At three occasions during the cruise (Stations 2, 4, and 9), an additional set of triplicate incubations for each measurement were terminated immediately (i.e. prior to incubation) and processed identically to the experimental incubations; data from these incubations were used to quantify background 33P signals in all of our measurements (i.e. analytical blanks). Background 33P was consistent at all three stations, and was averaged and then subtracted from all of the experimental results; the standard deviation of the background was propagated as analytical error. In all cases the 33P radioactivity recovered from the experimental incubations was three times greater than the background 33P radioactivity. Incubations proceeded for an average of 3.25 h before being terminated by vacuum (approximately 200 mbar) filtration on 25 mm diameter polycarbonate membranes (Millipore); a poresize of 0.2 um was used for whole community incubations and a poresize of 5.0 um was used for the Trichodesmium incubations. The membranes were quickly rinsed three times with freshly filtered (0.2 um poresize polycarbonate membrane) surface seawater. The membranes were then immediately placed in a liquid scintillation vial containing 10 mL of UltimaGold liquid (Perkin Elmer) scintillation cocktail, which was then shaken vigorously. After resting for a few hours, the 33P-radioacitivity in the vials was determined using a liquid scintillation counter (Perkin Elmer). A steady-state phosphate turnover rate was calculated by dividing the total 33P radioactivity retained on the membranes by the total 33P radioactivity added to the incubations and the incubation time. Turnover times (reciprocal of turnover rates) varied from between 15 and 50 hours (not shown), which is much longer than the incubation time and validates the steady-state calculation.

Phosphate concentrations - Phosphate in seawaters samples and incubations was quantified using MAGnesium Induced Coprecipitation (MAGIC) as described by Karl and Tien (1992).