All analytical and sampling methodologies are described in Burkhardt et al. (2014). However, summary of most relevant methods are included here:

To explore the relationship between POM source and remineralization rates and stoichiometry, the investigators conducted a suite of on-deck incubation experiments in the North Pacific Subtropical Gyre (NPSG) in March of 2011 near Station ALOHA. 20-L aliquots of seawater were collected from the 75-m depth horizon at Station ALOHA. Immediately after collection, seawater was stored in the dark in an incubator continually flushed with surface seawater for ~72 hours. Dried POM material (cultured Trichodesmium IMS 101, “TRICHO”, Prochlorococcus MED4, “PRO”, T. weissflogii, “DIATOM” and the natural POM from the Oregon coast, “OR-POM”) was added to the carboys with aged Station ALOHA seawater. Each treatment was prepared in duplicate except for the OR-POM. Concentrations of ammonium (NH4) and SRP were obtained every 5 min for roughly the first half hour following POM addition to capture any solubilization trends. This initial phase was followed by discrete sampling every 3 hours. Nutrient samples were run at OSU, NMR samples were run at the University of California, Santa Cruz.

Nutrients were analyzed using flow-through colorimetric methods on a Technicon Auto Analyzer II. SRP was analyzed using the phosphomolybdic acid reduction; ammonium (NH4) was measured by the indophenol blue method (Gordon et al., 1993); and nitrate + nitrite (N+N) was analyzed using the cadmium reduction method of Armstrong et al. (1967). Detection limits were 55 nmol L-1 for SRP, 22 nmol L-1 for NH4, and 8 nmol L-1 for N+N. Total dissolved P and N (TDP and TDN, respectively) were determined by the alkaline persulfate oxidation method (Valderrama, 1981) using a 1:10 oxidant to sample ratio. Dissolved organic P (DOP) and N (DON) were calculated as the difference of TDP and SRP and TDN less the sum of NH4+ + NO3- + NO2-, respectively.

Particulate C, N, and P content of each POM type was determined by collecting a subsample of the biomass onto combusted GFF filters, wrapping in foil, flash freezing, and storing at -80 degrees C. The filters were then thawed and dried at 60 degrees C overnight, folded into tin and silver boats, and run on a Carlo-Erba C/N Analyzer for particulate C (PC) and N (PN) content (Sharp (1974). For particulate P (PP) analyses samples were thawed and combusted at 450 degrees C for 4.5 hours, then extracted with 0.15 M HCl for 1 hour at 60 degrees C. PP was then analyzed as SRP in a 1.0 cm cell at 880 nm following Strickland and Parsons (1972).

Molecular characterization of PP compounds was performed using subsamples of each POM type with 31P nuclear magnetic resonance (NMR) spectral analysis as per Cade-Menun et al. (2005). Samples were freeze-dried, extracted with a 25-mL solution of 0.25M NaOH 0.05M Na2EDTA for 4h, and then centrifuged. 1-mL aliquots of the supernatant and digested residue samples were analyzed for P concentrations via inductively coupled plasma optical emission spectroscopy (ICP-OES) to determine the extracted P and fraction that was not extracted. The remaining supernatant was analyzed for 31P-NMR spectroscopy on a 600 MHz Varian Unity INOVA spectrometer equipped with a 10mm broadband probe at 20 degrees C and a 90 degrees pulse. Compounds were identified by their chemical shifts (ppm) relative to an external orthophosphoric acid standard. After standardizing the orthophosphate peak in all samples to 6 ppm, peak assignments were based on Tebby and Glonek (1991) Cade-Menun and Preston (1996) and Turner et al. (2003b,c). Peak areas were calculated by integration of spectra processed with a 5 Hz line broadening, using NUTS software (Acorn NMR Inc.) as described in Paytan et al., (2003). Finally, the relative contribution of surface-adsorbed P was assessed for remaining TRICHO and DIATOM POM samples via the oxalate rinse method described in Fu et al. (2005); not enough material remained from PRO and OR-POM for similar analyses.

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